For a more thorough explanation of this circuit and how it works, be sure to read the original article, "A 'Mini' full-featured Pulse Width Modulator for high-power LEDs and laser diodes" found at this Link.


The reason for this updated version:

This updated version of the "Mini" PWM LED/Laser modulator addresses two issues:

- It slightly simplifies the original schematic diagram by removing the transistor-based microphone preamplifier.
- Additional operational modes were added to the PIC's firmware that allow "Fixed" mode operation in which audio gain and tone selections may be made by setting the appropriate I/O pins to the appropriate state.
- By removing the AGC capability, a manual gain control can also be added, which can also allow further simplification of the circuitry by (optimal) removal of the low-pass (anti-aliasing) filter.

Prototype used to test the version 2/3 circuit of the "Mini" PWM LED/Laser modulator.
Click on the image for a larger version.

The prototype (see the picture to the right) was built to accommodate either the Version 2 or Version 3 circuit for testing of the firmware on phenolic prototype board of the sort with etched copper rings on the bottom side.  Using a single high-brightness LED, its output was sufficient to go across the room - or just a few inches - during testing to verify operation of the AGC algorithms, the various tone modes, the audio gain settings and the modulation of the circuitry and the modified firmware.

Three circuit diagrams:

Three circuit diagrams may be found on the right:  Click on a diagram for a large, printable version.

Version 2:  Updated Audio AGC circuit


This circuit, depicted in Figure 2, is functionally identical to the original circuit found on the page linked above in that it uses the PIC to set the gain of the first Op Amp stage - as well as internal gain switching in software - to provide about 25 dB of audio AGC (Automatic Gain Control) range.  This causes the audio from the microphone or line input to be always be adjusted such the overall audio gain is modulating the LED or laser at or near 100% all of the time to provide the highest possible signal-noise ratio.

Another feature of this firmware is a "Tone" mode in which one can vary a potentiometer from a low voltage, at which a 1 kHz tone is produce, up slightly to produce a variable audio tone from a few 10's of Hz to several kHz, up to the top end of the potentiometer at which point a dissonant tone sequence is produced that is designed to be heard amongst noise and be resistant to "ear fatigue."

The features of this firmware are described fully on the page of the original article noted above, linked again here for your convenience.

One minor disadvantage of this original firmware was that the AGC could not be disabled:  While one could have installed a potentiometer in lieu of the switchable gain resistors, the internal +12dB step in the firmware would still be in effect.

The updated circuit:

Top - Figure 1:  Version 2 of the Simplified PWM LED/Laser modulator with Audio AGC and potentiometer tone control.
Center - Figure 2:  Version 3 with manually adjustable audio modes and modes selectable via CPU pin logic stats.
Bottom - Figure 3:  "Minimized" version of the Version 3 PWM LED/Laser modulator.
Click on a diagram for a larger version.

The circuit depicted in Figure 1 has been simplified in that the original transistor-based microphone preamplifier has been removed.  Instead, the low pass filter has been re-worked to increase the gain by more than 10dB, providing most of the original system gain.

This circuit is completely compatible with both the original and updated firmware.

Version 3:  Manual audio gain control and "Fixed" tone modes

The circuit depicted in Figure 2 shows the variant in which the audio AGC has been removed and replaced with R309, a manual gain control.  In addition to this, the various modes - audio and tone - are selected by setting the logic levels on the pins of the PIC as follows:

If, when the PIC powers up, it finds that GP1 (pin 6) is tied to either GND or +5V, it will assume that the user does NOT want the original Audio AGC and tone modes (e.g. selected by potentiometer) but rather, to select audio and tone modes with logic levels on the pins.

The "Audio" mode and "Tone" mode is then determine by the state of GP3, pin 4:

GP3 is Low - Tone mode:
    GP4 - Low
        GP5 - Low:  1000 Hz (nominal)
        GP5 - High:  1020 Hz (nominal)
    GP4 - High
        GP5 - Low:  Tone sequence (C4, A5#, F4#, E6)
        GP5 - High:  Variable tone, according to voltage on Pin 7

GP3 is High - Audio mode:
    (GP5:  Don't care)
    GP4 - Low - Normal audio gain
    GP4 - High - High audio gain (+12dB)

A bit of explanation is in order:

• 1000Hz and 1020Hz are both available as 1000 Hz, exactly, is a harmonic of the 100Hz mains frequency in many parts of the world, but because the PIC12F683 is operating from an internal clock, its accuracy is typically +/-2%, so it can vary by several 10's of Hz.  Both frequencies are available to allow at least one of them to be far enough away from the harmonic of a mains frequency to avoid problems.
• The variable tone frequency is adjustable in the same manner as on the original firmware, as in:
• <= 0.5 volts:  1000Hz tone (nominal)
• > 0.5 volts to < 4.5 volts:  Variable, from a few 10's of Hz to approximately 2500 Hz.
• >=4.5 volts:  Tone sequence (C4, A5#, F4#, E6)
• The "Tone Sequence" is, as noted above, is designed to sound dissonant and as such, it sticks out of noise and interference, unlike a constant tone to which one's ear can become "fatigued".

Because this mode uses a potentiometer (R309) to set the audio gain it is imperative that the operator pays close attention to the transmitted audio level by monitoring the quality of the transmitted signal by listening to a sample of the emitted signal on a local optical receiver or listening via the "Audio Monitor" point.  During normal operation, the audio gain should be kept as high as possible without causing objectionable distortion.  Without an automatic gain control to keep the levels constant, it would be normal to hear the occasional bit of clipping on audio peaks.

Remember:  For the purposes of intelligibility it is always preferable to have a bit too much audio and a bit of distortion than too little audio and have your voice lost in the noise!

At any point, switching to a "Tone" mode can give a quick comparison as the modulation of the audio tones is always at 100%:  While it is difficult to judge solely by "ear", the audio peaks should sound about as loud as the tone.

Minimized Version 3 circuit:

The circuit depicted in Figure 3 shows how one can strip it down even more by removing the low-pass filter and a few of the features, leaving it nearly "bare bones."  By removing the low-pass filter a bit of audio distortion may be noted due to aliasing effects, particularly if pre-recorded audio or music is played back via the "Line" input, but this effect should generally be tolerable, but if it bothers you, you may be able to utilize the graphic equalizer feature of your audio player to remove frequencies above 5 kHz.  For most speech, the lack of the low-pass filter isn't generally noticed except, perhaps, on certain consonants which may start to sound a bit "spitty" - but this is highly dependent on both the person speaking and the microphone being used.

Because the low-pass filter provides about 16 dB of audio gain, removing it will require that R309 be adjusted to make up for the difference, and it is likely that one may also need to run software in the "High Gain" mode as well.  In the diagram in Figure 3 the pinout shows one half of dual op amp being used, but a single section op amp could also be used instead - but the pinout would, of course, be different that what is shown!


Interfacing this circuit to an LED, Laser or other devices:

The original page, linked below and to the right, includes quite a bit of information about how to interface this circuit to an LED, Laser diode or even a light bulb as well as information about how to use lenses and other optics to cast the light over a great distance.

For convenience, part of that information may be found in Figure 4, to the left - make sure that you click on the diagram for a larger, readable version.

Information about interfacing the PWM driver to LEDs and laser diode modules.
Click on the diagram for a larger version.

For a more thorough explanation of this circuit and how it works, be sure to read the original article, "A 'Mini' full-featured Pulse Width Modulator for high-power LEDs and laser diodes" found at this Link.

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