how to shield arduino uno from external mag fields?

I'm replacing the relay logic of an old flipper with arduino uno. My first idea was not to totally replace the logic but just some functions. The relay logic is based on relays (of course) and motor operated cams contacts. In addition there are big solenoids to reset flags and move flipper fingers. I used coax cable to connect contacts and I connected the shield to ground, but even if I place the arduino board outside the flipper it gets enough noise to activate the interrupt and related programming. I used a pull up resistor on the interrupt pin. Flipper counters relays are operated through optoisolated relays so I don't think that noises could come from there.

I then decided to cut out the relays and cam contacts and to use only programming logic but I'm afraid that would not be enough because of the other solenoids I must keep.

Question is : how can I shield arduino? Should I use filters and if so how to calculate values?

many thanks

The question is what value of pull up resistor did you use?

Magnetic fields are no problem, electro magnetic waves are another matter.

of course it is the variation of the mag fields that create problems. I used a 220ohm resistor.

OK 220R is very hefty so it looks like you need to reduce the emissions rather than any protection on the Arduino. Try snubbers on the power lines. Basically you need to slow down the rise time of the current into the coils. Have you got diodes across the relay coils?

coils are part of the old flipper and they operated directly with relays, no diodes nor condenser. Flipper fingers are directly connected to power and operated by push buttons.

no diodes nor condenser.

Then you need to fit some.

Yes, definitely - you have lots of arcing going on without free-wheel diodes or other snubbers, and anything arcing puts out tons of interference (try bringing an old AM radio near it when its operating!)

The same goes for the relay coils if they lack them too.

Diodes are needed it that kind of circuit to use it safe

unfortunately relays and solenoids operate with 30v AC and should react very quickly. How could I place diods? in addition I have to say that relays and solenoids are on elctrical circuit that is separated from arduino. Arduino was powered through usb port of a pc and the set of relays used to control flipper counters are also optoisoled. So the only point in common between solenoids and arduino is the 220v power.

Maybe a snubber can help in this case.

Ciao, Ale.

So the only point in common between solenoids and arduino is the 220v power.

You seem to be remarkably reluctant to accept advice once you have asked for it.

Interference can also be airborne as well as conducted. I am not sure what else we can tell you. You have an interference problem and unless you treat it at the source then your project stands little chance that it will work.

Grumpy_Mike:
You seem to be remarkably reluctant to accept advice once you have asked for it.

I do not accept suggestions that make no sense for me. Suggestion to use diode is not feasible in my case with AC relay.

Interference can also be airborne as well as conducted. I am not sure what else we can tell you. You have an interference problem and unless you treat it at the source then your project stands little chance that it will work.

I agree and I think that in my case the problem is generated by radio frequency ( because circuits are separated).
So the good suggestion is to use snubbers and that is what I will try.

And with this thanks to all nice guys that gave me ideas.

Hi, Can you post a picture of your project so we can see your component layout? Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom... :)

Tom thanks for your post, but it is not possible to post a drawing or a picture. Imagine a 60 years old flipper with the bottom part full of relays , a motor operate cam switches and under the top 2 big solenoids to operate the fingers and next to them contacts the ball will close when hit.

I then decided to disconnect all the relays and the cams, but I cannot avoid the finger solenoids and another pair of them to restore some flags.

Counters are in the bottom upper part and I think they will be not a problem.

Doing that the question is limited to 4 solenoids using 30 v AC related magnetic fields. From the power point of view arduino and the solenoids are on separated electrical circuits.

I hope now that using shielded wire for contacts and condenser and resistors for the solenoids I will fix the problem. It will take some time to me to test because the flipper is not in my home.

Hi, Keep all low voltage arduino wires well away from any AC wires associated with the flipper.

The closest the two systems should get is where the arduino wiring connects to the relay on one side and the relay to the AC on the other.

If you have a rats-nest of wires, then that won't help either.

Tom.... :)

TomGeorge: Hi, Keep all low voltage arduino wires well away from any AC wires associated with the flipper.

The closest the two systems should get is where the arduino wiring connects to the relay on one side and the relay to the AC on the other.

If you have a rats-nest of wires, then that won't help either.

Tom.... :)

thanks for suggestions gio

I would also think there may be other possible problems, but anyway…

The easiest and cheapest way to block magnetic fields is to just use sheets of plain carbon steel, such as furnace ductwork or chimney flue. Even perforated sheet will work, as long as it is made of carbon steel (that contains iron).

But let’s say you want to spend money for the best possible material…

The industrial-grade metal used for magnetic shielding is called mu-metal.

Industrially… mu-metal is used for blocking RF fields, where there is not enough physical space to use a thicker & cheaper piece of carbon steel.

It will attenuate static magnetic fields and lower frequencies (say, <10kHz) but for that it really doesn’t work any better than regular carbon steel would. And mu-metal costs a LOT more than regular sheet steel does.

It will attenuate static magnetic fields and lower frequencies (say, <10kHz) but for that it really doesn’t work any better than regular carbon steel would.

Not sure if is the English, but Mu-metal will block a static magnetic field where as a carbon steel sheet will not block it at all.