Science Buddies: "Ask an Expert"

Hello, I've rebuilt this project twice but I've have a couple issues with it working. When I turn it on often only one of the motors will work at a time, they've switched sides as well. When I remove the potentiometer, both of the motors work. I have played around with the potentiometers by turning them completely clockwise and counter clockwise and it seems to only make a difference sometimes. I'm not sure what I'm doing wrong or how to fix it. Thank you!

[Administrator note: Project url and circuit diagram: https://www.sciencebuddies.org/science- ... wing-robot ]

Hello,

It looks like you purchased your own parts instead of the Science Buddies kit. Can you provide the part numbers on your MOSFETs? From what I can see you followed our wiring diagram correctly but it might not work properly with different MOSFETs. Have you also tried the troubleshooting steps in the project's FAQ? There are some additional steps you can take if you have a multimeter available.

https://www.sciencebuddies.org/science- ... -robot#faq

Finally - those motors do not have any gearing and may have difficulty driving wheels directly when supporting the robot's full weight. The yellow motors in our Bluebot kit have internal gears that slow them down but make them stronger, so they are better suited for driving wheels. So even if you get the circuit to work properly, you may have trouble building a robot that can drive around with those motors.

Thanks,

Ben

Hi I really appreciate your reply! The Mosfets are labeled LB8721 5T 60 4221p. Also I have a multi meter available, I was just a little confused on where to connect it too. I just want to thank you again!

Ok - as far as I can tell from the datasheet, those MOSFETs should work for this project.

Before you continue, I would highly recommend watching our multimeter tutorial video on this page: https://www.sciencebuddies.org/science- ... multimeter

Then, I would specifically look at this part of the FAQ: https://www.sciencebuddies.org/science- ... #question4

The key to this project is the voltage at the gate pin of each MOSFET (the leftmost pin when viewed from the front). When that voltage exceeds a certain threshold, the corresponding motor (the one wired to that MOSFET) should turn on. The voltage at the gate depends on both the position of the potentiometer and the amount of light hitting the photoresistor. Ideally, you tune the potentiometer such that the motor is off when there's no light on the photoresistor, and the motor turns on when you shine light on it, which is what allows the robot to follow light when you do this for two motors (and criss-cross the motor wires, i.e. when the left photoresistor detects light, the *right* motor spins, causing the robot to turn left toward the light).

Using a multimeter, you should be able to adjust the potentimeter such that the gate voltage is low (less than about 1 volt) when no light is hitting the photoresistor, and goes higher (above about 3 volts) when light hits the photoresistor. The key parameter is the "gate threshold voltage" on page 2 of the MOSFET's datasheet https://cdn-shop.adafruit.com/datasheet ... 721pbf.pdf which is listed as a range of 1.35-2.35 volts. Below that, the MOSFET (and motor) should be fully off. Above that, it should be fully on.

I didn't say this in my earlier post but it may also help if you try the project in a darker room. If you're in a room with a lot of existing natural or artificial light, it can be harder to tune the circuit to respond to a flashlight.

Hope that helps, write back if you have more questions!

Thank you!