# Project Help 12v dc Amps/ Digital Voltmeter/Tachometer

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#### Rogeru

##### New Member
Hi, looking for some help on how to get readings from a 12v dc motor.

History, I own and race slot cars that run small 12 v motors with gears on them that run on a commercial track that can run from 155 ft to 250 ft long.
The cars are powered by 2 pieces of braid that gives a pos and neg side. We control these cars by a controller that allows you to control how much power goes to the car, resistors of 1 ohm to digitally controlled pods.

What im trying to do is find out what i would need to be able to measure how many amps the motor is drawing , how many rpms it is running at a certain volt, we have power supplies that show us how many amps the motor is drawing on a 10 amp-variable power supply 1- 12 volts dc. What it does it supply power to the motor to break it in, test it if it works, etc. If you turn the power supply up to 6 volts it generally pulls 2 amps. To give a idea.

A company use to make one that is a power supply and showed the amps it pulls, then showed you the motor rpms, but it is no longer being made.
Im looking for something that I could buy or make that would allow you to do all the above plus show the peaks of the motor, were it stops pulling and were it flattens out at. Maybe software for this, maybe just a automotive digital gauge ?? Some type of dyno

any suggestion would be great and appreciated.. Roger

here are a few links to help as to the motors and what we do:
Pro Slot Pro-Slot New Products

#### Mr RB

##### Well-Known Member
This is only half the solution but it might work for you, at least it's easy. You can get a "optical tacho" or "optical RPM meter" from Ebay, they are usually in the $30 -$40 range.

These are self contained you just need to point them at the wheel, or at the motor shaft. They do need a light/dark mark on the spinning object, but you can just put a white sticker on the black wheel or with a shaft you can attach a tiny disk with a light/dark side painted on.

So you could then see the motor amps with your power supply meter and see the motor revs with your optical tacho. It's not as nice as a proper "model car dyno" but it would get you up and running quite fast.

If you did want to build a specialised dyno, there is a low cost project here;
(see SH1_Tacho)
Shift1-LCD Projects
that option would require soldering skills and the ability to connect up an opto sensor etc, it's not really a beginner project even though the PIC is supplied pre-programmed ready to use.

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