This biestable touch circuit was found in American 1980s documentation. The circuit must not be powered by a source without a transformer. Sensitivity depends on the 10 M resistors.
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...