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picky motors?

 So I'm a complete beginner at this. I know ohm's law and generally what components do, but I've never tried to put anything together yet.

 I have these 3v motors that don't seem to want to be in a series with anything. They run fine in parallel with each other, but I can't put anything with them. I've tried resistors, other motors, voltmeters, and photoresistors (even when it's right under a light) (The photoresistors work right, I've checked them with my voltmeter). It's bizarre, any little thing and the motors just stop working. I've tried 4v and 6v and it, so there should be plenty of extra current around. So, what gives?

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The dropout voltage of that regulator is normally only 0.5V (i.e. the input can be as low as 5.5V), but it can get up to 1V, particularly if the regulator is loaded with high current, such as may be happening with your motor. You can confirm this by checking the output voltage before connecting the motor, as you've done already, and then checking again after the motor is hooked up.

Another thing that may be causing issues is the motor's stall current. When motors are starting up or loaded so much they can't turn the current rises up to a maximum value, which may exceed what your regulator can handle (500mA). If possible, connect the motor to a 5V source (or close to it, 6V will be useful enough) and measure the current flowing through the motor. Then grab the output shaft so the motor can't turn and check the current again.

it's an LM2937-5 from solarbotics (http://www.solarbotics.com/products/lm2937/). I'm supplying 9v 6v to the regulator, and it works (gives 5v, according to my voltmeter) unless I hook up a motor. This is when there is only the motor on the output circuit from the regulator.

did I say 9v? I meant 6v. Is it possibly causing the power source to dip below what the motor needs?

http://letsmakerobots.com/node/2679
http://letsmakerobots.com/node/8032

What type of regulator (part ID) are you using, and what voltage are you supplying it? Linear regulators need a supply that's higher than their output, often by a few volts, so you may need to supply >7V to the regulator input.

ah, I hadn't thought of that. So is there any way I can make the motors work with photoresistors?

 also, they don't seem to work with a 5v voltage regulator either. Any way I can make that work? 

Since the motors are designed to operate at a low 3V, they'll have quite low resistance motor winding coils. If you know Ohm's law, then you'll already know that in series the total voltage is shared across the components, with larger resistance components consuming proportionately larger amounts of voltage. In other words, whatever you put in series with these motors will almost certainly have a larger resistance than the motors, thus depriving them of enough power to start turning.

Oh, and while I think of it, voltmeters aren't made to work in series, they're always placed in parallel across the two points you want to measure voltage across. Voltmeters have very, very high resistance inputs, which stops current flowing into them which would mess up the readings, but it also means they have to basically be outside the circuit they're measuring.