Light tracking bristlebot

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HopePressler1
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Joined: Sun Oct 07, 2018 12:20 pm
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Light tracking bristlebot

Post by HopePressler1 »

My 9 year old daughter is trying to build a light tracking bristlebot. What kind of mosfet does she need? I see the kit has an n channel mosfet, but it does not give further details. When I was trying to buy a mosfet they seem to come in a variety of amps and volts, but that information is not on the Science Buddies directions for this project.

Thank you
Hope
bfinio
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Re: Light tracking bristlebot

Post by bfinio »

Hi,

The exact part used in our kit is also available for individual purchase from our partner Home Science Tools: https://www.homesciencetools.com/produc ... el-to-200/

It is rather expensive there, but honestly for such a cheap part, shipping might be more expensive anyway. Something similar will be available from other places, for example: https://www.sparkfun.com/products/10213

If you are already buying your parts from a certain vendor and you let me know where, I can help you look to see if they have something that works.

Just in case you want the full technical explanation (but this isn't really necessary): the specification you care about here is called the "gate threshold voltage," or the voltage at which the MOSFET will turn "on." This needs to be LESS than 3 volts, because this robot is only powered by 2 AAA batteries (each battery is 1.5 volts, so you get 3 volts total). You can find this if you look at the part's datasheet - it is on page 2 here under the "On Characteristics" section: http://cdn.sparkfun.com/datasheets/Comp ... 30N06L.pdf. You do not really need to worry about the amp and volt ratings for this small robot, because it is way below the limits (60 volts and 30 amps is a lot). You need to make sure you avoid MOSFETs with a higher gate threshold voltage - for example the IRF510 is a common one but the threshold voltage is in the 2-4V range, so can be too high and won't work with this robot: https://www.jameco.com/z/IRF510-Major-B ... 09234.html

Hope that helps! Please write back with any questions.

Ben
HopePressler1
Posts: 45
Joined: Sun Oct 07, 2018 12:20 pm
Occupation: Parent

Re: Light tracking bristlebot

Post by HopePressler1 »

This is helpful. Thank you.

She wants to make multiple bristlebots and buying multiple kits would be very expensive. We already have some parts, but we do not have Mosfets. Do you know if it is possible to buy them in multiple packs from Amazon? I found out the ones we bought need 5 volts for the gate. If she used a battery pack with more batteries would that work?

Would it make a difference if we used AA batteries instead of AAA? We have several AA packs from other projects.
bfinio
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Posts: 740
Joined: Mon Aug 12, 2013 2:41 pm
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Re: Light tracking bristlebot

Post by bfinio »

Hi Hope,

In general you can buy electronics on Amazon but I've found that you have to be careful, many times the product pages do not link to a datasheet and do not have all the technical specifications that you can find when buying from a place like SparkFun or Jameco. But if you know the exact part number you are looking for, then you might be able to find it. In this case, searching for the manufacturer part number of this MOSFET from SparkFun (https://www.sparkfun.com/products/10213 - click to the Documents tab), it does show up in multi-packs on Amazon: https://www.amazon.com/s?k=FQP30N06L

As for the batteries - this circuit will work with 6 volts (either 4xAA or 4xAAA batteries) - that is exactly what we do in the larger "Light-Tracking Bluebot" version: https://www.sciencebuddies.org/science- ... wing-robot. It has 4xAA batteries but is otherwise exactly the same circuit and parts (except for the bigger motors). The problem with the bristlebots is more about the physical size of the robot and fitting all the batteries. In theory, you could cram four AA batteries on there but I think you would have to mount them on top instead of the bottom. Everything would be fine electrically, but I'm not sure if that would get too heavy for the robot to move.

Another option would be to use AAAA batteries, which are even smaller than AAA, but not as common (I don't think I've ever seen them in a store, but you can get them online: https://www.amazon.com/s?k=AAAA+batteries). But I'm not sure if it would be cheaper to buy AAAA batteries and holders for them, or to just buy MOSFETs that are compatible with a lower voltage.

Hope that helps, please write back if you have more questions! We'd love to keep in touch and hear how the robots turn out.

Thanks,

Ben
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