for high-power LEDs
About this project:
After constructing the Pulse Width Modulator for High Power LEDs I needed to build another LED modulator for another optical transceiver. For this project I decided to take a different approach and use solely linear techniques for the audio modulation. Like the PWM circuit, this circuit also uses the "precision current sink" to regulate set the LED current but in addition to the linear modulator, a tone generator - based on the same source code as in the PWM circuit - was added to facilitate testing and aiming.
In order to linearly modulate the intensity of an LED it is necessary to vary the amount of current flowing through the device rather than the voltage across it. It is fortunate that the luminous output of an LED is linearly proportional to the current flowing through it: At higher currents, the "current versus light output" curve "flattens" a little bit, but this results on only a very small amount (a few percent at most - see the sidebar below) of distortion and is unnoticeable in voice communications.
 |
Comments:
- When you use an LED other than what is mentioned below, please take into account the current ratings of the devices that you plan to use.
- Philips is apparently phasing out the Luxeon I, III, and V lines in favor of the lower-power Luxeon Rebel devices: For the time being, however, the devices discussed in specific detail here - the Luxeon III devices - are still available as old stock from various sources. If you have any questions about interfacing the LEDs of your choice, feel free to contact me using the link at the bottom of this page.
- The circuits shown on this page are quite adaptable: With the techniques described below, I have constructed modulators for LEDs with current ranges from just a few milliamps to 10's of amps!
- Please heed the comments on the use of the LM324 for
these circuits - see the sidebar,
below.
First, a bit of explanation as to how the circuit works.
The "Precision Current Sink":
Several means have been devised to modulate high-current LEDs, variously using bipolar and power MOSFET devices, several examples of which may be found on the page "The 'Luxeon': New Light of Hope for Optical Comms". I decided to try a different approach - the Precision Current Sink - which is very similar to some types of current sources.
The basic current sink circuit may be seen in Figure 1. This circuit uses a single section of an operational amplifier "wrapped around" a transistor and using a current sense resistor and it works thusly:
- Assume that 1 volt is applied to the non-inverting input of the op amp at Vin.
- If the voltage across Rsense is lower than 1 volt, the output voltage of the op amp is increased, turning FET - Q1 - on until enough current flows through Rsense to achieve a voltage drop equal to that of Vin. Ohm's law tells us that if Rsense is equal to 1 ohm, we will achieve a drop of exactly one volt when the current through Rsense is one amp. Likewise, if the current is higher than one amp the op amp will reduce the drive to Q1 to reduce the voltage drop across Rsense until it is one volt.
- As the voltage applied to Vin changes, the op amp will change the drive to make the current through Rsense proportional to Vin. Because the drain current through Q1 is the same as the source current (ignoring the minute amount of gate-source leakage) we know that the current through the LED's current will be the same as that of Rsense.
- Cfilt is important in that it provides a
low-impedance source current. Without it, audio would
be imposed on the power supply leads and get into other
circuits. Also, there is no guarantee that the power
source - be it a battery or power supply - will provide a
low impedance path at all frequencies - especially if fairly
long power leads are used. For high-power (>1 watt)
LEDs, a recommended value is at least 1000uF.
To make this circuit practical for use as a linear modulator for speech, several refinements need to be made:
- A fixed DC voltage needs to be applied to Vin to establish an "idling current" for the LED. This idling current would be set to half that of the expected peak current - a value that would correlate with "100% positive modulation." For a red Luxeon III LED, this maximum peak current would be 2.2 amps or a "resting" current of 1.1 amps.
- Modulation needs to be applied. The voltage at Vin would, in addition to the fixed DC voltage, have the audio modulation applied to it.
- A few additional capacitors need to be added to make the modulator stable under varying current conditions.
Comment: While an N-Channel power MOSFET is shown in the circuit diagrams, a NPN transistor could also be used. The MOSFET was used because its drive requirements are negligible at audio frequencies (e.g. only capacitance) and because they are rather ubiquitous (even available at Radio Shack) and they are inexpensive. If an NPN transistor were used for a high-power modulator, a Darlington arrangement would likely be required as the current-sourcing capability of a typical op amp is likely inadequate for reliable operation. It should also be noted that if an bipolar transistors are used that a few other changes in the stabilizing resistors/capacitors would also be required to maintain circuit stability.
 |
The circuit shown in Figure 2 addresses the added requirements. The "heart" of this circuit is the "precision current sink" consisting of U1B, Q1, and R9. I don't recommend this circuit for serious use, but rather as one to illustrate the concepts involved for the reasons mentioned below: I would recommend the circuit depicted in Figure 3, instead.
In this circuit, the current through the current-sense resistor, R9, produces a voltage that is proportional to the current through the LED. U1B is wired such that, because of the closed-loop feedback, it will attempt to make the voltage on pin 6, the inverting input, the same as that on pin 5, the signal input: If the voltage on pin 6 is below that of pin 5, the output voltage of the op amp will increase, causing Q1, a power FET, to conduct more, until the voltages on pins 5 and 6 match each other. Because the luminous output of LEDs is proportional to their current, the result is an extremely linear modulation curve with excellent audio fidelity.
In order to properly modulate an LED one needs to establish a "resting" current: A "100% modulated LED" will experience a current excursion from zero to twice the resting current, with the resting current being half of the maximum current and to do this, R7 sets a DC bias. With the 5 volt reference supply shown this resting current could be set from zero all the way to 2.5 amps, but with a 3-watt Red Luxeon device, this would be set to 1.1 amps as measured by observing a drop of 1.1 volts across R9.
The audio is applied and modulated atop this resting current, coupled via C3, a 0.1uF capacitor. The value of 0.1uF was chosen purposely as it will limit the low-frequency response of the modulation to about 150 Hz or so at the -6dB point - and this small value will also allow the circuit to stabilize more quickly when it is powered up and to minimize effects of wind or breath on the user's microphone. The circuit consisting of U1A is a simple audio amplifier, the gain being adjustable using R4, with the audio coming from an inexpensive electret microphone. If the gain of this circuit is not adequate for the microphone being used, it can be increased by decreasing the value of R3 (to as low as 1k) or by increasing the value of potentiometer R4 to as much as 1 Meg.
