Electronics 2c - Transistors as switches

Question 1

How can we make a n-channel enhancement MOSFET act as a switch?

specifically to switch a light on when light is low

Answer 1

Circuit is shown below

Explaination:
potential divider type cicuit. As light reduces, resistance of photodiode increases, thus volts across the gate increases so does the source current across the lamp turning it on with only small amounts of amps across the photodiode section

Question 2

How can we make an n-type BJT act as a switch?

For a thermo switch circuit.

Answer 2

Circuit is shown below

Explaination:
The current across the base is dependent on the voltage supplied to R2. IB = Vtherm - 0.7 / R2. We know IB relates to IC by some factor. So when thermistor resistance changes with temperature, the voltage across the thermistor, Vtherm changes. The voltage into the base is always 0.7V so we can see how that Vtherm changing changes base current from the formula before, and thus IC changes. The diode is to stop surges of high voltage going into the BJT when the inductor (relay) collapses rapidly.