This circuit was found in a 73 magazine for radio amateurs from the 90s. The circuit uses high voltage transistors and the relay is high voltage as well.
This is a simple voltage tester for voltages from about 2V to about 15V with the indicated...
The gain of the Operational Amplifier in the figure is given by the ratio between R1 and R2. In...
This electrochemical circuit monitors the conductivity of the oil indicating when to change it. The...