Science Buddies: "Ask an Expert"

Having trouble with our bots. Teaching a stem class with 15 students. Only two bits worked correctly.

Trouble seems to be with the potentiometers. We purchased our parts in bulk and NOT the kit. The potentiometers we purchased have offset prongs. The bot motors work when these are removed. When we set them into position and turn the knobs the motors do not work. Keep in mind we have two that work perfect even after being calibrated for the light. So I can’t figure out why the other ones don’t work. Any thoughts?

Hi - if you purchased potentiometers with offset pins, it is very important that you still place the potentiometers such that each pin is still in its own row on the breadboard. If you accidentally rotate the potentiometer 90 degrees, then two of the pins will wind up in the same row which short-circuits them.

With 15 bots, if there was a 50/50 random chance of the potentiometer being in the correct position, I would expect more than 2 to work. But it's worth going through all of them to check the circuit against the final slide in the procedure here: https://www.sciencebuddies.org/science- ... #procedure. Note how the potentiometer's pins are all in column H, in their own row. If you have the type I think you do, then each one will have two pins in column H, and then one pin in column I or J depending on how far they're offset. But it's important to make sure that the top potentiometer has its pins in rows 1-3 and the bottom one has the pins in rows 15-17, or it will not work properly.

If that doesn't solve the issue, we can help further if you can upload some clear pictures of the circuit for a robot that's not working, along with a picture of the bottom of one of your potentiometers so we can be sure how the pins are arranged.

Ok. Still doesn’t work. So uploading pictures to see if this helps more. Prongs for potentiometer are in J 1,3 & I 2.

Hi - OK, I can't see every single connection in the picture but from what I can see, your wiring is correct. Next thought, and I'm sorry I didn't mention this in my first reply - can you send me the exact link for the MOSFETs you purchased? Since it's only powered by a 3V battery, this project requires MOSFETs with a very low gate threshold voltage to work properly (in the 1-2V range). Some MOSFETs are designed to work with 5V logic and won't work for this project. If all of that sounds like gibberish, don't worry about it - the point is that parts that look identical at first glance might not work and you can't tell unless you look at the datasheet.

I should have added - I assume the picture you posted above is of one that's not working? With 15 robots and 13 not working, it could potentially be 13 different problems. The picture you posted looks good but that doesn't mean all the others are wired correctly. Another thing I note from the picture is that it looks like you're pushing stranded wires from the battery pack into the breadboard, which can be difficult. If you don't have access to a soldering iron, you can use needle-nose pliers to crimp those stranded wire ends to a bit of solid-core jumper wire, wrap the connection in a short piece of electrical tape to insulate the exposed wire, and then press the solid wire into the breadboard for a much better connection. If you have some robots where the motors NEVER spin (even if you remove the potentiometers), it may be because they arent' getting power due to a bad connection to the battery pack.

Here is the link to the mosfet, https://www.amazon.com/gp/product/B07MW ... UTF8&psc=1

I should mention that all bots were tested and the motors work. Each one was tested individually. However, when the potentiometers are in position the motors stop and do not work even with turning the knob. As soon as you take the potentiometers out the motors run. It’s got to be those. The two that work are the same position.

Hi - I'm afraid that the problem might actually be the MOSFETs. Long story short, you should be able to fix the problem by buying different MOSFETs (I will give links at the end), but here is the complete explanation.

This gets a little deeper into electrical engineering, but here is the datasheet for that part:

https://www.infineon.com/dgdl/Infineon- ... 3b3a9f220d

The relevant spec is VGS(th) Gate Threshold Voltage on the second page, which has a *range* of 2-4V, meaning there is some variability in the manufacturing and it will vary from part to part.

I won't repeat it all here, but to understand how the circuit works, you need to read the FAQ starting with the "how does the light sensor work" question: https://www.sciencebuddies.org/science- ... #question6

When you remove the potentiometers from the circuit completely, you are removing R2 and R4 from the circuit diagram in Figure 4. We'll just talk about the left half of the circuit since they're identical. When you remove R2, that means the voltage V2 (which is the voltage that turns the MOSFET "on") is *always* going to be equal to the battery voltage, which is the full 3V. Since you said all the robot's motors work in this scenario, that means 3V is enough to activate all of your MOSFETs.

However, when you insert the potentiometer, the voltage V2 is going to be "pulled" even lower. If you turn the potentiometer all the way in one direction such that the resistance is zero, then V2 will go all the way down to 0V, and the MOSFET will be fully off. If you turn the potentiometer all the way in the other direction, that is going to push V2 higher. However, depending on the maximum resistance of the potentiometer and the gate threshold voltage of the MOSFET, it might not be high *enough* to turn your MOSFET on, which is what I believe is happening for 13/15 of your robots. So, the potentiometers are not broken - they are working exactly as they should, but their resistance is too low, and your MOSFET's gate threshold voltage is too high.

Where does that leave you? Here's what I'd recommend:

1. If you haven't already tried this, you should be able to confirm my theory by taking the MOSFETs out of one of the two working robots, and swapping them into one of the robots that currently isn't working. If that robot suddenly works, that confirms my theory that the issue is the manufacturing variability in the gate threshold voltage of the MOSFETs.
2. If you want to get all the robots working, you need to buy more MOSFETs with a lower gate threshold voltage. I have had good luck with these: https://www.amazon.com/gp/product/B07CTF1JVD/. Note how on the datasheet, the gate threshold voltage is lower (1 to 2V): https://www.sparkfun.com/datasheets/Com ... 0N06LE.pdf
3. Alternatively, you could buy potentiometers with a much higher resistance value. Assuming you bought 10K potentiometers, you could go up to 100K or even 1M. But I do not think that will be as reliable as option #2.

I'm not sure how much, if any of that will make sense - if it doesn't, don't worry, because this is really college level electrical engineering. Again the short version is that there is nothing wrong with the potentiometers; the circuit was designed to work with a specific type of MOSFET and you wouldn't know that without looking at the datasheets.