motor noise

hello hello. Does anyone have any suggestions for coping with motor noise? I am running a 3V motor which is triggered by sound input from a electret mic. The motor is getting 5V, as is the rest of the circuit which is built on a breadboard. The sound trigger circuit is working fine, I have an LED wired up to show when a 'trigger' occurs. When I connect the motor to the circuit the motor is triggered correctly but the noise which falls back onto the circuit means that it does not deactivate when the incoming audio signal drops below a threshold set by a 10k pot. I am guessing this is to do with the noise from the motor, I've wired up a small speaker to check a few things and when the motor is activate the noise is clearly audible above that coming from the electret mic. How much of a problem is it powering a 3v motor with 5v, other than overloading the motor? could limiting this to within 1.5 - 3v range get rid of the pesky noise?

any hints are greatly appreciated.



yes there are some easy rules for eliminating motor noise. In order of effectiveness:

  • put a .01uF cap across the motor terminals, at the motor;
  • put a scchottky diode like an 1N5819 across the motor terminals. Only works for single-direction circuits, not reversing circuits.
  • use a giant honking power supply capacitor on the motor supply: 470Uf or so; **
  • for single-supply circuits, use a diode in series with the motor supply circuit. this prevents spikes and noise form going back into the logic supply.
  • use separate power supplies for logic and motor circuits. **
  • use opto-isolated inputs to motor driver circuits, with above items.

The ones with asterisks are the ones that will fix your circuit lickety-split. D

Cheers Daniel, thanks once again for the knowledge booster.

I had a quick go at using a separate power source for the motor (before having to run off to france...) which seemed to work well although there was a bit of a delay between the motor deactivating and becoming ready for reactivation... Think I'll go with that method when I transfer it to the strip board. Is there any benefit to using two cells or would it be the same as running two sources from a single power suply (such as the 5v and 9v on the arduino, which is how I am testing the circuit for now)?

I'd also be interested in trying out the power suply capacitor method, once I'm back home, just for the practice. Is a power suply capacitor different to a normal capacitor? I assume it should be placed between the motor's positive terminal and V+ ?