Found in an EDN from the 1980s, this circuit uses a double 555. The circuit activates a relay that controls a horn or siren. Next to the diagram we have the timings.
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...