Limit Switch Noise!

Thank you for advice!

The resistors and capacitors in your original post are electronic filters that allow some frequencies to pass relatively unimpeded but absorb others. What you want is something that blocks high-frequency signal and passes low-frequency signal. This is called a “low pass filter” and here’s a technical explanation:

The values of the resistors and capacitors determine what frequencies to pass and which to block.

Basically, fast voltage spikes from interference pass through the capacitor without raising the voltage at the input to the point that the Arduino detects the switch has been triggered, but if the switch is actually activated, the capacitor quickly “fills up” and the Arduino triggers. There are resistors inside the Arduino processor (called pull-up resistors) but they are between 20K and 50K so the 4.7K external resistors in your diagram reduce susceptibility to noise.

The thing you show clipped around a cord is a “ferrite bead” and could also help; it is also a filter, but it works by resisting change in magnetic field. I’d start with the resistor and capacitor in your original post before clipping ferrite beads on the wire.

(@donkjr is the real expert here, I’m just translating. :grin:)


Actually, the cap has another function. It sort of acts as a resonant circuit with the coil. Try running your old car without a cap? Won’t run. No spark.

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And have a SEPARATE ground return for each and every power and signal line.

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The cap is shorted out when the points are closed. It is only connected across the points when they open so there is one dc pulse which charges it to supply voltage. So dc resonance?

I’m willing to try and follow you here… if you can explain…


If you stick a scope on the points, you will see a big ringing pulse at the moment they open. The cap is
trading energy back & forth with the coil. Don’t take my word for it - disconnect the cap and try to start the car.

Last time I burnt the points on my scoot the cap opened up. Didn’t notice much performance difference.

My 64 m3 jeep stopped out in the desert, the cap had shorted. Pulled it out and drove it home, sans cap.

I guess I’ve had a difference experience.

I don’t know what would be oscillating… I’ll have to do some research… hunt down a scope view of it like you suggested… don’t have anything with points on it anymore… :frowning:



In that case, I stand corrected. I know I read it somewhere…

I think if you drew the circuit with the points closing the circuit with one side of the coil and see the cap across the points you’d notice when the points are open, the coil field collapses and at that point the coil and capacitor are in series with the cap grounded and the high side of the coil at 12V. Points side is floating. Back EMF from the collapsing field buffered in the cap and might be some settling but nothing with enough power to affect ignition.

Great drawing… As I stated, it’s across the points, it purpose is to protect the points.

I’d like to ‘point’ out that there is a series lc circuit. Here’s a schematic, but it’s fed with ac, not dc and the capacitor isn’t ‘shorted out’ through 1/2 the cycle or part of the operation.

Everybody keeps bringing up back emf.

Nobody cared about ‘back emf’ since there is nothing there to damage when it does occur. Generally it only effects solid state equipment where a reversal of the voltage causes the device to conduct.

There is nothing in this circuit that a ‘back emf’ could damage.

There is also no high voltage, as many seem to think…


I mentioned back EMF because I thought that’s what the energy the cap was absorbing. When the points are closed the cap is shorted and xformer field saturated but when the points open, the field collapses but maybe it’s all about the abrumpt stopping of holes(electrons) which limits the arcing at the points the moment they open and the capacitor is free to charge. Ya, I answered my own comment. :slight_smile:

The only high voltage is in the xformer/coil secondary so you are correct, no HV in the 12V circuit, just some bursts of current.

Right arm (on)… :crazy_face:

If there is no capacitor to stop the arc… the arc is resistance, so it lowers the saturation of the coil, i.e. lower voltage. When the field finally collapses, less saturation results in less ‘spark’…

I think they figured this out in the early 19th century…

I dismiss claims of back emf having any involvement here.

This was the first ignition coil I fooled around with, grandfather had it in the basement. Took the batteries out of my grandfathers flashlight and series them to get a spark, actually a shock… Live and learn… I wish I could have kept it around. Always loved the wooden box, especially compared to ‘then’ current ‘black can’…

Screenshot from 2022-07-28 07-06-41

Same coating of oil… It’s a ‘buzzer’ based spark system…


Well, this really strayed far from debouncing switches and preventing noise pickup.