This circuit identifies the polarity of DC signals and if the signal is AC. The circuit is from 1969 US EDN documentation. Operational may be equivalent.
This commercial circuit is from a Japanese magazine from 1967. It uses germanium transistors, and...
This circuit can be used to drive relays, solenoids, motors and other loads according the...
This circuit is based on the small keychain that responded to the sound of a whistle and were...