In order to maintain stability of the circuit with varying power supply voltages, U2, a 5 volt regulator is used because without a stable voltage reference the LED's resting current would vary: A regulator is chosen over a simple Zener-based system as it removes from the reference supply any traces of modulation that might be imposed on the power supply, improving stability and preventing "motorboating." Another critical component is C5, a 2200uF capacitor and this provides a low-impedance path, preventing current swings from excessively modulating the power supply: Without it, the entire circuit could be unstable, and modulated audio might ingress any other devices operated from the same supply. R11/C4 are also used to improve stability of the circuit at higher frequencies. It should go without saying that Q1, the N-channel power FET, must be heat-sinked!
When building any circuit that mixes both low-level (microphone) signals with high-current ones (from the LED) it is important that single-point grounding techniques be employed. What this means is that all ground connections having to do with the LED, its power transistor (through R9) and bypass capacitor (C5) should be connected to one point. The other ground connections on the rest of the circuit should be connected to that "high-current" grounding point with just a single wire. The idea here is to prevent any voltage drops across any wires that carry current from being communicated to the low-level portions of the circuit. It's easy to forget that a few centimeters of small-gauge hookup wire could easily have a few hundredths of an ohm of resistance: If a couple of amps is flowing through that wire, the crop across it could be 10's of millivolts - a level not too different from that coming out of the microphone!
Current limiting:
Another component worth mentioning is the current-limiting resistor in series with the LED (RLimit.) If the LED module is not constructed with a means of Overcurrent Protection then the builder may feel more comfortable with the addition of this component. For a 12.0 volt supply (representing a partially-depleted lead-acid battery) using a Luxeon III, one would wish to limit the absolute peak current to 3 amps or less. Assuming a nominal peak current of 2.2 amps one would incur 2.2 volts across R9, about 1 volt across an inexpensive MOSFET, and 4 volts across the LED - plus another half volt or so due to IR drop in the connecting wiring for a total voltage drop of 7.7 volts or so - the "remainder" being about 4.3 volts. In this particular case, using a 1.5 ohm resistor for RLimit with a dissipation of 5 watts would be sufficient, as the average current would be 1.1 amps.
Another way to limit the maximum current to the LED would be through the use of a larger ohmic value of resistor for R9 - but remember that doing so will not only require a higher-power resistor, but still-higher peak-to-peak audio levels from U1A, requiring one to take into account that sufficient audio levels would be available.
| "How 'linear' is this modulator?" To determine the relativel linearity of the modulator, the amount of distortion was measured using a Hamamatsu S1223-01 photodiode in parallel with a 10k resistor, the output of which was connected to a computer sound card. In each case, a red Luxeon III high power LED was used as the light source, using the circuit in Figure 4 as the modulator. Before any other measurements were to be done, the amount of distortion of the current waveform applied to the LED was measured to provide a reference point as to how "clean" the signal being applied to the LED might be. First, the Spectran program was used to measure the distortion produced by the 1 kHz tone generator itself: The 2nd and 3rd harmonics were found to be at least 59dB below the fundamental. Next, the distortion of the modulator itself was measured by observing the voltage waveform across the current-sense resistor ( Rsense in Figure 1.) With the modulator set at 100% and a small amount of audio compression occurring, the measured distortion was:
The total distortion of the two harmonics was about 0.5% and did not change more than 1 dB between 1.1 amps and 100 milliamps. It should be noted that the above test measures the linearity of the modulator only: Remember that the curve relating and LED's luminous output versus current is not perfectly linear, so we need to run a few more tests. Next, the distortion was measured using a plain photodiode, illuminated by the LED and configured as noted above: Care was taken to avoid saturating the diode. To determine the effect of nonlinearity of current-versus-luminous output, two sets of measurements were taken as noted below: Measurement 1: 1.1 amps of average current, peak current of 2.2 amps
Additional testing was done using the KA7OEI Version 3 optical receiver with the lowpass filter bypassed and light attenuated to avoid saturating the receiver: LED Current of 1.1 amps:
|
The use of more than one Luxeon in series:
This circuit, as shown in figure 2 (assuming that RLimit is omitted and is zero ohms) is capable of driving two Luxeon III LEDs in series while operating from a 12.0 volt supply. If one is driving a pair of LEDs in series, more careful attention must be paid on the value of RLimit, if one chooses to use this resistor in the first place. If voltage drop is to be minimized, the selection of a FET for Q1 with a low ON resistance is also advised.
It is also worth pointing out that reducing the value of R9 further-reduces the amount of voltage drop across the modulator, allowing one to modulate a pair of Luxeon III LEDs in series with a power supply voltage below 10 volts. If, for example, R9 was changed to 0.25 ohms then only 0.55 volts of peak-to-peak audio would be required at C3 to modulate one or more Luxeon III LED's (in series) to 2.2 amps. If the value of R9 is decreased, it is advisable to increase the value of R5 (to about 33k or so in the case of R9 being 0.25 ohms) to put a "safe" upper limit on the amount of the LED's resting current as adjusted by R6.
Setup:
It is recommended that one uses a current-limited supply (maximum of 2.5 amps or so) or replace the LED with, say, a 1 ohm resistor, when checking out the circuit and setting things up: It would be easy, due to a wiring error or misadjustment, to destroy the LED. The first step would be, with the gain set all of the way down (R4 at minimum resistance) to set the idling current at 1.1 amps with R6: These recommended currents are those suggested if one is using a 3-watt Red Luxeon: Different devices may require different current.
The next step would be to adjust the audio gain. For this, one starts out with minimum gain and simply increases it until 100% modulation is achieved. Even without any test equipment this is easily done as one can tell by ear when full-modulation is achieved by noting that audible clipping starts to occur.
It should be noted that if the resting current is adjusted using R6, the audio gain should be readjusted at the same time to assure 100% modulation.
Notes on operation and construction:
- It is worth noting that, with this circuit, there is essentially no limit as to the amount of upwards modulation that can occur: While the "negative" (downward) modulation is obviously limited at zero amps, if the audio gain is too high, the peak current could achieve many amps, so it is recommended that one be careful to avoid overdriving the circuit to excess: Through sad experience, I have noted that the bond wire on a 3-watt Luxeon will fuse (e.g. burn open) somewhere between 4 and 7 amps of average current! This can happen with careless adjustment of R6 or, possibly, if a sustained audio overdrive occurs.
