This circuit is from a Popular Electronics from the 90s. It uses an opto-coupler with SCR directly controlling a 20 mA current load.
The circuit presented is based on a mechanical tuning fork. The circuit is suggested by National...
In this circuit of Texas Instruments an incandescent lamp is used as a light source, and lenses are...
One touch of the push button S1 and the circuit LEDs shown in the figure start to flash rapidly. The...