This is an infrared remote control tester circuit which gives both an audio and visual indication that a remote controller is functioning. D1 is a reverse biased photodiode which forms an IR detector.
It’s output is buffered by IC1a and then enters a pulse stretcher comprising C2, R2 and IC1b.
The pulse stretcher enables even the shortest of pulse trains to trigger the following oscilator. D3 is a low current red LED which provides the visual indication when the infrared remote is directed to the tester circuit.
An oscillator is formed of IC1c and IC1d which drives a piezo disc element X1. When an IR pulse is detected, IC1b goes low and the input of IC1d sees only a high impedance from the diode and oscillation starts. The input of gate IC1f is grounded for stability. The IR remote tester runs from a 9V PP3.
Article sent by Ian Johnson (Sw), thanks.