Board of Education Power Supply Question

FrankA_645

Hello,
I just finished the what a microcontroller book. fun fun fun ..I'm hooked! So I have decided to build a little tank robot with the board of education from my kit.
The parts are pretty simple, i have a little hobby tank chassis and the dual gearbox shown here.. motor specs are on the bottom of the page.. www.pololu.com/catalog/product/61/resources
I also have this motor controller built and tested.. www.solarbotics.com/products/k_cmd/resources/

Now, I have connected the two enable lines directly to vdd, and the control lines to 12 through 15 (12 and 13 one motor, 14 and 15 the other)
And I have written a little test program to go forward and backwards for 2 seconds with 2 second pauses in between, and the program works great when i connect a 9v to the BOE and use the 4xAA holder to power the motor controller. I took the plug off the holder and put it back on with two bare wires added in to do this.

However, when i use the 4xAA holder to power the motor controller AND the BOE, the BOE resets itself several times a second. I think it is doing this beause the led on the BOE is pulsating and it will only go forward (first instruction in the program). Also, If i power the motor controller from the BOE with vin and vss, the same thing happens, weather I use the AA's or a 9v in the BOE . I would love to use just one power source in my robot, how can I do this?

Thanks in advance for your help!
-Frank

Mike Green

You need a bigger power source.

You didn't say what kind of AA batteries you're using, but I suspect they're alkaline. The BOE requires at least 5.5V, maybe a little more to produce regulated 5V for itself from the unregulated Vin supply. Under heavy load from the motors, the output voltage of the battery pack probably drops well below 5V and causes the Stamp to reset. At the very least, you should use a 5 or 6 cell NiMH rechargable battery pack for your BOE. That would yield 6 to 7.2V and NiMH cells can provide higher peak current than alkaline cells. You can wire a single cell AA holder from RadioShack in series with the 4xAA holder on the BoeBot or use Parallax's 5th cell adapter

Try fresh high current alkaline cells (the "titanium" AAs or other premium AAs made for motorized toy use). You will probably find that they'll run both (the motor and BOE) for a short time

Mike Green

You probably need some filter capacitors across the input of the motor controller. Something like 1000uF 10V minimum would help a lot. The motor controller already has some smaller capacitance across the input to the controller's 5V regulator, but, with the amount of current used by the motors, you should have more.

FrankA_645

Thanks Mike!
Yes they were nice fresh Industrial Alkalines

I happen to have a 7.2v nimh (6 cell) sitting right here.. should I try to connect it to the 9v holder pins on the BOE? I had tried using this rechargeable pack as the motor controller power (with separate power to the BOE) and the motors went way too fast and it stank like ozone(?). maybe with the voltage drop from the BOE it will be just right.. but I still had the resetting happening with a 9v battery in the BOE, and the motor controller wired into vin and vss..

Also, I had tried to solder some filter capacitors (.1 .."104's" from the kit) across the leaders of the motors and from the leads to the motor chassis, but the chassis ones didn't stick so well and they're just kinda hanging there.. just on a recommendation of a friend to reduce noise. How exactly do filter capacitors work in these situations? I still have a 1000uf 10v cap from the kit, but I'm unsure of how to hook it up to the input.

Thanks Mike! you're like.. smart and stuff..