This is a variation on the astable multivibrator. Circuit was recently developed to test for N-mosfets(the power kind e.g irf830). I don?t claim circuit can test all bad mosfets or all fault mosfet conditions. If mosfet is working it will operate in the astable multivibrator circuit causing the Led to flash. A bad mosfet will not cause the LED to flash. Below is the circuit diagram, the other half of the astable utilizes an npn transistor to make the circuit cheap. Almost any npn transistor will work in this circuit. The npn transistor to the right is used as a common emitter buffer that also drives the led as it receives pulses from the mosfet drain.