- Again, note that the use of an LM324 (or similar) for U1
is absolutely essential: This op amp -
unlike many others - is capable of handling an input voltage
of zero volts and outputting a voltage down
to zero volts on the output. If another op amp is
chosen make certain that it is a "rail-to-rail" type on both
its input and output - or at least capable of
operating down to the negative rail like the LM324 - but
keep in mind that most common op amps (like
the LM1458, LM358, NE5532, TL084, etc.) are not
able to do so. If an op amp was chosen that could not
operate down to the negative rail, U1 could have to be
operated with a negative supply at pin 11, but this need be
only 2-3 volts at very low current - something that could be
provided by a pair of 1.5 volt cells in series or a simple
voltage converter. Also, the phase/frequency response
of op amps other than the LM324 may not result in stable
operation with the components shown.
- This circuit does not include any sort of audio processor. If communications is anticipated in which a low-signal/noise ratio is likely it is recommended that one connect an audio compressor or "speech processor" at the microphone input to improve the peak-to-average ratio
- It is possible to strip the circuit down even more of
so-desired. One could, for example, simply apply a
source of audio to C3, dispensing with the preceding
circuitry associated with U1A. To fully-modulate a
Luxeon III, 2.2 volts peak-to-peak of audio (correlating
with 2.2 amps of current at 100% positive modulation) would
be necessary.
- The circuit depicted in Figure 2 is intended to
demonstrate how one might modulate an LED, but from a
practical standpoint, it is a bit awkward in terms of
setting up the LED's operating current in that it is fairly
easy to accidentally overdrive and burn out the LED - which
is why I recommend the circuit in Figure 3 described
below.
Minor circuit refinements - improving current limiting and adjustment:
If you are going to build a simple modulator, I would recommend the circuit in Figure 3 over that in Figure 2 as it is much easier (and safer, from the LED's viewpoint) to use.
The LM324 is cheapest, most commonly-available op amp available that can operate with an input and output down to zero volts, and with it we have four available op amp sections. By using just one more section than the circuit shown in Figure 2 and adding two resistors we can take advantage of the op amp's supply-limited voltage swing to put an absolute limit on the maximum amount of current that could be applied to the LED and provide a very easy means of adjust LED current while maintaining full modulation: Figure 3 shows such an adaptation.
In this circuit, U1C is used as a microphone amplifier, capable of a gain of up to about 20dB, followed by U1B which is wired as another op amp section with a fixed gain of about 13dB - or a voltage gain of 5: Note that the value of R7 could be increased (to several hundred k-Ohms) if it turns out that U1A/R4 do not yield enough gain from your microphone to adequately drive the LED to 100% modulation. It is important to notice that U1C is AC coupled to U1B to prevent cumulative DC offsets from being amplified from U1C. Wired in the way shown, U1B's output voltage is centered around 5 volts as referenced from U2, the 5 volt regulator, with modulation riding atop it with 100% modulation being 10 volts peak-peak.
When operating from a 12 volt supply, the output of U1B is limited in its swing from zero volts, representing zero current (or "0%" modulation) to just short of 11 volts - this upper voltage being limited by the supply voltage and the ability of the LM324's output to swing only within about a volt or so of the positive supply rail. Because of this intrinsic, positive upper limit of the audio voltage swing, the maximum LED current is limited to a reasonable positive value - about 110% when operated from a 12.0 volt supply, or about 125% when operated from 13.8 volt source - no matter how much audio drive is applied. Of course, if one operates U1 from a higher voltage supply (say 24 volts) this intrinsic "positive modulation" current limiting does not necessarily apply.
 |
The actual LED currents are set using R8 and R9 to scale down the voltage output from U1B: R8 is used to set the maximum current obtainable when R9 is set to full current and this arrangement has the advantage that if R9 is front-panel mounted, one may use it to continuously adjust the LED current from "full" current (1.1 amps of idle current, for example in the case of a red Luxeon III) all the way down to just a few milliamps - still have 100% modulation of the LED at any selected current. If one does not require the need to adjust the LED's idle current from the front panel, the one can eliminate R9 completely, using R8 to set the idle current once and forgetting it.
Because of a quirk in the way the LM324 works, R5 and R8 are used to load the outputs of U1C and U1B, respectively, to prevent crossover distortion. In the case of R8, the resistance value is not critical - but it should be less than 5k. Practically any value of potentiometer may be used for R8 from 1k up to 100k, but if more than 5k is used, simply parallel a lower-value resistor (anything from 1k to 4.7k is fine) across it to maintain loading at pin 7.
Setup:
- To avoid damaging the LED, Use a current-limited supply or replace the LED with a 1 ohm resistor.
- Remove audio input.
- Turn R8 all of the way down (e.g. wiper to ground.) R8 should not be user-accessible as it is a "set once" adjustment.
- Turn R9 all of the way up (e.g. wiper connected to R8.)
- Slowly turn up R8 while monitoring the LED current until an idle current of half the peak rating for the LED is obtained. Since the peak current rating of a red Luxeon III is 2.2 amps, this would be set for 1.1 amps.
- You are done!
A few comments applicable to all circuit variations on this page:
In order to maintain stability all of the completed circuits have capacitors at the output. Taking the schematic in Figure 3 as an example:
- A large, good-quality capacitor should be used for C3. This prevents large amounts of modulated audio from appearing on the power supply lead. This capacitor should be placed physically close to the connection to LED1, Q1, and R11 and good, common-point grounding techniques should be used.
- Capacitor C4 is used to assure stability of the current sink itself. Without this capacitor, high-frequency oscillations may appear on the output causing audible distortion to occur under certain circumstances. Note that the values of capacitance used have no affect in the audio frequency range.
- The output transistor should be attached to a heat sink. If it gets hot enough to boil water, it needs more heat-sinking!
- Remember: Whatever you use for U1, it must
be capable of operating down to the negative supply
rail. The LM324 is the cheapest and easiest-to-find
op amp that can do this. Again, with the components
shown only the LM324 will result in stable operation.
Up to this point "100% modulation" has been mentioned - but not explained. For the purposes of this discussion, 100% modulation refers to the fact that the LED's current varies about a "resting" current from zero to twice the resting current. As it turns out, this range represents the maximum that the amplitude-modulated LED can be driven and avoid distortion of the original signal.
