Sensor interferance and USB

[roli]

So back in the old forum I posted a question right before the upgrade to the new forum. Since I haven't solved my problem yet I am reposting this topic.

Link to the old forum:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1295719923/

[MikMo]

I would try the same setup, but with another PC.

[Grumpy_Mike]

From your other post

As for common mode choke - what could that be?

A common mode choke is two coils wound together on a single core but in opposite directions, any large transient signal on both lines will be suppressed as each line generates a magnetic field in opposite directions and they cancel out.

What I don't get is this - sensors themself are digital. They output 1 (if IR light is detected) or 0. So the only way that the interrupt gets triggered is if somehow the voltage on the arduino pin drops (the interrupt is configured to trigger on falling voltage) when I turn on the light.

No :- the sensor is normally a high impedance and can pick up transitory signals like a radio antenna does as well as the signal it is supposed to pick up. Try putting a 0.1uF capacitor from the arduino input to ground, that will help suppress pickup of interference signals.

[roli]

I don't think pluging it into another computer would work. I just noticed that my wacom USB tablet blinks when I turn the light on. So proabably it has something to do with voltage drop.

Idea with capacitor looks like it might work. Of course this will be a little messy to do since the PCB has been made. But I will try it.

[MarkT]

From your other post

As for common mode choke - what could that be?

A common mode choke is two coils wound together on a single core but in opposite directions, any large transient signal on both lines will be suppressed as each line generates a magnetic field in opposite directions and they cancel out.

Really?

[Grumpy_Mike]

http://www.butlerwinding.com/store.asp?pid=28349
Yes

[MarkT]

Look again, common mode chokes are parallel winding, differential-mode are anti-parallel.  If your unwanted signal produces no flux in the magnetic circuit, how can there be any inductance to choke it?

[roli]

Try putting a 0.1uF capacitor from the arduino input to ground, that will help suppress pickup of interference signals.

I did this a few minutes ago and for now it seems to be working. It may just be luck since this problem sometimes doesn't appear but I think that it is working.