This circuit, with two operationals, was obtained from an old American publication. Operational amplifiers can be of modern equivalent types.
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...