If you were to attempt to exceed 100% modulation, the most obvious side-effect would be that some portion of your waveforms would try to go below zero current - something that is clearly impossible - and that portion of the audio would be hard-clipped, causing distortion. On the "positive" side of the modulation, it may be possible that the current could go above twice the resting current - at least until the op-amp, transistor and/or power supply ran out of voltage/current "swing" at which point it would "hard clip." As mentioned above, the circuits described, when operated from 12-14 volts - a range that encompasses portable battery operation - can produce around 125% positive modulation.
Practically speaking, however, one can overdrive the audio circuit and cause a significant amount of clipping (perhaps 10-20%) and actually improve intelligibility somewhat under noisy/weak-signal conditions. While the fidelity of the speech is obviously reduced, the impact on its intelligibility is generally negligible and the fact that the "peak-to-average" ratio is reduced (that is, the "quiet" portions of speech are louder by comparison than the loudest portions) and the overall "speech power" is actually increased and can be better heard when conditions are poor.
Exactly how much "clipping" is "too much"? That's mostly a matter of taste. Clearly, if one runs the audio too "hot" then excessive clipping will result in enough audio distortion to reduce intelligibility. Again, it should be noted that the circuits in Figure 3 and Figure 4, when operated from a 12-14 volt supply, can be run into clipping without much fear of damaging the LED as they intrinsically limit the maximum amount of LED current to a reasonably safe value. The same cannot be said of the circuit in Figure 2, however!
What about lower than 100% modulation? If one doesn't fully-modulation the LED, the result is that the amount of audio being conveyed by the lightbeam is reduced and it will sound "quieter." Where this becomes an issue is where the signal is already weak or competing with noise: A badly-undermodulated (or "quiet") signal is at a significant disadvantage and is clearly not being used to its full potential.
In other words, in terms of overall effectiveness it's better to run the audio a bit "hot" and put up with a little bit of distortion than run it too low!
Why I recommend the circuit in Figure 3 over those in Figures 1 or 2:
As noted above, when first setting up the circuit with the LED, R8 is set to a setting correlating with the maximum "idle" current for the LED and if this control is not disturbed (e.g. not accessible from the front panel) one cannot easily damage the LED by either overdriving it or through misadjustment, making the circuit nearly foolproof!
As is also mentioned, when properly configured, one can also adjust the LED's drive current from the pre-set maximum down to zero, but not have to adjust the audio level to keep the LED fully-modulated at any setting. With this capability, you can do things such as:
- Link margin tests. By measuring well your
signal can be copied as you lower the current, you can
determine how much "extra" signal you have at "full
power" and with this capability you can determine how
good your path is, how well your receiver/transmitter is
working, etc. With the circuits shown in Figure 3
and 4 the depth of modulation is independent
of the LED current as determined by the "Idle Current Set"
potentiometer! What this means is that you can adjust
the LED's power up and down without having to adjust
anything else to maintain proper modulation: By
knowing the idle current as you make your adjustment you can
calculate your link margin and determine the actual
performance of your gear under the extant atmospheric
conditions!
- Reduce power consumption. If you are
operating from battery power and you have
particularly good signals, you can turn down the current and
save battery power while maintaining communications:
Again, with just one knob to adjust this is very easy!
- "Safely" run 100% modulation on the LED. For
best link margin/performance, it is desirable that one runs
100% modulation of the LED (e.g. current down to zero and up
to twice the LED's resting current) as dropping to just 50%
modulation will reduce the recovered signal at the receive
end by 6dB! With the circuits in Figure 3 and
Figure 4 the downward modulation is limited by the
fact that one cannot go below "zero" current while the
upward modulation is limited by the available voltage swing
of the driver stage (e.g. U1B in Figure 3) which, when
powered by 12 volts, cannot exceed 11 volts or 110% of the
"full" modulation. Practically speaking, one can set
the modulation to frequently clip the voice peaks without
causing objectionable distortion - a practice that increased
reduces the peak-average modulation and can increase
intelligibility on links with low signal-noise ratio - much
like a speech clipper on a transmitter.
Adding a few features:
The "simple" modulator shown in Figure 3 work nicely, but in order to improve intelligibility and facilitate alignment of the optical gear a few extra features would nice to have:
- Audio compressor/AGC. Maintaining as close to 100% modulation as possible is very helpful when trying to communicate over a link with a low signal-noise ratio - especially as one might move closer-to and farther away from the microphone.
- Tone generation. Being able to generate tones can aid in alignment as well as provide a reference when measuring and analyzing the link's properties, such as scintillation.
- Variable drive level. In the circuit below, the current can be adjusted over its entire range while still maintaining 100% modulation.
- Line-level input. A separate line-level input is a nice feature to facilitate inputs from other audio sources - such as audio from a computer or digital audio player. This facilitates use of the transmitter for audio-based modes such as WOLF, WSJT, PSK31 and SSTV, just to name a few possibilities.
- Additional monitoring features. These allow
easy measurement of the LED's operating current as well as
providing indications of "full" audio modulation and the
monitoring of the "transmit audio" either via a headphone or
to a recording device.
Circuit description - "Fancy" version:
Modulator:
 |
Signal input stage:
The audio input can be one of three sources: A built-in electret microphone for those situations where you forgot to bring one, an external microphone via J1, or an external line input via J2. S1, an SPDT switch, selects which source is to be used, and if a "center off" switch is used, the center position can serve as a "mute" setting. Note that J1 is a "disconnecting" type of jack and is wired to disable the internal microphone when an external one is plugged in. In experimentation, it has been noted that computer-type microphones are wired in one of two ways: While the audio is always on the "tip", some connectors apply bias to the tip and leave the ring disconnected while others apply the bias voltage only to the ring. The circuit shown accommodates both wiring schemes.
Note also that J2 is wired such that the two resistors will sum (and attenuate) a line-level stereo input (from a computer or an audio player) to a monaural signal. The resistors (R3 and R4) are necessary in many audio amplifiers because it has been observed that with many stereo-output audio devices, simply shorting the left and right channels together often results in distortion as the two amplifiers will "fight" each other.
