Toy motor interfering with display

Hello everyone,

I'm controlling a toy motor with PWM. Every time you press a button it increases the value used in the AnalogueWrite command. However, I've also got a 16 x 2 character display to show what the present value is...

The problem is that as the motor increases speed it appears to cause interference which leads to erratic behaviour on the display, from garbled characters to going blank completely.

I know this has been discussed before and I've searched the forums already with limited success, although I did find a link to an article by Grumpy Mike (

I've followed his suggestions of a 100nF capacitor across the motor terminals and I've put a 470uF electrolytic across the motor supply. The stall current of the motor is in excess of 1 ampere so a 1mH choke would be relatively expensive.

Any ideas anyone?



The stall current of the motor is in excess of 1 ampere so a 1mH choke would be relatively expensive.

Any sort of inductor is better than nothing. However as it is in series with the load then the inductor only has to stand the current of a short with its own resistance. It could be it will never see anything like 1A.

If you have a scope meter attach it to the pins on the LCD. Check to see if the LCD is getting inadequate power and that is the reason it is not behaving properly. If that's the case, you should probably have some decoupling capacitors as close to the lcd as possible.

Is the motor drawing current directly from a battery, from the Arduino's onboard regulator, from USB, or from an external voltage regulator?

Hi, Have a look at the links in my signature below, if your not using separate power for the motor this could be your problem. Both links relate to servos but apply equally to motors, shields or anything else power orientated.

While you circuit may not be as extreme as the ones in the video, it could be enough to cause junk data.

The decoupling suggestions are also very good and should be applied.

Duane B

Thank you for the suggestions!

I had already put decoupling caps on the display, and put a large(ish) cap on the supply in general to try and filter it. However, I'll look at the power supply issue shortly, presently it's a 9 volt 600mA wall wart. The whole project is just a lash up on a breadboard to get some nominal values for later use.

I also think that it's causing interference to digital radio signals, but to be fair the reception can be poor so it's difficult to tell!


The problem may be related to layout, in particular how you have the ground wires connected. A circuit diagram and a photo of your lashup would make it easier for us to help you.