Science Buddies: "Ask an Expert"

I am currently working on a science buddies project called, "Dealing with Diabetes: The Road to Developing an Artificial Pancreas." I have completed my circuit and my conductivity sensor and my peristaltic pump is able to pump the liquids. The problem I have stumbled upon is my potentiometer can not turn my pump off. I have played around with it and I have turned the knob both clockwise and counter clockwise. I checked all of my wires to see if any of them were bumping, and I also checked to see if all of my wires were inserted correctly. Do you have any other tips to see how I can fix this problem?
Thank you, sphan.

Hi Sphan,

Sorry you are having a problem with the circuit. It will be hard for me to trouble-shoot without being able to see your breadboard with the components. Check all your connections again to make sure you have all components and the battery in the right holes. Make sure none of the wires are touching. Make sure the alligator clips are making good contact with the wires.

Try removing the potentiometer ('pot') from the circuit. Does the pump stop? Try reinserting the pot in the opposite direction. Did that make any difference? The pot may be defective.

Did you put the MOSFET in the correct holes with the black side facing right?

Maybe another expert or one of the students who has done this project can suggest something else to try. You could post your question on the physical sciences forum as it involves electronics and the experts on that forum are probably more electrically savvy than we are.

Let us know when you get it working and find out what was wrong so we can use that information in the future.

Good luck!

Sybee

Hi Sybee,
I tripled checked everything and all my wires, MOSFET, and potentiometer are all in the correct places. I had the pump running, but when I had taken out the potentiometer it had no affect. The pump still continued to run. I will most likely purchase a new potentiometer as well as a MOSFET to see if they are the problem to why my pump will not turn off. Thank you for your help and I will take your advice and post in that forum!
-Sphan

OK. I hope you succeed in getting the circuit to work. I know it can work because many other students have used it. Please let us know when you find out what caused the problem so we will be able to advise others what to look for.

Good luck!

Sybee

Replacing the MOSFET ought to fix your problem. The likelihood that your potentiometer is bad is extremely small. On the other hand, MOSFETs are extremely sensitive to static electricity and must be handled *very* carefully to avoid killing them.
Static electricity is mrs and more of a problem as the weather cools down, the humidity in our houses drops and we turn on the heat. In an engineering lab you would construct your circuit on an anti-static workstation and ground yourself with an anti-static grounding strap on your wrist before opening the gray or pink plastic bag.

Since you don't have one of those, the next best thing is to find a wooden surface (avoid plastic of any kind) and place a piece of aluminum foil down on it. Strip the ends of a long wire and tape that to the corner of the foil, strip the other end and wrap it around a metal water pipe. If you can get a cheap grounding strap do that (http://www.jameco.com/webapp/wcs/stores ... 2259366_-1 or something similar) and use it. If you don't have the grounding strap, keep as much of your hands and arms in contact with the foil as possible while you're wiring. Once you have the circuit built and the batteries in place you can relax a bit. But do keep in mind that the harmless and fun static zaps you might get by shuffling your feet on a carpet will fry your MOSFET.

As Sybee said, let us know if you get your problem fixed.

Howard

Thank you Howard for your suggestion! I did put in a order for a new MOSFET so as soon as I get that in I will see what happens then.