Compressor/AGC:
C1 is used to limit the low frequency response to about 30 Hz and U1C, a non-inverting amplifier, contains a gain cell, OC1 that consists of a Cadmium-Sulfide (CdS) photocell and LED, optically coupled in a light-proof package. The CdS cell (which is in parallel with R9, the "Max Gain" control) will decrease in resistance when the LED, driven by U1D, is illuminated at high audio levels, thus causing the resistance to go down and reduce the circuit gain. In this way, an "AGC" or Automatic Gain Control circuit is formed, keeping "loud" signals from overdriving the modulator and bringing up quiet audio to a higher level, keeping the level fairly constant. Under poor conditions, it is advantageous to keep the audio level as "loud" as possible - but not so loud that overdriving and distortion occurs.
U1B is used as a summing/gain amplifier (a voltage gain of 5 is set) to provide additional amplification. The output of this amplifier is fed to U1D which has a variable gain setting that effectively sets the maximum amplitude of signal that can be present at the output of U1B. The output of U1D goes to a full-wave bridge rectifier (D1-D4) that allows the LED to be illuminated on either positive or negative excursions of the audio waveform. C3/R12 prevent "clipping noise" from being put onto the +5 volt supply line as well as preventing excess positive excursions of the +5 volt rail by averaging out the conduction current through the LED.
There is another input to U1B: The signals from an audio tone generator (see below) are input via R21 and C8. Note that the audio levels from the tone generator are not affected by the AGC amplifier, but the AGC does detect the presence of those tones being generated and will decrease the gain of U1C accordingly to prevent overmodulation of the composite signal.
Comment about the gain control device:
It is possible to use other devices for gain control: The CdS cell and LED method mentioned above has the advantage of consisting of components that are probably already in your parts box as described below! Other suitable devices include JFETs, MOSFETs, gain-control devices like the NE/SA570 (or the '571) and those could have been used as well.Pre-emphasis:
Switch SW3 provides high-frequency boost to the transmitted audio - mostly to accommodate conditions in which the signal-to-noise ratio at the receiving end is quite low. While the frequency response of the transmitter and accompanying receive system are fairly flat, the human voice has relatively little energy at higher frequencies (above 1 kHz or so) but this is the same range in which un-voiced consonants have their energy. With weaker signals it was found that these un-voiced speech components - those that allow one to tell an "f" from an "s", for example - were among the first to be lost, making deciphering speech a bit more challenging. This capability, along with the liberal use of phonetics, can help improve intelligibility under such adverse conditions. If digital modes were used, pre-emphasis would probably be disabled.
Peak Indicator:
The Q2/Q3 circuit forms a "peak" indicator to let the user know that the audio level is sufficient to fully modulate the transmitter. In this circuit, R27 sets a threshold voltage: When audio from the output of U1d drops below this voltage, it causes Q2 - and, in turn, Q3 - to conduct, turning on the LED. Even though this indicator works only with the downward modulation, it serves the purpose of letting the user know that something is happening.
The purpose of this circuit is simply to let the user know that an "adequate" amount of audio is present: It is expected that it will flash frequently under normal conditions, with the AGC amplifier preventing gross overmodulation. What is most important is that the user note if the LED is NOT flashing occasionally - a conditions that means that either there's no audio at all (which could happen for a number of reasons - such as forgetting to move S1 to the proper position, having a microphone disabled, etc.) or too little audio to properly modulate the system - which could occur if R9 is set too low or if the external audio source were running with an inadequate output level.
About OC1:
OC1 is simply an LED that is optically coupled to a Cadmium Sulfide photocell. While these are commercially available (many are made by Vactec) they can also be homebrewed rather easily. All that is required is an LED (a high-brightness red one will work fine) and a small CdS cell. This CdS cell should exhibit a resistance of several megohms after a few moments in total darkness and under 10k in bright room lighting.
To construct a homebrew gain cell:
- Use clear epoxy cement to glue the face of the LED and the CdS cell together, making sure that the LED lights up the face of the CdS cell.
- After the epoxy cures, slide a piece of black heat-shrink tubing over the entire assembly - but do not shrink the tubing just yet! Use some black silicone adhesive (RTV) to fill in the open ends of the heat shrink tubing to prevent light ingress along the leads - especially on the LED side.
- After the silicone has cured, shrink the tubing.
- If black silicone and/or heat shrink tubing is not
available, you can carefully make do with black "spaghetti"
tubing or even black electrical tape - just take care that
there is no light ingress. If the circuit is housed in
a light-tight enclosure, this is less important, although be
aware that light leakage can cause confusing results during
testing.
Finally, some CdS Cell optical couplers contain a pair of LEDs connected back-to-back so that they respond to either positive or negative voltages. If you are constructing your own optical coupler, you can do this and avoid the need for D1-D4 completely - just make sure that you try to position the two LEDs so that they illuminate the CdS cell more or less equally: It is possible that the 2-leaded "bi-color" LEDs (e.g. one that lights with red with for one polarity and green with the other) will work nicely as well.
Comment: With this linear modulator it is more important that the downward modulation not exceed 100% too often - that is, the LED current cannot go below zero - as this will cause distortion. On the other hand, occasional excursions above 100% (that is, above twice the average, resting current) may be permitted. If the full-wave rectifier (D1-D4) is omitted and a single LED is used, the LED could be connected so that it responded only to downward ("negative") peaks of the audio waveform. Note that both U1B and U1D invert the audio, so the LED would, in fact, be wired to illuminate as the output voltage of U1D went downwards.
Output driver and monitoring:
U1A is wired as a "Precision current sink" and with R20, the 1 ohm "sense" resistor, one volt of input to pin 3 will result of one amp of current flow. Using U1A in this way guarantees that LED current will be proportional to the voltage present across R18.
Being that the output from U1B is AGC-limited to an amplitude pre-set during the adjustment of R16, this signal is always representative of one with 100% modulation. If one wanted a resting current of 1 amp, this output, through R17, a trimmer potentiometer, is set to provide 2 volts peak (1 volt "resting") voltage at its wiper. In this way, R18 may be used to provide a 100% modulated signal over a continuous range from full current to just a few milliamps.
Two monitoring points are provided: The junction of C11/R22 may be used to measure the average LED current, where 1 volt = 1 amp, while J3 is used to monitor the modulated audio with headphones, the level being adjusted by R23. S2 is in series with the LED, allowing it to be shut off without having to power down the entire circuit and PB1 allows simple on/off keying of the LED: If the LED is being modulated with an audio tone, MCW keying can be done using PB1.
