How does an IR module work?

Question

Why is a capacitor connected between the input and ground terminals of the module in circuits?

Answer 1

The capacitor is present to remove voltage spikes from the supply. One thing all these modules have in common is a high gain amplifier built in. Any noise on the supply at the carrier frequency will get amplified and cause a false output.

The module is designed to respond to a carrier (one typical carrier frequency is 38 KHz). The IR emitter is typically turned on and off at the carrier frequency to encode one logic level and the IR emitter is simply turned off for the other logic level.

The device contains a pre amplifier, a high gain amplifier with automatic gain control, a bandpass filter, a demodulator, an integrator, a comparator, and an output buffer. See block diagram from
http://downloads.solarbotics.com/PDF/PNA4602_datasheet.pdf

Answer 2

The capacitor is probably there for ESD protection, so the internal workings of the module don't get "zapped".

The receiver module has an IR detector, probably an IR phototransistor, and possibly and amplifier.

The IR transmitter has an IR light emitting diode (LED), and some level shifting and "drive" circuitry (amplifier for the LED). The transmitter takes a series of logic pulses and produces the same sequence as IR light "on" and "off".

The receiver is set to recognize the IR frequency (matched to the LED), and produces the same pulse train at its output (back to logic levels).

It is then up to a logic controller (such as a PIC microprocessor) to decode the information in the logic pulse train and decide what the command is.

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