1. The problem statement, all variables and given/known data Firstly, this is more of a schoolwork issue, not coursework or a set task. It is more to do with understanding the circuit seen in the image provided: I am trying to understand why this circuit does what it is supposed to; activate the lamp when a certain light intensity is incident on the photodiode. 2. Relevant equations 3. The attempt at a solution My logic is that whether or not the transistor will allow a current to flow depends on the voltage difference between the gate and source nodes into the transistor (horizontal wire and lower vertical wire). As i believe these points and the photodiode lie in parallel, the potential difference between gate and source is the potential difference induced in the photodiode. So, my belief is that the following will take place: 1. intensity of light increases. 2. photocurrent increases proportionally. 3. voltage increases as the current increases (not sure if proportional, but it is a positive relationship). Since the photodiode is connected to ground, the potential just after the diode is the potential at the gate. 4. when the voltage induced by the photodiode is large enough, an inversion layer forms in the MOSFET, current flows and the lamp is lit etc. The book talks about the resistor and how the potential difference across it relates to the Gate-Source voltage we need to know about. However as you can see i don't even consider the resistor. Am i right still? P.S i have another small query. why do we need a larger, 6V supply voltage overlooking the whole circuit? Guidance appreciated as ever. Regards, James.