Modifications to minimize voltage drop:
The following comments about minimizing voltage drop are generally applicable to both the "simple" and "fancy" circuits although specific component designations refer to those in Figure 4.
In testing, it has been noted that the LM324 used will properly operate down below even 10's of millivolts input and because of this, it is possible to reduce the value of R20 down to at least 0.1 ohms. If a low on-resistance MOSFET is used for Q1, one can construct a circuit that will fully-modulate the LED with less than 0.5 volts of additional voltage drop. What this means is that with these lower resistances it is possible to run a single Luxeon III from a 6 volt supply (with appropriate circuit modifications) or up to three Luxeon III's in series from a 12 volt battery supply!
With the circuit shown and a value of 1 ohm selected for R20, there is adequate headroom to modulate two red Luxeon III LEDs wired in series to 2.2 amps peak while using a single 12 volt "gel cell" as a power source.
With a 1 ohm resistor used as R20, it is possible to set the LED's resting current to 5 milliamps and still achieve nearly 100% modulation!
Comments about the circuit:
- I plan to revisit the audio monitor and install a buffer amplifier so that it will be insensitive to the load of the LED: As shown, the audio output decreases when R18, the current setting, is decreased - or just goes away altogether when S2 is opened.
- Do NOT omit R10, a 4.7k resistor that goes from the output of U1C to ground. Because a peculiarity with the LM324 as used in this circuit, objectionable crossover distortion may occur if you omit this resistor. If you chose another operational amplifier, it may not require this extra resistor, but whatever amplifier you chose, it MUST be capable of operating down to the negative supply rail - MOST OP AMPS CANNOT DO THIS!!!
- Remember that circuit stability can be assured only
with the LM324 using the components shown in the
diagram. If you use a different op amp, be prepared
to have to re-work the circuit to avoid oscillation!
- With a fairly high level audio input (a 500mV pk-pk 1kHz sine wave works nicely) on the line-in connector (with S1 appropriately set) use an oscilloscope on pin 7 of U1B to adjust R16, the "Limiting Level" control so that the output *just* hits the negative rail. If you don't have an oscilloscope, a low-frequency (30-200 Hz or so) tone may be used as any distortion due to hitting the negative rail is easily audible: R16 would be adjusted to the point where the distortion just goes away.
- Now, shorting the LED connection and monitoring the LED current test point (or across R20) turn R17 down all of the way (wiper to ground) and R18 up all of the way (wiper at R17) and then adjust R17 for a resting current of 1.1 amps. Do NOT do this adjustment using an LED as it is possible to exceed the LED's maximum ratings by accident and destroy the LED.
- R27 is adjusted so that the LED illuminates when the audio hits the 80-90% downward point, but it can also be adjusted by simply looking at the LED while modulating the unit at maximum gain, turning R27 until the LED turns off - and then backing it off until the LED just comes on again.
S1 is used to select between the line input or microphone input while R9 is used to adjust the maximum gain that the AGC will allow. If a high gain setting is used on a fairly high audio level, overmodulation will not occur, but there will be considerable compression - depending on the audio level at the input: High compression may be desired under conditions of low signal-noise in order to maximize intelligibility.
Note that in all positions of S2, the audio input is still active. This means that even when generating a tone the microphone will pick up and transmit audio, although the level will be reduced by the AGC amplifier because of its detection of the tone. If S1 is a "center-off" type of switch, the middle position can be used to mute the audio inputs or, if no line input audio source is present, muting can be accomplished by selecting that position.
The LED current may be set with R18, providing 100% modulation at any current from the full setting all the way down to a few milliamps.
Comment: Again, 100% modulation is defined as modulation that goes all the way from zero up to twice the average (unmodulated) current as set by R18.
Tone generator:
 |
On the same board, I also built a tone generator using an 8-pin PIC, the 12F683, which has an onboard A/D converter as well as a hardware-based PWM generator that can be used as a D/A converter. Much of the source code was "borrowed" from my PWM-based modulator, hence the similarities in operation.
Clocked at 20 MHz, there is a 19.53125 kHz interrupt that is used to generate sine waves using DDS techniques and a 10-bit sine lookup table, using the PIC's built-in PWM hardware as a D/A converter to produce 78.125 kHz square waves of varying duty cycle. The lowpass filter for the PWM-generated audio consists of R103, R104, C105 and C106, effectively removing most of the 78 kHz PWM energy while causing little attenuation of the highest-frequency audio tones.
Modes are selected by applying different voltages to GP1: These voltage are created using a resistive divider on the rotary switch and the digitized voltage is used to select the appropriate mode. Also present is R102, a pot that produces a variable voltage read by the PIC's onboard A/D converter and is used to select the various tone modes and frequencies.
On the output of the PWM network are two pots, R105 and R106, that are used to set the output tone levels to correspond with 100% modulation in the case of the test tones, and 25% modulation in the case of the pilot tone: The second half of the rotary switch (S1B) is used to select the tone level in the case of the test tones, the attenuated tone in the case of the pilot tone, or no tone (in position "E") should the pilot tone not be used at all. Note that C108 isn't needed if you use this circuit with Figure 4 as there's already a blocking capacitor, C8, in that drawing.
Pin 4 (GP3) is used to select whether or not the 1 kHz tone should really be 1 kHz. This arose when this circuit was to be used in areas with 50 Hz mains (e.g. most areas of the world other than the Americas) where a mains harmonic would coincide with the 1 kHz tone. By tying pin 4 high, the 1 kHz tone selected in "Tone Mode A" and the one available in "Tone Mode C" (see below) is, in fact, 1000 Hz. If pin 4 is grounded, a 1020 Hz tone is generated at those positions, instead. Note that even though the 4 kHz "pilot" tone is also harmonically related to 50 Hz mains, its frequency is not altered as it serves simply as an amplitude reference.
The nominal voltage used to select the tone mode is noted below and assume that the PIC is operating from a 5.0 volt supply. The actual voltage thresholds for selecting the modes are midway between the voltages specified.
