We found this circuit in a 1967’s Electronics World. The circuit can be made with modern equivalent transistors. The selection is made by the tuned coil.
This experimental amplifier was found in the 1960 Mullard transistor reference manual. The...
This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit...
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...