Softpot and interference problems..

Hey all!

The facts:

  1. I'm building a midicontroller in an aluminium chassis. The controller is based on Arduino.
  2. Inside the chassis I've got an ATX power source from an old computer. This is only used to power some cold cathode lights and is totally separate from the Arduino circuitry. However the power source does physically touch the chassis.
  3. On the outside panel I have a Softpot linear potentiometer. This is isolated from the metal chassis with plastic strip and glue. Also the wires are covered with adhesive tape.

The problem:

When the power source is turned off the Softpot works fine. However, when I fire the source up, the Softpot goes insane and starts fluctuating like crazy. So obviously there's some interference from the power source. I just don't see how this is possible, since the Softpot is in no direct contact with the chassis. I tried grounding the whole chassis too, with no results.

Some pics:

Any help? Thanks!

This is clearly a problem of interference caused by high impedance pick up. What circuit have you got with your softpot? What is the impedance going into the analogue input?

I would try a 200R series resistor and a 100nF capacitor on the analogue input. The other thing you could do is to improve the overall power supply decoupling, including the supply feeding the softpot.

For further information about decoupling see:-

In addition to being connected to the analog port, the softpot’s wiper is connected to ground via a 100K resistor to pull it down close to zero when not touching the Softpot. There’s no resistor between the wiper and the anlog port. The + and - are of course connected to Arduino’s +5V and GND.

I’ve also got some pots on the device which I haven’t tested yet. They’ll propably have the same issues…

I’ll see if the capacitor and resistor help, thanks!