Tone modes:
A - (0 volts) 1 kHz tone, fixed. The 1 kHz tone is a "standard" tone used for peaking the receiver using the audible S-meter system, for MCW keying, for measurement of scintillation, or just as a source of audio. Even though it is possible to set Mode C to generate a 1 kHz tone, this mode allows the convenience of simply turning the switch all the way to one end of rotation - something that's easy to do in the dark! In this mode, R102, the "Tone" pot has absolutely no effect. Note that if pin 4 is grounded, a 1020 Hz tone will be produced instead.The 8 fixed audio tones available in Mode C are:
B - (1.0 volt) The tone pitch is variable from 20Hz to about 2.5 kHz using R102. Note that inside the PIC this adjustment is de-linearized so that when a linear pot is used, the rotation is "stretched" at the low end and compressed at the high end, making frequency adjustment seem more "natural."
C - (2.0 volts) Eight fixed tones are selectable - see below.
D - (3.0 volts) Tone sequences. Below the midpoint of rotation, a descending tone sequence is generated, while above the midpoint an ascending tone sequence is generated. The rate of repetition is faster the farther the pot is turned toward either end of rotation.
E - (4.0 volts) No tones generated: Used for "normal" audio mode.
F - (5.0 volts) A 4 kHz tone is generated as a "pilot" tone. Using R106, the level of this tone is typically reduced by 12dB to 25% of the maximum level (referenced to "100% modulation") and this tone is mixed with normal microphone/line audio. As shown in Figure 4, the modulator's AGC is connected such that the amplitude of this pilot carrier is taken into account and prevents overmodulation of the LED with the combined audio sources. At the receive end, this pilot tone can be filtered out and is available for analysis and/or compensation of scintillation.
1 - Musical note B0 (actual freq. = 30.9944 Hz) (Lowest voltage on pin 7)Notes:
2 - Musical note E1 (actual freq. = 41.1295 Hz Hz)
3 - Musical note C4 - middle C (actual freq. = 261.6674 Hz)
4 - Musical note F4-sharp (actual freq. = 369.8468 Hz)
5 - Musical note A5-sharp (actual freq. = 932.26912 Hz)
6 - Musical note - E6 (actual freq. = 1318.52896 Hz)
7 - 440 Hz - Musical note A4 (actual freq. = 439.907 Hz)
8 - 1kHz tone (actual freq. = 999.9242 Hz) or, if pin 4 is grounded this will be 1020 Hz (actual frequency = 1020.13 Hz) (Highest voltage on pin 7)
- As mentioned, the audio tones are fed into the audio chain at U1B, past the audio gain compression of U1C. What this means is that the audio tones are unaffected by any compression or gain settings. Because the AGC amp is connected to the output of U1B, it does see the tones and the AGC action includes the tone amplitude. What this means is that when the pilot tone is active, the contribution of the audio is reduced by about 1dB. This also means that if a test tone is selected, it will - with its more-or-less 100% modulation - will "steal" most of the AGC action and the microphone audio will be greatly attenuated.
- Instead of a 6-position rotary switch, a potentiometer (anything from 1k to 50k) could have been used in lieu of S101a. The obvious disadvantage with a pot is that it lack detents so it is a bit harder to know exactly what setting one is using or if it is on the edge of switching between two modes if the voltage is at a threshold. If a potentiometer were used, one would have to add another switch to perform the function of S101b to select amplitude of the "pilot" tone.
- The lowest tones in Mode C (30.9944 and 41.1295 Hz) are
chosen to be below mains frequencies and are within the
range of low frequency response detectors such as the K3PGP
types. As such they may be used for single-tone
signaling and path
- analysis, even in the presence of mains harmonics.
 |
 |
 |
Operation in the various modes
Using DDS techniques, low-distortion sine waves can be generated at practically any audio frequency below the Nyquist limit with a resolution of 0.298 Hz.. Having this capability allows several tone generation modes:
- Continuously variable frequency (Mode B.) Using R102, the audio frequency can be adjusted from 20 Hz to about 2.5 kHz (2457 Hz, actually.) In this mode, the rotation of R102 is "de-linearized" to make it easy to adjust the tone frequency over a wide range. With a 20 MHz crystal and assuming a 5.0 volt supply, the output frequency is based on the following formula:
- Frequency (Hz) = ((3270403125 * V^2) + 670000000) / 33554432 (Where "V" is the voltage applied to pin 7, GP0.)
- The A/D resolution is 10 bits and the frequency step size is approximately 0.298 Hz. Because the calculation uses integer math, some rounding-off errors will occur.
- Selection of fixed frequencies (Mode C.) When in this mode, one of 8 fixed tone frequencies may be selected using R102 as noted.
- Ascending or descending tone sequence (Mode D.) The tone sequence consists of four dissonant tones that are very easy to pick out of the noise. R102 is used to adjust the sequencing rate and direction (e.g. an ascending or descending tone sequence.)
- Activation of a pilot carrier (Mode F.) In
this mode, a 4 kHz tone (recommended setting of 25%, which
is12 dB below 100% modulation - the precise level being set
by R106) is mixed with the microphone (or line input)
audio. Because the sample point of the
compressor is at the output of the audio mixing stage, the
presence of the pilot is detected - along with the other
audio - and the compressor prevents the combination of the
"audio plus tone" from overmodulating the LED. With
the pilot tone being 12dB below the peak audio, its presence
reduces the available power for modulating other audio
sources by less than 0.5 dB.
The tone sequence mode (Mode D) can generate either an ascending tone sequence consisting of tone #'s3, 4, 5 and 6 (in that order) or a descending tone sequence using the same tones in reverse order. The tone mode (and sequencing rate) is adjusted via R102:
- If R102 is set below mid-rotation, a descending tone sequence is generated, the sequencing rate increasing as R102 is turned counterclockwise (e.g. wiper toward ground.)
- If R102 is set above mid-rotation, an ascending
tone sequence is generated, the sequencing rate increasing
as R102 is turned clockwise (e.g. wiper toward +5 volts.)
Adjustment of the tone generator:
- Setting to tone Mode B and to "line-in" mode with no audio source, adjust R106 so that the sine wave just touches the bottom op amp rail as measured with an oscilloscope at the output of U1B (pin 7) of the linear modulator. If no oscilloscope is available, increase the level until the distortion just becomes audible - backing it off slightly to the point where the distortion just becomes inaudible. After adjusting R106, rotate R102 to change frequencies and adjust R106 again as necessary with different audio frequencies to assure that U1B is never overdriven.
