Science Buddies: "Ask an Expert"

Need help with this project. In step 10 C of the procedure, it says the following:

c. Once the pump is running, very slowly turn the potentiometer's knob in the opposite direction to turn the pump off. Stop turning the knob when it is just reaches the point that makes the pump turn off. You can play around with adjusting the knob until you are satisfied that the pump does not run in the neutralized solution (but will still run if turned slightly). I. While you are adjusting the potentiometer, identify which pump tube has liquid flowing out of it. When the pump is not running, dry the end of this tube and mark it with a small dot using a permanent marker. This will help you in the next step when you need to pump a liquid into a different bowl.

Problem is, pump does not stop no matter how much we adjust the potentiometer. We tried with two different sets of breadboards, resistors and potentiometers but we are not able to stop the pump as it was mentioned in step 10 c. Would you be able to tell us why this could be? Thanks!

Sorry you are having a problem with the circuit. It is hard to trouble-shoot a problem like this when I can't see your breadboard, but it sounds like you have done everything correctly and even tried it twice.

The pot adjusts the V on the second voltage divider as it says in the FAQ section of the procedure https://www.sciencebuddies.org/science- ... shtml#help

but it is the MOSFET that actually turns on the pump. If the voltage on it is above the threshold value then the pump will run no matter how much or which way you turn the pot. My guess would be that the MOSFET is the problem. The negative terminal of the battery is connected to the lower terminal of the MOSFET and power is not supposed to flow across the gate unless the sensor circuit is closed and there is a certain V coming from the pot. If the MOSFET is defective or in backwards maybe current is able to flow all the time and keep the pump running.

MOSFETs are solid state devices and can be shorted out by a static charge. If you are getting a lot of sparks to your fingers when you touch metal then you need to ground yourself before working on the circuit. Get another MOSFET and try it, being careful to ground yourself and tools before touching it. If you have a digital voltmeter you can measure the voltage on the MOSFET to see if it is correct.

Let us know what happens. If the MOSFET is the problem we would like to know this.

Good luck!

Sybee