This circuit was found in an 80's EDN. The circuit uses a current common CMOS, the 4093 powering an LED. Trigger adjustment is done in a trimpot.
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...