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.
When the sensor, an ordinary photo-transistor, receives light, the relay closes its contacts. The...
This circuit consists of a bass filter that can be used as a subwoofer in a sound system. The...
This circuit has a traditional configuration often found on this site. The transistor must be...