Science Buddies: "Ask an Expert"

Hi all,
Looking at the circuit diagram, there are two resistors in the circuit. What is the role of these resistors? Could I not eliminate them and simply go to the source of the Mosfet with the negative wire of the battery? Does the Mosfet need negative charge as well?

Hello and welcome to the forums:

The Resistors are necessary. R3 makes a resistor divider circuit with "R1" (and similarly for R4 & R2, but I will just discuss the first pair). MOSFETs have very low leakage currents into their "gates" (theoretically zero), so one way to look at this circuit is that R3 "bleeds" off the current through R1. The value of R1 when in the light and when in dark varies from around 1K ohm to > 10K ohms. It would be a good exercise to figure out the voltage at the gate of the MOSFET under these two conditions. If you eliminated R3, the voltage at the gate Q1 would always be the full voltage and the motors would always be on.

I'm not understanding the second part of your question: "Could I not eliminate them and simply go to the source of the Mosfet with the negative wire of the battery?" The source of the MOSFET does connect to the negative wire of the battery. The resistors are between the two poles of the battery and the gate.

Please let us know if you have any other questions.
Keith

Thank you, sorry the second part was a bit badly worded, but your response answered my question anyways. Thanks again.