# How can I control two motors using only two wires? [closed]

Yesterday I bought an old RC Car from a junk sale. I assume this circuit has a built in motor driver, which is how it gets the forward/reverse of the motor working. This circuit is running of a 9v household battery, not the car battery for the motors.

I want to scale this up so I can use it on an old electric mobility scooter, but here's my problem:

The motor is driven by switching the polarity of the driver output wires.

In order to control the 12v motors (Powered from a car battery), I need to use a relay device (I'm using an opto-coupler) and know I need the below circuit:

However, if I were to complete my circuit using the yellow wires in the above diagram, the other relay device would be activated and both motors would turn. I don't want this to happen, just one motor should turn.

Unfortunately, I don't know how I can achieve my expected result, if it's even possible. Perhaps some kind of logic IC is needed? I'm not sure which one.

## closed as unclear what you're asking by Andy aka, Trevor_G, brhans, Voltage Spike, Enric BlancoMay 1 '17 at 19:52

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• Could you try and clarify exactly what you want as expected behavior? I'm not quite getting what you are trying to achieve. What do you want to happen when you push "forward" or "backwards"? should only one motor run? (asin, forward turns on motor A, backwards turns on B) – Joren Vaes May 1 '17 at 13:17
• @Jodes The asker wants to only use two wires, I believe. Ground would be a third. – Hearth May 1 '17 at 13:21
• @JorenVaes The diagram that says "forward" and "reverse" is incorrect. Should say "left" and "right" Only one motor should turn. – Harvey Fletcher May 1 '17 at 13:24
• I think your requirements defined here are way out of whack. Presumably both motors need to be independently driven at various speeds either forwards or backwards. What you are suggesting here is far short of that. Unless all you want the scooter to do is go round in circles. – Trevor_G May 1 '17 at 14:11
• @Trevor I am attempting to have the motors run at a single speed (on/off) in a single direction (forward) – Harvey Fletcher May 1 '17 at 14:20

What do you mean when you say

However, if I were to complete my circuit using the yellow wires in the above diagram, the other relay device would be activated and both motors would turn.

Unless I am misunderstanding something here, it should work, provided you add the required circuitry (limiting resistors for the optocouplers).

If you are in the "forward" position, we have the following circuit:

The left optocoupler is forward biased and it's LED will conduct current. The second optocoupler is reverse biased, and it's LED will not conduct any current. Hence the left one is turned on and the right one is turned off. If we reverse the supply, we turn on the second optocoupler, and turn off the first one.

We can actually get rid of one resistor by doing the following:

Now, I would like to point out that if you are using actual optocouplers, it's a bad idea to use them to directly drive the motors. Instead, use them to drive a MOSFET or other switching device, that will in turn actually switch on the motor.

• The OP mentioned 'tank like' control in a comment. I think this means that forward motion would be both motors on, turning with just one motor on and stopped with no motors. Using this arrangement it is possible to have turning control with one polarity of signal on the two wires. Using and alternating signal it is possible to have both motors on for forward control. Driving two suitable relays with selector diodes and holding capacitors would be the simplest and not require further electronics at the motor end. – KalleMP May 1 '17 at 15:39
• The "Tank-like" comment was only added after I wrote this answer. My response is to the original circuit question. From the description of the problem and my lack of knowledge about mobility scooters, I didn't know /why/ the OP desired to do what they wanted to do - I simply showed them how it could be done. Granted, it's not ideal, for many a reason. – Joren Vaes May 1 '17 at 15:43

As long as both motors only run in one direction, the basic idea is pretty simple. Joren Vaes has priority. However, his answer can be expanded.

First, let's say you're willing to drive your motors in bang-bang mode (either on or off). Then

simulate this circuit – Schematic created using CircuitLab

will do the job, with a few reservations.

1) The motor will not start to turn on until the joystick is pretty far from the zero position, so there will be a large dead zone which will make precision operation difficult.

2) The motor will go from full off to full on over a fairly narrow deflection range. This is actually a good thing, since

3) If you try for partly on and succeed, the MOSFET (M1) will start to get very hot very quickly unless you provide a good heatsink for it. Like, it will die. And if the motor draws multiple amps of current (and a mobility scooter will indeed do that) you will need a much bigger heat sink than you think. Just a warning. On the other hand, if you just use the controller to drive the motors full on or full off, you won't need much heatsink.

Also note that I'm assuming you use a single 12 volt battery. This is important. If you use a higher-voltage setup, like 2 12-volt batteries in series, you will produce 24 volts, and you will be able to kill the MOSFET by applying too much gate voltage. If that's a problem, start another question.

Finally, keep in mind that, as I stated at the beginning, this will only work if you are willing to drive your motors in one direction. There is no simple way to allow reverse drive for this sort of setup.

EDIT - And you should also keep in mind that driving one motor at a time is unlikely to do anything you want to do. With this setup, the vehicle will spin in place (approximately) and your only choice is which direction it spins.

• I think your first statement is already false. Depends on how you read the poor quality of the question, but I cant see a mobility scooter not having both motors reversible. Presumably it has two motors for steerage. – Trevor_G May 1 '17 at 14:09
• The motors are individual. They are reversible. I don't mind on and off (no speed settings) and it will work like how a tank works. – Harvey Fletcher May 1 '17 at 14:19
• @HarveyFletcher - OK, the motors are reversible - do you need to operate them in reverse? I expect the answer is yes, in which case you are out of luck for anything remotely describable as simple. There are various ways to do what you seem to want, but they all take much more electronics knowledge than you seem to possess. – WhatRoughBeast May 1 '17 at 14:23
• @WhatRoughBeast although the motors are reversible, I don't need them to be. – Harvey Fletcher May 1 '17 at 14:24
• @HarveyFletcher - No. Leave it in place. You asked, and you learned. – WhatRoughBeast May 1 '17 at 14:59

I could show you how to do what you ask, but the fundaments of this solution are in error.

Scooters are driven with two motors for a reason.

The speed of the motors is adjusted by the user in order to steer the scooter. Since one motor will ALWAYS drive at a different speed than the other without user intervention, or a "smart" control system, the scooter will continue to veer in one direction. Worse since you are only allowing the controller to apply full power or none, it will do that at a violent and dangerous speed.

Your control system MUST therefore provide the ability to drive each motor proportionally, based on the joy-stick position.

It also needs to have the ability to drive either or both motors in reverse when required. e.g. One forward one back to turn on the spot, both back to get away from the obstacle you just ran up to.

Of course, this is all doable. However, it is far more involved than the simple solution you intimate in your original question and beyond the scope of an answer here.

I suggest you look more closely at how your RC model works and how the scooter control system works. Once you fully understand that you may find integrating one with the other is actually simpler than you think.

If you have difficulties understanding what you find out during that study, by all means ask more questions on here with sufficient details and I am sure someone will be able to explain it to you.

• Most major brand mobility scooters only have one motor. Also remember that many of them are front-wheel (and therefore one-wheel) drive. See the following manual for a common rear-drive model with motor - link – Bert May 1 '17 at 17:50
• @Bert, yup good point. – Trevor_G May 1 '17 at 17:57

This could be done, but with a lot more electronics. Basically, consider the two wires as providing binary outputs, in which case you have 0-0, 0-1, 1-0, 1-1, which is four possibilities (0 = negative, 1 = positive). So, off/stop, left only, right only, both. Converting those four possibilities into two-motor controls is what the additional electronics are for.