Found in a 1966 documentation, this “coin finder” uses components from the era operating at 2.1 MHz. The circuit uses an audio amplifier for detection.
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...