- Note: When adjusting the tone amplitude in Tone Mode B, it is easiest to hear low amounts of distortion and aliasing with a low frequency (<100 Hz) tone as the first several harmonics and digital artifacts are within the optimal hearing range.
- Setting to tone Mode F (audio with pilot tone) -
again, with no external audio input - adjust R105 so that
the the voltage is 1/4 that of the tones in the other modes
(12dB down.) This may be done with an oscilloscope or
AC voltmeter. Note: If you are
using an AC Voltmeter, first switch to mode A (the "variable
tone" mode) and set the potentiometer for the highest
frequency and note the voltage.
Operation:
S2 positions A, B, C and D are intended to be used to generate audio tones at 100% modulation. Position F generates a low-level 4 kHz pilot tone for reference/analysis purposes and position E is for generating audio with no pilot tone at all.
When in modes A, B, C and D, R102 is used for setting the tone frequency, selecting one of eight fixed tones or selecting the tone sequence mode and rate, respectively.
Note that in all positions of S2, the audio input is still active. This means that even when generating a tone, the microphone will pick up and transmit audio, although the level will be reduced by the AGC amplifier. Again, if S1 is a "center-off" type of switch, the middle position can be used to mute the audio inputs or, if no line input audio source is present, muting can be accomplished by selecting that position.
Note that the (partial) component list below applies only to Figures 4 and 5.
Components:
- U1 is an LM324 quad op amp: DO NOT SUBSTITUTE
unless the device you use for U1 capable of operating down
to the negative supply rail on both the input and
output! If you substitute a different operational
amplifier for U1, make sure that it is capable of
"Rail-to-rail" operation on both its input and output.
(Note that LM324 can work only down to the negative rail
- which is good enough for our purposes. If an
"ordinary" op amp is used, you'll need to produce a
negative supply of at least 2 volts for things to work
properly.) Remember also that if a different op
amp is used, you may need to rework the circuit slightly to
assure stability (e.g. prevent oscillation.)
- U2 is an 78L05 (or 7805) 5 volt regulator. This is used as a stable and "clean" reference voltage to set the LED's idle current. It is also used to run the tone generator.
- Q1 is a power MOSFET capable of handling at least 5 amps and 50 volts or more. Practically any N-channel power MOSFET will work - as long as it can be attached to a heat sink so that it may safely dissipate up to 10 watts of heat. As can be seen from Figure 6 the transistor was mounted to the aluminum front panel of a Radio Shack project box and has been proven to provide adequate heat-sinking. Common examples include the IRF510 transistor available at Radio Shack.
- Q2 and Q3 are NPN and PNP general-purpose transistors, respectively.
- OC1 is "gain cell" consisting of an CdS cell optically coupled to an LED - see text.
- D1-D5 are small signal silicon diodes, such as 1N914 or 1N4148.
- D6 - About any 3 amp diode. This is used as a reverse polarity protection in the event that the power supply is inadvertently connected backwards.
- U101 is an appropriately programmed PIC12F683
microcontroller.
- R107-R111 could be anywhere from 1k to 22k - as long as they are all of the same value.
- LED1 is a high power LED. The use of a red (or red-orange or amber) 3-watt Luxeon is assumed here, but other units may be used provided that R7 is adjusted for maximum safe current. It is strongly recommended that the LED itself be equipped with a current limiter to protect the LEDs.
- LED2 is a plain, ordinary LED and it need only be bright enough to be seen during normal operation. I used a standard "dim" red LED, but about any color would be fine.
- TH1 is a self-resetting fuse with a rating of 3 amps that is used to protect the circuit in the event of an internal power supply short, or in conjunction with D6 to provide power supply reversal protection. These fuses are very inexpensive and are available from almost any component supply house in a wide variety of current ratings. (I use them in almost all of my projects anymore and rarely use "normal" fuses, except in special circumstances.)
- S1 is an SPDT switch - preferably a "center-off" type - used to select the audio source. If you have one with a "center off" position, that can also serve as an "audio mute" position.
- S2 disconnects the LED to allow "muting" of the light output, but leaves the rest of the circuit powered up. This keeps the circuit active, thus eliminating the need to wait for things to stabilize were the entire circuit powered down: The current consumption with the LED off is about 40 milliamps. Note that as the LED current is decreased, the audio output from J3 will also decrease and when S3 is opened, the audio output will also go away.
- S101 is a 2-pole, non-shorting rotary switch with at least 6 positions. I used a 6 position switch (from Radio Shack - P/N 275-1386) because that was what was available. If necessary, several toggle switches and fixed could be used to set the various modes, as could a simple potentiometer. (See the note above.)
- R20 should be a 1 ohm resistor with a power rating of
2-watts or greater. A wirewound resistor is fine for
this purpose as low frequencies (e.g. audio) are being used.
Comments:
- This tone generator circuit may be used to generate test tones for other types of communications circuits. One example of this is to aid with alignment of a 10 GHz wideband FM communications system: The use of distinctive tones is invaluable in establishing the aiming of the antennas between the two stations attempting to make a QSO. This tone generator may also be of use as a general-purpose bench top audio signal generator.
- At very low frequencies (<100 Hz) and in the background, it is normal to hear digitizing artifacts on the generated sine wave. Even though the distortion of this audio generator is only 1-2 percent, the human ear is very good at picking out the extraneous sounds amongst a pure tone.
- For details about a modulator capable of driving extremely
high power LEDs such as the Luminus Phlatlight (tm)
series. See this page: A Modulator for very
high power LEDs. For that modulator, I
used two of the four pins on the DIN connector used for
connecting the LED to provide "raw"modulation to the
external, high-power modulator as can be seen in Figure
6. One of these pins was connected to ground and
the other to Pin 7 of U1b via a 100 ohm resistor and this
provided a 10 volt peak-to-peak audio signal with a resting
"center" voltage of +5 volts. Doing this allowed the
use of this unit's built-in tone generator capabilities and
the audio compressor on the new, higher-power
modulator.
Return to the KA7OEI Optical communications Index page.
If you have questions or comments concerning the contents of this page, or are interested in this circuit, feel free to contact me using the information at this URL.
Keywords: Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector
This page and contents
copyright 2007-2013. Last update: 20130719