This Schimitt trigger with MOS transistors was found in an old American documentation. Modern equivalent semiconductors can be tested.
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...