Science Buddies: "Ask an Expert"

Help!

We're doing the Light-Tracking Bristlebot project. We thought everything was progressing nicely until we finished constructing the robot and tested it. Nothing happens. The motors never come on no matter what we do. We tested the motors separately (connected them directly to the batteries) and they work, so it's a connection failure somewhere in the breadboard. Problem is - how do we figure out where exactly the problem is? We've double- and triple-checked everything.

The motor wires are VERY thin and flimsy and we had a very hard time getting them into the breadboard. I suspect this is where the problem lies, but even after we got (what we thought was) a good breadboard connection they still wouldn't turn on. I am considering soldering the motor wires to jumper wires to make them more sturdy and easier to put in the breadboard, but I'm scared the motor wires will burn or melt because they are so frail. Any suggestions on that?

I couldn't buy the kit from Science Buddies because it's out of stock and our project is due Feb. 2 so I bought all components individually. I emailed Science Buddies first and asked a couple of questions to make sure I got all the correct parts. The ONLY part I got that doesn't exactly match what's in their materials & procedure pictures is the switch. They show a slide switch (only referred to as a toggle switch with no other specs given); the one I found at Radio Shack is this one:
http://www.radioshack.com/spdt-micromin ... ch&start=7

It has 3 pins and it fits in the breadboard, but the middle pin is slightly longer than the other 2 pins. I don't know if this has any impact on anything or not. I can't find a slide switch like in Science Buddies' picture anywhere locally; I'd have to order one online and now I'm nervous about time.

Help? ANY advice or suggestions appreciated.

Hi Meggle,

I saw that you emailed us so I will respond to that separately. A few quick responses though:

1. Yes, it will be a huge help if you can solder jumper wires onto the motor wires to make them easier to push into the breadboard. This is already done for you in the Science Buddies kit since most of our users don't have access to a soldering iron, but if you know how to solder you can do it yourself.

2. Do you have a link to the exact type of MOSFET you bought? The type of MOSFET included in our kit is designed to work with the 3 volts supplied by the 2 AAA batteries. There is a type of MOSFET commonly available at Radio Shack, I believe the IRF510: http://www.radioshack.com/mosfet-irf510 ... U&tab=tab1, that will NOT work at 3 volts. It has a higher "threshold" voltage of somewhere around 4-5 volts, so it will never activate with only 3V available from the batteries.

3. Have you tried adjusting the potentiometers at all? The potentiometers act sort of like a "dimmer switch" for the motors. If they are turned "down" all the way, the motors will never turn on.

4. I know you said you already double and triple-checked...but quadruple-check your connections. 99% of the time when I teach robotics workshops in person and someone's robot "isn't working", it's because just one wire is misplaced. That's all it takes to stop the circuit from working.

5. Here are a couple switches that should work, if you have time to order online:

https://www.sparkfun.com/products/102

http://www.jameco.com/webapp/wcs/stores ... 2192384_-1

Thank you, thank you for your reply.

Yes, that is the transistor we have. What should we have instead?

We have a soldering iron and can attempt to attach the motor wires to jumper wires, if that can work. I'm really nervous about ruining the motor wires!

Yes, we have been adjusting the potentiometers as we go. No luck.

Is our switch a no-go? Do I need to go ahead and order one of those others?
THANK YOU, THANK YOU, THANK YOU.

https://www.sparkfun.com/products/10213

Will that transistor work?

Either of these transistors from SparkFun or Jameco will work (not sure who has better shipping options if you are tight on time):

https://www.sparkfun.com/products/10213

https://www.jameco.com/webapp/wcs/store ... 2001373_-1

It's hard for me to tell from the Radio Shack page whether or not that switch will work (they don't include a data sheet with dimensions). Have you tried pushing the switch into the breadboard yet? If all three pins fit snugly into the breadboard's holes, then you should be OK. If the outer two pins don't really go in very well because the middle pin is longer, that could be a problem. If your switch doesn't work, then either one of these should:

https://www.sparkfun.com/products/102

http://www.jameco.com/webapp/wcs/stores ... 2192384_-1

Finally, I saw you had some general questions about breadboards in your email. It might help to look through some general guides about using a breadboard (at least look at the pictures, even if you don't have time to read them all):

https://learn.sparkfun.com/tutorials/ho ... breadboard

http://www.instructables.com/id/How-to- ... readboard/

https://www.sciencebuddies.org/science- ... ml#summary

The key point is that there are little metal clips inside the breadboard, like this: https://dlnmh9ip6v2uc.cloudfront.net/as ... 000001.JPG. When you push a wire into one of the breadboard's holes, those clips hold it in place. Very flimsy wires (like your motor wires) aren't strong enough to easily push into the clip, which is why it's easier to use thicker jumper wires.

Please let us know if you have any more questions!

Ben (I'm the author of the project, which I didn't mention before)

Ben, thank you so very much.

I'm going to order new transistors and a new switch and then we'll try again. I'm hoping to be able to get stuff here in 2 days - fingers crossed. I will also have access to a multimeter so we can use that for further trouble-shooting if necessary.

I'll check back in when we have our new parts! THANK YOU.

We got the new parts today and IT WORKS!!!

We still have to do minor adjustments, but the motors come on when the light is shined and the pot.s are turned (and they only have to be turned a little bit!). My son is so excited!

Thank you again for all of your help. We'd have been stuck without it!

That is great to hear! We love hearing "success stories". Would you (and your son) be interested in possibly answering some interview questions and providing pictures or videos of the robot you made? If so, let me know and we can figure out how to get in touch outside of this thread (please do not post personal contact information in the forum).