Modulating Audio on a LED

[I made some changes to also support a microphone]

In this tutorial, I will demonstrate how audio (music or sound) can be transmitted over a light beam generated by a LED. This project includes 2 parts:

1. The transmitter, that takes the audio and modulates it using the LED
2. The receiver, that reconstruct the audio from the LED's light

This method and project can be expanded to a LED bulb as well.

How does it work?

The input audio is amplified and then modulated using a LED by changing the current that lights the LED based on the audio wave. The receiver translates the light into voltage and reconstructs the audio wave.

Part List

R1 = 100 Ohm
R2 = 1.5K
R3 = 3.3K
R4 = 1.1K
R5 = 1.1K

R6 = 10K (required only if you use a microphone)

C2 = 100 nF (104)
C3 = 10 uF
C4 = 10 uF

D1 = 1N4733A or other

Q1 = 2n222A

L1 = Basic LED (any color)

Electrect microphone OR a mono audio cable

Solar panel Speaker with amplifier

Step 1: Building an Audio Amplifier

The audio amplifier circuit is based on the LM386 audio amp chip. Vss (source voltage) is 9V although the 386 may require less depending on the IC type (see https://en.wikipedia.org/wiki/LM386). The audio source can be either an electrect microphone or a mono audio cable (3.5mm) that connects to a phone / PC / radio audio out jack.

The blue part in the schematic diagram is required only if you use a microphone.

Step 2: Step 2: Building the LED Modulator

The transistor and its resistors were selected so that the LED would have sufficient current to be turned on all the time (~20mA) while the audio is modulated over this current, causing it to fluctuate. R2 and R3 are acting as voltage dividers (so that the voltage on the emitter (and collector) is around 2.5V.

Step 3: ​Step 3: the Receiver

The receiver is composed of a solar panel (in which light is converted into voltage) that is connected to a speaker with an amplifier (otherwise the audio will not be heard). Alternatively, you can build another audio amplifier (with LM386 IC) and feed it with the solar's voltage. In that case, connect the output of the amplified audio to the speaker (make sure the solar panel's output voltage is not high enough. If so, use a voltage divider to bring it to a reasonable level suitable for the amplifier circuit). It's very important to filter the DC voltage received from the solar panel by putting a small capacitor (104 will do) in series with the solar panel's positive cable, before you feed it into a speaker or another amplifier.

Comments

[Xrayrep]

1. Although R2 and R5 are in different parts of the schematic diagram, electrically they are in parallel. Was that a mistake or was it intentional? Please explain.

2. What is the purpose of C4? Placing a fairly large electrolytic capacitor in the signal path to ground would severely limit the high frequencies in the signal bandwidth. Please explain.

Thanks!

[mvidal01]

I wonder if this could be used as the "strobe" part of a strobe tuner?

[asjpr9832]

Can I use only LED Modulator (module) and connect it directly with jack stereo? (without amplifier on LM386)

[TheICMan]

The voltage at the music jack (headset) is very low and needs to be amplified before it is used for modulation, but you can use other pre-amp circuits (search Instructables for 'preamp') if you don't want to use LM386.

You can find circuits that are based on 2n2222, 2N3904 transistors or others.

[remedios667]

actually....i think this might be the circuit I've been looking for...

I have the LM 386 chip's on hand and some 2N222's on the way from Amazon...have you tried using a microphone for the input? (take a look at my recent post to see what I was trying to do. Basically making LED's react to the thumping downbeat of dance club music...optimally the LED would be "barely on" and the amp would have a sensitivity adjustment (possibly a variable r4?)

[TheICMan]

I made a small change to he circuit to support a microphone. Have fun...

[remedios667]

thanks...that makes sense, still I'll have to adjust all of the resistor values to compensate for my choice of VCC but if this works.

another question (may have posted earlier) what application do you use to draw your schematics ( I have a few art programs but would be nice to just copy and paste symbols rather than hand drawing every resistor....etc.)

[TheICMan]

For this schematic I used a free online tool and then captured the screen as an image and edited the image with paint.net

[TheICMan]

Check out LM3916 for lighting LEDs based on audio amplitude. See https://www.instructables.com/id/LM3915LM3916-VU-Me... for example.

[remedios667]

a quick question, did you use a program for drawing schematics (step 1)...I usually hand draw them then take a picture and download it. I have a "rudimentary" setup in my art program but it's usually more trouble than it's worth.

[remedios667]

see "wearable tech, sound triggered LED by remedios667" (on instructables) ....I'm trying to keep the circuit small and powered by lithium cells (what is the minimum VCC for the 386). I can always Surface mount the 386 (minimal external components is a plus) but I want to avoid having to run a power line down to a power pack, stacking 2 -2032 button cells is better than a 9 volt or 4 AAA's in a plastic holder.

there is so much info on the net I have problems finding the exact wording for searches.... thanks

[TheICMan]

Check out https://en.wikipedia.org/wiki/LM386 for information on the minimal supply voltage. It depends on which 386 IC you are using.

You can also check out the datasheet: http://www.ti.com/lit/ds/symlink/lm386.pdf

[remedios667]

thanks....most types have a median VCC of 6 Volts so I can stack a couple of 3 volt button cells for power, my other "go to" chip is the 4017 decade counter, check out what I did with them about a decade ago at my you tube channel (Bob667).....I'll follow you to see what other amazing things you come up with

[DIY Hacks and How Tos]

Fun project. You can also use laser pointers for longer range.