Science Buddies: "Ask an Expert"

Hi,
My name is Sai and I am in 6th grade. For my science fair project, I decided to do the light-tracking bristle-bot. I followed the materials list and bought the items from Amazon. After building the circuit, the circuit motors simply didn't work. Since it didn't work, I tried some troubleshooting methods to test the parts if they were faulty. When we gave direct connection to the motors, they worked but when the connection was given on the circuit it didn't work. So I asked my parents to buy new parts like Mosfets, Photo resistors from Frys. Even with the new parts it didn't work. Can anyone please help me to fix this problem? I have a week before my project is due.

Following are the items bought from Amazon for the first trial.
https://www.amazon.com/eBoot-Photoresis ... B01N7V536K
https://www.amazon.com/Uxcell-a12013100 ... B007QAJUUS
https://www.amazon.com/HELLOYEE-Breadbo ... B01IK6GT1E
https://www.amazon.com/Gikfun-Packing-R ... B01GNORMWA


The details of the items bought from Frys are listed below.
https://www.frys.com/product/1975891?si ... IN_RSLT_PG
https://www.frys.com/product/8555249?si ... IN_RSLT_PG

For the mosfet bought from Amazon , we could fit in the breadboard only after twisting the legs .
But the one bought from Frys went in easily without twisting.

Regards,
Sai

Hi Sai,
I am very sorry to hear that the circuit is not working. Can you post a picture of the circuit that you have built please? (I think you can also email the pictures to [email protected] if that's easier.)

Do you have access to a multi-meter that we can use to measure different voltage at different points on the breadboard? For the motor to turn on, and based on the datasheet of the MOSFET from FRYS, you should have somewhere between ~2 to 3 volts at V2 or V3. (Please see figure 4 of https://www.sciencebuddies.org/science- ... lebot#help)

One other thing to watch out for is that I think you are using two different type of photo-resistor. The Amazon one have a resistance range of Resistance 50 to 100kohm whereas the one from FRYS is 5-10 koHM. The Amazon one will probably not work because your voltage divide will give you an output voltage of about ~0.5V. (when the 10k potentiometer is set to 10kohm). The FRYS one should work better. (Here's an online calculator that should help you figure what is your output voltage. http://www.ohmslawcalculator.com/voltag ... calculator

Good Luck and please post again if you have any additional questions.
Willey