This circuit that generates signals between 1.5 and 2 kHz was found in a documentation from the 90's. The capacitor C1 can be changed to obtain other frequencies.
If the input voltage falls below 12 V the LED glows up. Other voltages can be monitored by...
Found in an old Popular Electronics, this circuit warns when a remote lighting system is not...
This generator is more complete than the previous one, being able to generate pulses as indicated...