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Analogue Infra-Red AM Transmitter

• 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.