This ingenious circuit was found in a 1990s Popular Electronics. The oscillator is a 555 and the keyer uses an optocoupler in the configuration shown.
Found in an issue of Elektor magazine in the 90s, this battery tester is based on the LM3914. The...
I found this circuit in a July 1982 English magazine. The circuit is still viable by simply using...
This circuit is from a 1966 Motorola Field Effect Transistors Manual. The circuit makes use of...