This circuit is from the 80's but uses common components today. The relay is voltage sensitive according to the power supply used. Transistors may be general purpose types.
This circuit was found in a 1968 Motorola documentation. The circuit can be implemented with...
This 1000: 1 voltage-controlled filter circuit was found in an Electronics Today International in a 1980s...
The Toshiba TLP577x series consists of an infrared photo-emitter (IR) and a sensor integrated circuit...