# What do i need to get this motor running off a car-battery?

I'm going to build a motorized barn-door mount for astrophotography, but have ABSOLUTELY no experience with motors, circuits or anything else related to the motor.

This is the motor I bought: https://www.sparkfun.com/products/12219

I've read about resistors, voltage, wire gauge, etc, etc... I need it at 1 rpm consistently from I turn it on until I turn it of. I plan on having a snap-switch on the thing.

Now, what do I need??? I have the motor, motor mount, switch and that's it. What gauge wire do I need? Resistor? Fuse? One of those boards that look like they belong in a computer? I've searched in here and took a look at the videos where I bought the motor, but I'm too green in this.... Hope someone in here can help.

• It really depends on your application. What are you planning to use it for? How do you envision it working? A flow chart or state diagram would be handy. – darudude Jan 5 '15 at 18:50
• [as an aside] A car (or motorcycle) battery can work for powering this motor. But a deep cycle battery will last longer (greater number of recharges, see also this). Of course you can start with a car battery, wear it out, then get a deep cycle battery. – Nick Alexeev Jan 5 '15 at 19:03
• Will it be the battery in your car, or just a car battery? If it's in active duty then its voltage will vary a fair amount, from first driving to the site, then standing still for hours, perhaps enough to upset your speed calibration. – tomnexus Jan 5 '15 at 19:08
• 1 rpm? Are you sure you want the motor to take a whole minute to make just one rotation? – Will Jan 5 '15 at 21:26
• @Will The question states it is for astrophotography, so I'm guessing the idea is to track the movement of the stars. One minute for one rotation sounds reasonable given how fast the stars move. – Phil Frost Jan 6 '15 at 13:28

You need just motor, switch, wire and fuse.

The motor has a stall current of 0.5A. That's the most it'll ever expect to draw. So you need a fuse that'll blow at more than that (1A would be fine). You also need wire that can take 0.5A minimum.

You also need to consider how far from the battery the motor will be.

Under normal operation it'll be somewhere between 95mA and 500mA (0.5A) current draw depending on the load it has to move. So we'll take the worst case of 500mA.

24AWG is the absolute thinnest wire you could get away with for 500mA. Any thinner than that and it'll fuse. 24AWG has a resistance of 25.67mΩ per foot. Using the formula: $$V=R \times I$$ we can work out that, per foot at 500mA, you would drop: $$V = 0.02567 \times 0.5 = 12.8mV$$ So after 10 feet of wire your 12V would be (12 - (0.0128 * 20) * 2) 11.744V. Note, it's times 20 not times 10 as you have to remember, it's "there and back again", so 10 feet distance is 10 feet of wire there, and 10 feet back again, so 20 feet of wire.

But, choosing thicker wire, say 18AWG, which is about twice as thick as 24AWG, the resistance drops right down to just 6.385mΩ per foot. So for 10 feet distance (20 feet of wire) you'd be losing just 64mV, so your 12V would still be 11.94V.

• Can you write the units using the Metrical system, please? (If not possible, can you PLEASE post a comment explaining them?) – Ismael Miguel Jan 6 '15 at 1:12
• I hope to have the battery no more than about one meter away from the motor. I have NO experience with electrisity, so if i say one meter of wire (2 meters there and back again), to have enought power to keep the motor at 1rpm, what size wire would you use to minimize drop? I think the weight it has to push will be around 3 kg, but that's 3 kg if i would lift it. I don't know what it will be since the motor is turning a screw that ends on a smooth surface... Hard to explain but hope you understood some of it... – Oliver Jan 7 '15 at 16:45

You could hook it directly to the 12v battery, with the switch in between.

If you want to be a little bit better, a common 1n400x diode in reverse across the motor terminals, and a slow blow 0 6 to 1 amp fuse would be nice.

That motor only takes a tenth of an amp, so 22 or lower gauge wire will do. Common 2 pair lamp electrical wire will be 16 to 18 Gauge and work fine.

A car battery will last for weeks on this motor alone.

It probably won't move a barn door though.

• Good point about the diode. Maybe the barn door comment is a joke, but a barn-door mount is a different story. – Samuel Jan 5 '15 at 19:04
• Both. Depends on the weight of the camera or scope, materials used, ops specific mounting. I figure that motor isn't as strong as a generic microwave motor. – Passerby Jan 5 '15 at 19:10
• The OP is talking about using a car battery. That fuse should be a requirement, not a nice addition. – Matt Young Jan 5 '15 at 19:29

You stated:

I need it at 1 rpm consistently from I turn it on until I turn it off

The product page states (emphasis added):

They have a gear ratio of 3000:1 and operate up to 12 volts and deliver a stall torque of 2995 oz-in. and a max speed of 1 RPM.

Maximum speed means it might be something less. In fact, if there is any mechanical load on the motor, I can pretty much guarantee it will be less.

If you don't actually need the speed to be consistent, then you can follow the advice in the other answers and hook it up to a battery. But if you actually do need a consistent speed, this motor is probably not going to work well for you.

For a consistent speed, you need something to regulate the speed. You have generally two options:

1. Measure the speed, and adjust the drive voltage to adjust the speed towards a target speed. This is called a servo.
2. Use a synchronous motor, one that turns at a rate synchronous with an AC driving waveform. A stepper motor is one kind, but there are others.

In either case, using those devices is a little more complicated than can be explained in the scope of one answer, but now you have a direction to research.

• Since the load is constant, an old-school technique is to gear the motor to roughly the speed you want then adjust the supplied voltage manually until you get exactly 1 rpm. This can be done to within 1% error margin. – slebetman Jan 6 '15 at 4:03
• @slebetman sounds like a servo with a meatware controller. – Phil Frost Jan 6 '15 at 13:26
• Hello. I should probably have been a bit more detailed. The "barn door" i want to move is a device designed to move a camera so that it tracks the stars in the sky so i can get pictures without startrails. With the right screw it should move 1 mm/min, the same rate as the sky move. – Oliver Jan 7 '15 at 16:34