Stumped student with DC motor problem

Moku

Heyas all,

I'm taking the What's A Microcontroller class and for my project I took on the (then) simple task of making the basic stamp board the controller for a cheap remote car. It has the generic DC motors for steering and speed. My initial thought was to use the I/O pins to turn on/off the motors using the onboard battery. That doesn't work. My instructor said I should use the 2 AA batteries on the car and a transistor for switching using the I/O pin on the base. Wiring the collector to one side of the motor and the other side of the motor to + terminal of the batteries. Grounding the neg terminal and transistor emitter to Vss.

I can get that to work with 2 significant problems.

1. The motor doesn't want to start spinning on its own. A little nudge and its good to go.

2. For left/right and Fwd/bkwd I need to switch the 2.5V to neg. I tried using an LED as a checkvalve between the batteries and motor, so I can connect the opposing transistor to the opposite motor terminals, but that only lights up the LED <with only 1 transistor wired in>. No joy with the motor.

Any help would be appreciated.

Added what I think would work as a schematic. Using 2N3904 transistors. The motors are Best Motor. Feel free to rip me a new one if it's a no brainer.

Mike Green

The LEDs are limiting the amount of current available for the motors. They're normally designed for a current of 10-20ma which is way too low for most motors. Do a web search on "wiki h-bridge" and you'll see a discussion of what is typically used to make motors go on and off and, particularly, switch direction. By the way, you do need diodes connected backwards across the motors to protect the transistors from "back EMF" (a reverse voltage spike when the motor's magnetic field collapses as the current to it is cut off). If you don't need to reverse the motors, your current setup is fine (as long as you add the protection diodes).

Andy Lindsay (Parallax)

A search you can do at www.google.com is:

DC motor site:www.parallax.com

Check out the link titled "Electronic Control of DC Motors Using Discrete Bridge Circuits". It's got some excellent background information for your project.

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Andy Lindsay

Education Department
Parallax, Inc.

Moku

Thanks Mike and Andy.

KatyBri

You might find this well written tutorial helpful-

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/index.html