This circuit included in the category astable multivibrator. This circuit is used to test N-Mosfets (the power kind e.g irf830), whether it works or not. If it is not working, the LED will not flash. If Mosfet is working it will operate in the astable multivibrator circuit causing the Led to flash. Here is the circuit diagram:
To make the circuit cheap, the other half of the astable utilizes an npn transistor. This circuit can test almost any NPN transistor. As a common emitter buffer that also drives the led as it receives pulses from the Mosfet drain, this circuit uses the NPN transistor
Diode that used in this circuit is a light emitting diode.