This circuit was found in a publication by Edelman from 1924. The power comes from the battery and the listening is through a high-impedance headphone.
This circuit was found in 1980s Radio Electronics. The circuit includes the power supply. The...
This circuit was found in a former Popular Electronics. The timing is set on the potentiometer and the...
This circuit has appeared on our site several times as a battery status monitor. In this...