This circuit was found in an American 1990s documentation. It can directly trigger a triac when light is detected by the sensor. The integrated circuit is common, like the 741.
Found in a Hands-On Electronics from the 90s, this circuit produces the dial tones of an old...
Found in Radio Electronics from the 90s, this circuit has several outputs with different pulse...
This circuit, found in 2002 documentation, triggers LEDs when a phone call is received. The LEDs...