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boba
03-03-2009, 03:07 PM
I apologize in advance, I'm trying to help my son (5th grade) build his first robot and don't have a background in it. We have built a circuit using some photo resistors and transistors that will turn on and off one of two drive motors when a light shines on the respective photo resistor. The idea is that these will be the "eyes" of the robot, and that essentially(with the motors connected to a wheel on each side) it will follow the light source.

My problem is the speed of the motors. These are Radio Shack 1.5-3V hobby motors and they have a really high RPM. I need some way of mechanically or electrically lowering the RPM's - without significantly lowering the torque. I am happy to buy some new motors, if they are sold with speed control or have low rpm motors. Otherwise, I have read about "frequency modulation?" speed controllers but I can't seem to find one that I can adjust manually and can put in a circuit with the motor and battery. Option 3 is some sort of ready-made mechanical gear set that would reduce the RPM's significantly.

Please let me know if any of you have any suggestions. I don't really want to get into CPU's etc, I'll save that for junior high. Thanks for any help.

Bob A.

lnxfergy
03-03-2009, 05:12 PM
I apologize in advance, I'm trying to help my son (5th grade) build his first robot and don't have a background in it. We have built a circuit using some photo resistors and transistors that will turn on and off one of two drive motors when a light shines on the respective photo resistor. The idea is that these will be the "eyes" of the robot, and that essentially(with the motors connected to a wheel on each side) it will follow the light source.

My problem is the speed of the motors. These are Radio Shack 1.5-3V hobby motors and they have a really high RPM. I need some way of mechanically or electrically lowering the RPM's - without significantly lowering the torque. I am happy to buy some new motors, if they are sold with speed control or have low rpm motors. Otherwise, I have read about "frequency modulation?" speed controllers but I can't seem to find one that I can adjust manually and can put in a circuit with the motor and battery. Option 3 is some sort of ready-made mechanical gear set that would reduce the RPM's significantly.

Please let me know if any of you have any suggestions. I don't really want to get into CPU's etc, I'll save that for junior high. Thanks for any help.

Bob A.

DC motors like ones you find at Radio Shack are lacking a gearbox, what you need is a geared DC motor, no control structure in the world will make that radio shack motor work like a gearmotor. I would recommend something like these http://www.solarbotics.com/products/gm2/ Solarbotics makes about a dozen varieties, and they are available from sources all over the place.

As to controlling them, under load they will likely eat more current than your current motors, thus you may have to beef up your transistors, or look for something like a relay.

Also, for inspiration, I might suggest looking at Braitenberg's Vehicles Amazon.com: Vehicles: Experiments in Synthetic Psychology: Valentino Braitenberg: Books You'd be amazed how many different configurations you can up with to make different bots with just a few pieces (note this book doesnt have anything about wiring/building bots, its all a thinking excersice, but a very good one... if you google around, lots of people use Braitenberg's ideas with analog bots and/or mindstorms for younger children). Your local library might very well have a copy, its a classic...

-Fergs

boba
03-03-2009, 10:20 PM
Thanks very much for your suggestions. I looked up the Solarbotic GM2 and it is exactly what I need for this (the Tamiya twin motor gearbox is dead-on also). It seemed like they must be out there, but I kept getting radio controlled car parts or other items). I used pretty heavy-duty transistors (they have a heat-sinks anyway) so I'm hoping they'll work.

You reading suggestion is also helpful - I took my son to the first lego league (I think) state championships a few weeks back and he was in heaven, so it appears that there may be more of this in my future. Thanks again.

Boba

lnxfergy
03-03-2009, 10:35 PM
Thanks very much for your suggestions. I looked up the Solarbotic GM2 and it is exactly what I need for this (the Tamiya twin motor gearbox is dead-on also). It seemed like they must be out there, but I kept getting radio controlled car parts or other items). I used pretty heavy-duty transistors (they have a heat-sinks anyway) so I'm hoping they'll work.

You reading suggestion is also helpful - I took my son to the first lego league (I think) state championships a few weeks back and he was in heaven, so it appears that there may be more of this in my future. Thanks again.

Boba

Personally, I think the solarbotics GM2/3/8/9 are far better drivetrains than the Tamiyas. Also note, you can get these with a wheel, about $15 for a pair of motors and wheels: http://www.solarbotics.com/products/gmpw_deal/

As for the transistors, the motors probably won't be pulling too terribly much current if the robot isn't too heavy... its more than a microcontroller can source from a pin, but probably any transistor in a TO-220 case will work, and I'd imagine that any to-92 case with heatsink will be fine (as long as the voltage is only 4-9V or so)

-Fergs