Pulse Width Modulation (PWM) Laser/LED Communications Circuit

The post briefly shows a circuit that could be used to transmit data optically (through the air) to a receiver. In this case, we made a circuit to transmit music. The topology of the system is shown in Figure 1. This post will only show the transmitter part.

 free space communication
Figure 1: Topology of an optical communication system.
The proposed circuit can drive a laser or an LED. The values of the output may need to be tweaked to provide enough current for your laser/LED. The laser module that can be used are the ones that comes in laser pointers and the usually cost around a couple of £.
Encoder Schematic
Figure 2: Circuit to Encode and drive a laser/LED
For the circuit shown in Figure 2, the signal (in our case, music) is applied to the port A1. The circuit is power using a PP3 9V battery connected to J3. The LED or laser module is connected to J1. U1 is a 555 timer that provides a triangular waveform.
U3 amplifies the sound waveform injected into A1. RV1 allows the gain of the inverting configuration of U3 to be adjusted. U3 is a comparator which compares the amplified music signal and the triangular waveform from U1. U2 then drives the base of BJT Q1, which is effectively configured as a switch.
A partially assembled PCB is shown below:

Encoder PCB

This post was meant to only give an introduction, more will be posted later. Keep in touch for regular updates. 🙂

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