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My Robot’s Wheels

January 24, 2012

My robot is going to have wheels, not legs. To have a 4-wheeled robot, it will need:
4 wheels
2 motors
4 NPN transistors
4 PNP transistors
sufficient power source

Why do I need 8 different transistors? Well, anytime you want to make your DC motor spin backwards, you need to reverse the + and -. Now, in order to do that, you need to use an H-bridge. Here is my VERY basic H-bridge schematic:


It’s a very clever design. The top two transistors would need to be PNP (because they’re between the motor and the power source) and the bottom two transistors would need to be NPN (because they’re between the motor and ground). But hmmm… my robot is going to be very simple and primitive. This might cost me some time and money. I want to make this as cheap as possible.
Well, lucky for me, I was in the downtown toy district and I picked up this remote control truck for a measly $10!


That’s right, $10. If I’m lucky, I may be able to strip this thing for some usable parts. So I took this new toy home and I took it apart. Lucky for me, it comes with 4 very usable wheels, 2 motors, a power source, and 2 H-bridges! Now, I didn’t expect the front wheel steering to be powered by a DC motor just like the rear wheels. I was expecting a servo motor.

This is great! You can see the 8 transistors to power the two motors.

Now, it’s extremely easy to fry one of these things. At first, I was trying to stare at the back of the circuit board to see what was connected to what, so I know how to utilize this H-bridge.

It not only gave me a headache, there was print on top of the circuitry so it made it really difficult to figure that out. I could figure out where the collector was, but which was the emitter and which was the base? So before I get careless and start soldering wires to the wrong connectors, I needed to do a little probing with my handy little multimeter.

After some probing and testing different connections, I figured it out.

I took some notes on the whiteboard and now, I know how to put this bad boy into some good use!

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