This circuit of the 70's uses transistors of the time. The circuit can amplify signals from 10 Hz to 100 kHz and modern equivalent components can be used. The circuit is from Ferranti.
This setting appears with variations at various points on this site. This is from a Motorola...
This circuit indicates through LEDs when an automotive fuse is open. ...
The circuit shown in the figure generates a rectangular signal whose frequency depends on C1. For...