This crystal or galena radio was found in documentation from the 1990s. The earpiece must be a high impedance crystal or magnetic earpiece and D1 can be any germanium diode.
This circuit functions as a thermostat triggering a relay when the boil control temperature...
The figure shows how to connect a relay to obtain the latch effect. The circuit is from an...
With this circuit it is possible to use a DC buzzer (not a transducer) as an extension of the...