This switching circuit makes use of a MOSFET transistor for the purpose of switching a lamp. We can use this for the control of a low-voltage porch lamp which is switched on automatically at dusk. The type of sensor is a photodiode, which could be either of the visible light type or the type that is especially sensitive to infra-red radiation. It is joined so that it is in reverse-biased mode. Only leakage current can pass through it. The current changes according to the amount of light that is falling on the diode. In darkness, the current is just about 8 nA but it rises to 3 mA or more in average room lighting.
Because the gate requires very little amount current, and therefore the MOSFET is the ideal type of transistor for use in this circuit. If we were to use a BJT instead of MOSFET, we might knew that the sensor network was unable to provide sufficient current supply to turn it on.
The lamp that is specified for this circuit has a current rating of 100 mA. The transistor being used has a rating of 270 mA, so there is no danger of burning out. There is no need to use a power MOSFET.
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