With this transmitter the infra-red LED glows at constant brightness. This is the carrier signal.
The input information signal increases and decreases the brightness of the LED.
This is amplitude modulation.
The line input could be connected to a computer sound card, phone or any similar device.
To drive a higher power LED, a source follower should be added after this amplifier, possibly with separate biasing.
You can find out if your IR LED is lit by viewing with a mobile phone camera.
Using a mobile phone camera to "see" infra-red
Analogue Infra-Red AM Receiver
The infrared detector is a "current to voltage converter".
The op amp inverting input (2) is a virtual earth ( held at +6 Volts by the two 10K resistors wired as a voltage divider ).
A BPW41 photodiode is used here.
The output voltage alters to keep the pin 2 input voltage as close to 6 Volts as possible.
The 10M feedback resistor is a compromise. A bigger resistance increases the sensitivity but also increases the background noise (hiss) produced by the circuit.
A low noise CMOS FET op' amp' is preferred here but the 741 chip works well enough.
The photodiode can be replaced with a phototransistor. Bright infrared light might make this circuit saturate.
The 10M feedback resistor could be made variable if the sensitivity needed to be adjusted.
The LM386 chip works on up to +12 Volts.
This circuit produced little noise or hiss as long as all the lights were turned off. The fluorescent lights were the worst for noise generation.
Media - Using Lenses to Improve Efficiency
In free space, these circuits have a rather short range.
Using lenses to focus the beams increases the range greatly.
A visible LED can be used to set up the optics and then return to using infrared afterwards.
Some types of optical fibre are good at channelling infrared.
Using LASERs, the distance record is 24 000 000 km through interplanetary space. On land it's about 150 km.
Alternative AM Transmitter
This uses a summing amplifier to combine the two stereo inputs into one monophonic infrared beam. The input resistance is 47kΩ which matches most audio line devices.
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