Avoiding EMI when bunching together multiple wires  - Page 1
Forum section: electronics / projects

author: amp2
author_rank: Newbie
timestamp: January 10, 2017, 02:30:20 pm
content: Hi,I'm trying to control a kitchen appliance with an arduino, so I'm replaced all the buttons (about 7) with 1 m solid wires (l need them long to route them to the relay card). However the buttons work reliably only when the wires are spread apart. My guess is this is due to EMI when the wires are bunched together.To mitigate this, I have tried twisting pairs together and covering individual pairs with Al-foil and grounding them, But I haven't had any success. Any thoughts?

author: Ian.M
author_rank: Super Contributor
timestamp: January 10, 2017, 02:45:33 pm
content: Probably cross-talk due to excessive capacitance is messing up the key scanning. Also it may not even be safe to extend the wiring outside the case. (e.g. if the appliance's controller uses a non-isolated PSU). Odds are you'll need to use optoisolators as close as possible to the connectors for the buttons, but you'll have to do some experimenting to determine the correct polarity for each opto's phototransistor to button connection hookup.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 10, 2017, 02:55:18 pm
content: The appliance inputs from the keypad are likely not debounced or hardened against emi as they were intended for short runs inside the machine. Strongly suggest as Ian did that you put optoisolators at the board input.You may need some pull up resistors as the leakage of the opto's might be too much for a reliable off state.You might try relocating your relays to very close to the card inputs as well.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: January 10, 2017, 04:15:52 pm
content: Though the suggestions above are better overall solutions you might be able to simply use some thing coax for the wires, and ground the shielding.

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2017, 08:01:45 pm
content: Quote from: Ian.M on January 10, 2017, 02:45:33 pmGood idea. You could use an opto-isolator with a FET output, then the polarity won't be an issue. The downside is they're more expensive, than BJT opto-couplers.http://docs-europe.electrocomponents.com/webdocs/129d/0900766b8129d5be.pdf

author: Benta
author_rank: Super Contributor
timestamp: January 10, 2017, 08:27:33 pm
content: Problem is more likely that your circuit is too high-impendance and susceptible to all kinds of pick.I just checked the schematic of an Arduino Uno, and there are no pullups/pulldowns on the I/O pins, meaning that you are relying on the internal 50 kohm pullups.This is far too high. In an appliance environment, you need to pull a few mA at least through the circuit for reliable signalling.Try adding pullups at the inputs to the Arduino, forcing this current through the switch loops.Start with 2 kohm pullups and take it from there.Yeah, I know, bye-bye low-power design, but that's the way of the world :-)

author: Ian.M
author_rank: Super Contributor
timestamp: January 10, 2017, 08:34:12 pm
content: Quote from: Benta on January 10, 2017, 08:27:33 pmYou've got it ass-backwards.According to the O.P., the Arduino is outputting to a relay card and the relay contacts are across the appliance buttons, to duplicate their action under Arduino control. If so, adding pullups at the Arduino would have no effect whatsoever except to fractionally increase power consumption..

author: Benta
author_rank: Super Contributor
timestamp: January 10, 2017, 09:53:34 pm
content: QuoteThank You for your friendly, courteous and well-mannered reply.You may be right, the original post is not clear on the functionality, and mentions "1 m solid wire", "buttons" etc.Rereading it, I'm none the wiser, a sketch on a napkin might help.