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I am introducing electric circuits to Grade 9 high school students as part of a dedicated engineering subject. The students have no prior exposure to electric circuits.

As part of this module I want to design a motor driver for the students to build, either using a breadboard or a soldering iron. The motor driver is needed to drive some mechanical doodads later in the course.

The motors in question are the DC hobby motors shown below. My choice is driven by price (these motors are going for $1/ea on clearance.)

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1.5VDC, 650 mA M-13-2270-1.5V

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12 VDC, 650 mA MF-26CS-18165-12.0V

Requirements

My problem is to design a DC motor driver to the following specifications:

  • Bi-directional (forwards and reverse drive)
  • Control by pushbuttons / toggle switches. Either a pair of "forwards/reverse" buttons, or a "forwards/reverse" direction toggle and a "go" button.
  • Drive either of two kinds of motor - 1.5VDC, 650 mA M-13-2270-1.5V or 12 VDC, 650mA MF-26CS-18165-12.0V. I am planning to use the 1.5V motors to actuate small geared mechanisms, and the 12V motor as a "winch" for a model elevator.
  • Cheap - less than $5 in parts. Cheaper is better. A class size of 30 students may be presumed, so I can order bulk parts in lots of 50 or 100 which should help with cost.
  • Preferably breadboard-able so the parts can be re-used for next year's students.

Possible solutions

I have considered a SN754410 Quad Half-H Bridge IC which is cheaply available ($0.80/ea in bulk from eBay, i.e. Hong Kong.) However this has an output voltage of 4.5-36 VDC. I believe this may cause the 1.5V motor to emit smoke.

On the other hand, I considered a H-bridge built out of cheap discrete transistors like the 2N2222. However, I am uncertain as to the power limits of such a H-bridge. It also involves at least a dozen parts, which may be a bit difficult for students who have never dealt with electronics before.

The problem

What transistors, IC's, or other switching components can I use to build a DC motor drive that handles 1.5 & 12 VDC motors? If I can get an idea of the main components, I can design all the auxiliary stuff (switches, biasing resistors, what have you) myself.

It must be cheap (we need 30 of them) and buildable by Grade 9 students and their teacher. (I am not the teacher.)

If the output voltage of the SN754410 could be reduced to be friendly with a 1.5VDC motor, that would be a very easy way to solve my problem.

I myself have very little practical electronics experience but I am willing to RTFM if directed.

The school can make basic PCB's on a milling machine if required.

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Li-aung Yip
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  • Does it especially have to be an H bridge or similar? A DPDT switch can reverse a motor (just connect either end to the motor the other way around). – PeterJ Sep 23 '13 at 09:38
  • @PeterJ: It doesn't need to be a H-bridge, no. Would you care to post "Use a DPDT switch" as an answer and collect your upvotes? – Li-aung Yip Sep 23 '13 at 09:52
  • For what it's worth I am still interested in an "electronics answer" for interest's sake. Mainly for extension to the more advanced case where electronic control is desired. Maybe when the students get to Grade 12 and learn how to program microcontrollers... – Li-aung Yip Sep 23 '13 at 09:59
  • No problems, if you like there's nothing wrong with un-accepting my answer for the moment to see what other answers you get in that regard. Sometimes if questions have an accepted answer they can get ignored a bit. – PeterJ Sep 23 '13 at 10:01
  • @PeterJ Noted. I'll put it back later. – Li-aung Yip Sep 23 '13 at 10:02
  • `However, I am uncertain as to the power limits of such a H-bridge.` The power limit to transistor H-Bridges are based on the parts you use. A 2N2222 (and it's PNP compliment) are 600~800mA maximum Continuous Collector Current. They can handle the no-load current of your motors, but any load on them, especially the stall current (If unknown, 2x Operating Current is a good thumb of measure). Hence you want transistors that can handle 1.3A. 1.5A on the safe side. – Passerby Sep 23 '13 at 17:34

2 Answers2

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One simple way would be to use a DPDT switch using the following arrangement. You could also include a pushbutton after the battery if required for momentary operation and there are also switches available that have a center position that is off. Just make sure to use a break-before-make switch so that you don't get any shorts:

schematic

simulate this circuit – Schematic created using CircuitLab

PeterJ
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  • Is a diode or snubber required to prevent inductive sparking in the switch, or can this be ignored for small 12V motors like this? – Li-aung Yip Sep 23 '13 at 10:13
  • @Li-aungYip, it's probably technically a good idea to but I think in reality it can be ignored both because of the relatively small motors plus presumably it's something that won't be used for a very extended period of time. It might be a good thing to include though from an education point of view explaining why they are used and why they are important with larger motors (and when driven by transistors etc). – PeterJ Sep 23 '13 at 10:29
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I'm looking for something similar for a micro-controller driven robot that needs to drive two tiny DC motors that I believe operate at around 1.5v. Until recently I had only been familiar with the L293D (Dual H-bridge) which is considered a low voltage motor driver, but not low enough for my application. I was advised to look into using a mosfet driver as they can handle lower voltages. Luckily, I found one that can handle a range of supply voltages from 0 to 11 volts. I don't know if I'm too late on this question or not but you might want to look into the Texas instruments DRV8835. Texas Instruments DRV8835

There are pre-made motor drivers available that use this integrated circuit and there are also breakouts you can get that the IC can be soldered to. It's a relatively small package so it would be a lot easier to breadboard if it is already attached to a breakout.

It looks like Ti is selling these at 70 cents per unit, but if I'm reading right, that's assuming you are buying 1000 of them. There are likely other vendors out there that will sell them in smaller quantity. Hope this answer helped.

Edit. To add to the above, as I was looking for a similar solution when I found this thread, I ended up finding an 8835 package already mounted and including the other essential components complete with usage instructions at Pololu. They are going for $4.49 each and support a voltage range from 0 to 11 volts. I picked one up and have successfully implemented it in a tiny robot that fits in the palm of my hand driven by two tiny DC motors. this unit was easy to hook up and it works great, you might want to check it out. DRV8835 Dual Motor Driver Carrier

I'm also somewhat using a thread on the Arduino.cc forum as my progress report, there is a sort of ugly picture of my robot (So far) if you're curious. Tiny robot using the 8835 driver Beware use of zip-ties and electrical tape. It's a prorotype ;)

Steve Robey
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