Let's Make Robots!

motor driver idea

Hey all,

I'm trying to create a bot that uses a lot of forward/backward motor action, and I'm hoping to make a low-power, small, lightwieght motor control using switch transistors. However, my electronics experience is -5. I came up with this idea that seems simple and effective, but I'm sure there's something I don't understand about the process. So with that in mind, here's my ultra-simple H-bridge.


The idea is that both "A" transistor bases are connected to one outpin, ditto for both "B" transistors. Obviously, I'll need some resistance along the way. But could something this simple really work?

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In one of my previous classes, we made a forward/reverse motor driver. Of course it was on a larger scale (driving a car antenna motor) we used relays that were used in the EXACT same way. The concept is there, that is for sure! So i'd say with a little work it could be very effective. But im no genius at electronics.

The main problem with your circuit is that it is not "smoke proof" If you accidentally turn on both A and B them the transistors will be shorted to the rails and let out the magic smoke.

For some alternative H bridge designs for small motors -- http://www.solarbotics.net/library.html 

under motor drivers.

Personally, I like to use a pair of 555's as a H bridge for small motors


that's some good stuff. I hadn't even thought about the "smokeproof" factor. That changes my approach a great deal (meaning: break down and just buy an IC). How do you use a 555 for an H-bridge?
A direct link to the h-bridge starter page mentioned above. One thing that needs to be considered before making a driver, is the requirements of the motor(s) to be driven.
Isn't that a type of beer? Or a "stout" to be more precise...? Speaking of which... <glug, glug>
Ya know it could be, just don't know though. Cheers!

Hi Calculon32,

I know -6 about electronics, so what I do is to hook up 4 relays. It is very easy, there is no loss in power, and you can chose whatever power you want for the motors.

Let me know if you want more info. It is used on the wall racers!

This bot i'm working on probably won't be BEAM, but it will be very light and lean, so if i can use up less space and weight than an L239, I would be down. Plus if I get smoke, I'm only out a few transisors and not the motor, right?
to avoid the smoke factor you could do something like this: make the top motors PNP and the bottom motors NPN, then connect the motors on the left to the same output A and the ones on the right to the output B. This way you can be sure that transistors on the same side won't be on at the same time. One problem about this setup is that, unless you disconnect the wires to your transistors, you won't be able to coast the motor.
The first setup i posted or the second one (same but with PNP at the bottom) should work fine if, instead of BJTs we used FETs right? With p-channels substituting the PNPs and n-channels substituting the NPNs. There would be no shorting since the gate of the MOSFET is isolated from the source and drain.