Light-Based Transmitter/Receiver Pair


These circuits serve as a simple introduction to infrared communications. For the transmitter (top schematic), the 741 op-amp serves as the pre-amplifier whose gain is set by the 1M potentiometer. After amplification, the signal is fed to an LM386 configured to give an additional amplification of 200. The result is used to AM modulate an infrared LED.

The receiver is almost identical. A photo-transistor converts the incident infrared light to a voltage that is amplified by a 741 op-amp. The signal from the 741 is fed to an LM386 with a gain of 200. The 5K pot serves as a volume control. The output is sent to a pair of headphones.

Build the receiver first. You can test it by pointing it at an incandescent bulb and listening for the 60 Hz hum. Adjusting the 1M pot for a higher resistance will give more gain, but setting it too high will cause distortion. You should try to shield the phototransistor from ambient light, but surprisingly, a small amount of ambient light will increse the sensitivity since it serves to bias the phototransistor to a more sensitive part of its operating curve.

Once the receiver is working, you can use it to test the transmitter. Any audio source will serve for the input. The output from a transistor radio works great.

Don’t expect miles of range from this pair, but they are fun to play with. When coupled with a simple lens system, you may be surprised at how far the signal will go.


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