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# Electric motor

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#### pcordoba

##### New Member
Hello!

This might be a silly question, but I have no idea.

I need to control an electric DC motor (those little motors you can find on a toy, that can work with a 1.3v battery) so:

-It will inverse the rotation every X seconds (I would like to be able to define/change X)

-I can control the speed of the rotation.

Can somebody help me out with this (the simplest way if it is possible).
Thank you!

Pablo

Use an h-brige, it uses two phase pwm. Φ1 in the attachment is the phase to
make the motor go foward Φ2 to reverse.To produce the pwm a mcu is necessary,it is also necessary for switching between foward and reverse.

#### Attachments

• pwm.doc
23.5 KB · Views: 290
What is doing the controlling... you with switches? Or is it supposed to autonomously run in one direction for X seconds and then reverse, etc?

Break the problem down into several smaller problems.

1. How do you control the direction of a DC motor -- look up H-bridge circuits as well as H-bridge ICs and motor driver ICs. A simple way for a small low current motor is to just use the motor driver IC and be done with it.

2. How do you control the speed of a DC motor -- the *best* way to do this from an efficiency, torque standpoint, is to use Pulse Width Modulation (PWM). You can look that up next. The lame way to do this is to use a variable resistor of appropriate wattage. The better way to do this is using some type of multivibrator operating at > audible frequencies with adjustable duty cycle. The 555 timer would be one option...

3. How do you send a signal that is on for X seconds and off for X seconds -- look up 555 timers and how to use one as a multivibrator. You can create a pulsetrain of adjustable frequency and duty cycle. Actually there is a dual 555 called the 556 ; one could do PWM, the other could do the forward/reverse signal.

4. How do I interface the 555 signal with the H-bridge -- depends on what you are using for your H-bridge... ask when we come to that point If you're just using an L293 or SN754410, it is probably fairly simple.

Michael

Use an h-brige, it uses two phase pwm. Φ1 in the attachment is the phase to
make the motor go foward Φ2 to reverse.To produce the pwm a mcu is necessary,it is also necessary for switching between foward and reverse.
Just helping you distributing the image. PNG is much better than doc

How are you going to find MOSFETs that will give the required current with a gate voltage of just 1.3V?

As I understand it, you'd need circuitry to drive the gate voltage much higher...

I think using a Joule thief style circuit to give about 5.5V for the controller. Most PICs will work from 5.5V and you can get MOSFETs (for the high side) which can pass a reasonable current with a gate voltage of 4V.

You don't really need a 555 timer if you have a microcontroller,because you can implement both the pwm and X seconds to switch between the two states (reverse and foward) in the microcontroller.

Thanx Grossel

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