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Arduino Interference in Vintage Adding Machine

Apr 18 2013 | 9:25 pm

Evening everybody; I’m wiring up a 1950s electric adding machine so that I can read the buttons. My dream was to run this with the adding machine plugged in so that the motor would still whir and the gears would click – it sounds awesome! Here’s the problem: I get all kinds of false readings and regular Arduino freezes when the motor runs.

The whole adding machine chassis carries a serious voltage when the motor runs. As such, I’ve taken great care to insulate my contacts on the button undersides, but it only helps a bit. Even if I unplug every input the motor causes the Arduino to transmit if they’re near one another (grrr)! Is there induced current at fault? I identified a ground loop problem between the adding machine and my computer, but even when I’m on battery power the bad readings occur.

I’ve been working on this for a week and I’m about to move on without using the motor, but thought I’d see if the brain trust had a suggestion before I bag this idea.

Apr 19 2013 | 12:10 am

I’d guess that the shielding on such an old motor is pretty poor – and as a result the motor is putting out some pretty serious EMR (electro-magnetic radiation) which interferes with other electronics nearby (guitar amps, mixer mic input channels, etc. – anything that amplifies low-level signals). The arduino inputs are pretty senestive to random fluctuations, so this makes sense. The only advice is to keep them as far from each other as possible, and see if re-orienting one or the other (turn it so it’s facing a different direction) reduces the interference. Beyond that, building some sort of shielding around either the motor or the Arduino to ground the interference would be the next step.

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