Let's Make Robots!

How do determine power requirements of salvaged motors?

Hello LMR, this is my first post so I will start with an introduction. My name is Sion and i'm from South Wales, UK. I follow the youTube posts and thats what lead me here.

I recently purchased a Sir Killalot RC toy from a local charity shop for £2. I managed to salvage a great base for my robot, 2 motors and gears, but it didn't have the remote control or battery pack.


My circuit is a simple 2 motor setup using the L293D and an Arduino. I have the Arduino powered by 9v and I have also connected the L293D to 9v. The whole circuit is ground looped. I'm confident the wiring is correct as my sketch is running as expected, but with each iteration of the sketch the motors get slower and slower as if the batteries are dying(despite being new).

How can I determine the power consumption of the motors I have salvaged? There was no battery to give me any indication(however the compartment was quite large) and I dont see any markings on the motors.

Any help greatly appreciated.

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I can tell without even looking that the motors are drawing much more current than the 9v can handle. Swap out that 9v going to the motors with a (4)AA pack and you will (probably) be all set.

Thank you kindly, this has indeed solved my problem. Would you mind sharing how you knew this would be the case?

I'm going to be putting an ultrasonic sensor and servo on the robot (as per "Start Here" robot). Do you recommend doing the same for the Arduino power source, that is upgrade to a 4xAA setup?

Well, you got yourself volts and amps and Ohm's law. Put simply, both your motors and your brains use similar voltage, however your motors draw a lot more current. Voltage is like water pressure in a pipe and current is like the diameter of the pipe. The 9v batts you are using only have a small pipe connected to them. The AA pack allows for a much larger "pipe" for the power to flow through. This is what your motors needed. In terms of aa's for the arduino itself --you probably can run off of a 9v for quite a while. The power required for thinkin' is not much.

I was 95% sure you needed a pack that could put out more current. I suggested a 4-AA pack because you said you had bought your toy car for just 2 quid. At that price, I assumed it was a super-cheap little toy and not one that would need a 7.2v or 9.6v "big" pack. Even if it were a RC car that normally would take a larger pack, the AA's would still give a voltage close enough to what is "normal" for that car to get the motors moving.


thanks for taking the time to reply. I guess ultimately it is experience that helped you decide then ;-)


Always better to be lucky than good.

--And it didn't hurt that you had a pretty common problem.