Science Buddies: "Ask an Expert"

Hi! My partner and I have been spending 2-4 hours every day this past week trying to get our project to work! We built a charge detector, and are trying to test sensitivity by changing the length of a wire (magnet wire) but the LED isn't changing when we hold up an object to the antenna. We need to use different types of wire in this experiment but we realize we don't know how to use the magnet wire in the charge detector! (We assumed we could use it in the experiment . . .) This project is due tomorrow!! PLEASE HELP SOON!!

1) Why isn't the LED changing depending on charge when we hold objects to it hanging on a cotton string?
2) How can we incorporate trying different lengths of magnet wire into the experiment?!

UPDATE: Good news! I talked to our teacher and our due date was extended. But we still need a lot of help!
Thank you,
liz2015

P.S. Please hurry!!

Hi liz2015,
I am afraid I need a little bit more information about this charge detector that you built please. Can you post the circuit diagram that you used please?
Willey

I have talked to a couple more teachers, and my partner and I think that we could have damaged the transistor. What should we do? Can this be considered a failed experiment? We weren't even able to do the actual experiment . . !
The circuit diagram is on this page: https://www.sciencebuddies.org/science- ... background
-liz2015

I am very sorry to hear that, liz2015.
Any idea on how the transistor may have been damaged? If it's at all possible, can you post a picture of your finished circuit board? I may be able to help you better troubleshoot your circuit with that information.
In the mean time, would it be possible to get another transistor from your teacher?
Willey

Thank you so much for helping. But my partner and I talked to my teacher again and he agreed that the transistor was probably damaged. He is accepting our project as a failed experiment.
Ah well . . . at least we tried.
Thanks again,
liz2015

liz2015 - There is a possibility that the transistor may have been damaged, but you might want to try a few simple things to see if you have a polarity issue with your LED and the transistor in the circuit. I suggest that you test the LED by replacing the MPF-102 with a resistor, somewhere between 470 ohms (Ω)and 680Ω. The LED should light with this setup. If it doesn't, the LED may be connected with the wrong polarity. Just reverse the leads to see if it lights. Once you have seen the LED light, you know the battery and LED is working and your polarity is correct. Reconnect the MPF-102 observing the proper polarity (PDF of the terminal assignments attached) to see if that corrects the problem. The MPF-102 is a Junction Field Effect Transistor, or JFET, and while somewhat sensitive is pretty robust unless seriously mishandled. It's possible that if you momentarily connected the 9 volt battery to the leads of the MPF-102, improperly, you could have burned out the transistor. If you can get your experiment working, your teacher will likely be impressed with you pursuit and success.

Rick Marz