Found in EEE documentation, this circuit can have the duration and frequency adjusted separately. SCRs can be modern equivalent types.
This different configuration for a crystal galena radio or modernized since a diode is used, has a...
This circuit is from 1990 documentation. When we press D1 the lamp remains on for a time interval that...
This version of the test lights a lamp when the ring touches the tortuous wire. The SCR can be the...