Science Buddies: "Ask an Expert"

hi i was wondering if this wasnt a good thing but when my potentiometer isnt in the circuit the pump still runs and even if the sensor is unplugged the pump still runs, its like if the power goes straight to the pump, help me please :/

thank you,
kevin

Hi Kevin,

I wish you could hold up your breadboard in front of a webcam so i could see how you have it wired!

Something has to be connected wrong. The pump gets its power through the sensor, which is attached to the battery; so if the circuit was correct there would be no way that the pump could run unless the sensor was placed in a conductive solution.

My recommendation would be to take everything off the breadboard and then put it back together again with help from your brother or mom/dad or someone who can read the instructions to you exactly step by step. If all the components are working properly and you wire the circuit correctly, the pump has to work according to the instructions in the project.

Let us know what happens.

Good luck!

Sybee

Hello, your advice has helped, i am able to stop and start the pump yet the sensor still doesnt have an effect on the circuit, i can unplug it and it will still work, i rellly dont know what to do

Hi Kevin,

Maybe if you explain a little more about your circuit I can figure out what's wrong. You said you can stop and start the pump. Exactly how do you do that? The pump circuit runs through the sensor wires so there should be no way that the battery power can get to the pump until the sensor is put into a conductive solution. Check your wiring connections again. Is the MOSFET inserted correctly? Maybe it is defective.

If you follow the circuit, the negative terminal of the battery pack connects through the breadboard to one terminal of the pot while the positive terminal goes directly to the pump. The other terminal of the pot connects to one side of the the sensor and the lower terminal of the MOSFET. The other wire from the sensor goes to the resistors at the top and then to the top terminal of the MOSFET. The middle terminal of the MOSFET connects to the other lead from the pump and the power should be coming from the MOSFET to the pump. The pot is used to control the gate voltage on the MOSFET and stop the power flow when the solution is neutralized.

Try tracing your circuit on the breadboard. Maybe it would help if you drew a diagram of the circuit in the old-fashioned way on a piece of paper http://opencircuitdesign.com/xcircuit/g ... schem.html
Note on your drawing where the connection points should be and make sure that these are the ONLY connection points on the breadboard.

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 until the sensor circuit is closed. If the MOSFET is defective or in backwards maybe current is able to flow all the time and keep the pump running.

Do you have any friends who are electronics geeks? Have one of them take a look at your circuit. There are ways to test MOSFETs to see if they are operating properly but I don't know how to do that since I'm not an expert in electronics.

Good luck and let us know what you find out.

Sybee