This 80's Exar circuit has a carrier detector for frequencies up to 500 kHz. The circuit uses a company 567 which is still a common component.
This additional bell circuit was found in the February 1994 Electronics Now magazine. The...
A delay of a few seconds to a few tens of seconds can be achieved with this circuit found in a...
This circuit uses a transistor to trigger the alarm from a low quiescent current sensor. The relay...