This circuit indicates when the voltage of a 12 V battery is low. With changes, for example, the zener, the circuit can be used to monitor other voltages.
This 1950's circuit used one of the first commercial transistors and had two stages for audio...
Using integrated circuits from the time, we found this audio compressor in an English publication from...
This is an experimental crystal radio that uses no power source. The antenna must be very long (at...