This circuit, found in 2002 documentation, triggers LEDs when a phone call is received. The LEDs will flash alternately in these conditions. Common LEDs are used in this circuit.
This circuit was found in a 1980s American documentation. The circuit is based on integrated CMOS 4001...
This 1970 Philips circuit uses a mechanical counter. The circuit is triggered by a reed switch...
This traditional photodiode amplifier configuration uses an operational one that is no longer...