Deciphering a DC Motor

Hey guys,

I was shopping for a RC car that have servo on the front and a dc motor on the back to use as a platform for a arduino project. I bought one which i thought had a servo on the front but when a disassembled it, i found a dc motor with a gear box and a magnet, depending on the direction of the current i apply to the dc motor i can turn the wheels left or right but i can not make them straight.

My question is what is the magnet used for it has two cables going in to it(orange and green), is it used to make the wheels straight? bigger question is what exactly do i have?

I will be using this car with a ardumoto board to move a line follower.

what is the magnet used for it has two cables going in to it

Magnets don’t have wires going to them, well not unless they are electro magnets.

It might be a limit switch so that the current is cut off from the motor when it has turned to the full extent. A reed switch can be used to turn on or off a connection in the presence of a magnetic field. This is used a lot to see if doors are open. A reed switch is place in the door frame and the magnet in the door. When it is closed the magnet is next to the reed and the contacts close (or open depending on the type of reed)

That's a "toy" version of a servo, it's going to be useless to you.

You would have to alternate the phase to the magnet to get it straight, ie, on one cycle apply positive to orange, and negative to green, then the next cycle switch the phase, apply negative to orange and positive to green... and you won't be able to drive it directly off the arduino you'll need some sort of driver circuit, and the code to steer would be plain nasty, if you could make it work at all.

It's going to suck, your better off with a servo, find yourself a cheap "hobby" grade rc car, there are plenty of Chinese clones of brand name cars these days, Tamiya's are really popular to clone, you should be able to find one super cheap, used cars are often cheap as well, kids nag their parents for one, drive it 3 or 4 times then stick it the cupboard and forget about it.

The problem with toy rc cars is as you've noticed they don't use servo's or separate speed controllers, because those components cost to much to put in a toy, if you get yourself a hobby style car you will be able to use the supplied servo and speed controller as a interface to the motor, which would save having to build your own circuit to control the motor.

Mike, I'll bet that it's an electromagnet... Toy makers are too stingy to put a proper servo in a toy, it costs too much, I have a 1/8th Scale 4wd Buggy, it's steering servo is worth around $120au, you can buy toy rc cars in shops for about $40au here... it eats too much into profits to put a servo in them.

Grumpy_Mike: motor has a teeth on it, so it can not physically do a full turn.

bircoe: i do have a motor driver circuit (ardumoto) to drive the motors, would it work if i use digital pins to apply +5v to the wires to straighten the wheel or do i run the risk of damaging arduino?

I'll bet that it's an electromagnet.

Yes but how does he know it is a magnet? If it's an electromagnet it will only be magnetic when current passes through it.

I agree though it is unlikely to be a servo. However part of engineering is to cut the cost, while preserving function. My PhD supervisor told be (a long time ago):-

"An engineer is someone who can do for a shilling what any fool can do for a pound" (Said in a Scottish accent)

Translators note for the young and non UK citizens:- There are (were) twenty shillings in a pound.

motor has a teeth on it, so it can not physically do a full turn.

Yes but is current still applied when it is at full steer?

would it work if i use digital pins to apply +5v to the wires to straighten the wheel or do i run the risk of damaging arduino?

Don't connect an arduino's output directly to a motor, it will draw too much current.

Grumpy_Mike: when i touched it with the screw driver it attracted it thats why i called it a magnet. i wasn't talking about connecting the dc motor to the arduino, i have ardumoto for that, i was talking about the other two wires orange and green bircoe talked about.

I see your point mike, yet i've never seen a toy RC car that does full proportional steering without a proper servo, not that i've seen that many toy cars, I do the hobby RC stuff, ever seen an electric RC truck do 90kph? it's pretty amusing!

So the part with the wires going to it is actually a motor with a pinion gear (the part with teeth)?

Might be good if you can provide some pictures of what you are talking about.

i was talking about the other two wires orange and green bircoe talked about.

Always risky connecting wires up that you don't know what they do. Try measuring the resistance between these wires first. It might be a reed switch in which case wiring it up to an output would cause a short.