RF Micro-switch

November 7, 2018 6:26 AM

I would like to buy a switch for industrial applications which, when pressed, sends a radio signal to a receiver up to 50' away. This switch would be a small button, no larger than an inch or two in any dimension. I would attach it to a torch head of a TIG welder. The dimensions of the receiver would be less important. I'll explain more inside.

My company has about 60-70 TIG welding machines being used in the shop and in the field, mostly Miller Maxstars. I build and repair the welding leads as needed. These consist of a heavy wire that carries the current and the argon gas, as well as a second, 18 gauge 2 conductor wire, which attaches to the 14 pin connector at the welder end, and a low voltage micro-switch at the other end. When this wire breaks the wire gets pulled from the field, and I replace it. It happens a few times a month.

I envision doing away with this 50' of wire by simply having a switch by the torch capable of sending a radio signal, detectable at the welder's 14-pin connector. The receiver would simply open or close a circuit depending on whether the transmitter button was pressed. Extra points if the transmitter could be set to either Normally Open or Normally Closed. More extra points if I could buy both components for <$50.

posted by jwhite1979 to Technology (13 answers total)

Some TIG welders produce copious RF noise, so your wireless signal might get lost on the way.
posted by scruss at 6:41 AM on November 7, 2018 [1 favorite]

Are there more than one welder in the area at a time? If there are, you'll need to be able to distinguish one from another/address them or you'll be in an all-on/all-off situation every time you hit the button.

In any case, if you're home-brewing, an arduino, relay, and 433 MHz module would be a good place to start. There's also a pretty good selection of RF-controlled switches on ebay that could you get close to what you're looking for.

It may go without saying, but there will need to be careful planning around safety, RF interference, and the like. In the 433 MHz portion of the spectrum, for example, devices must accept interference from other devices. Which means if some other thing down the street is transmitting at the same time, there's no real recourse for anyone. That's just the way of it.

It's a shared segment, and there's tons of other gadgets using it these days for all manner of wireless communication (smart meters, thermostats, etc). Getting into different parts of the radio spectrum will get more expensive in terms of either components, licensing requirements, or both.
posted by jquinby at 7:02 AM on November 7, 2018 [1 favorite]

I thoroughly distrust wireless communication devices, especially when there are multiple devices within radio range of each other and double-especially when there are electric arcs emitting wideband noise nearby. I would expect that re-engineering the control wires so that they're not so easy to break would be far, far easier than engineering a comparably reliable wireless control for the task you describe.

Wires have essentially three failure modes only: they can break, they can short or they can arc. Radio has many, many, many more.
posted by flabdablet at 7:19 AM on November 7, 2018 [3 favorites]

I've never had much luck with 433MHz kit. I'd go with a Wemos D1 mini (or mini pro, which would enable a better aerial), and use wifi - (subject to RF interference concerns mentioned above). You can get a single relay shield for them.

Two units, plus a button, cases and power supply(/supplies) (USB powerbank) should be under $25.
posted by pompomtom at 2:09 PM on November 7, 2018

I'm guessing a failure mode of stuck-ON would be very very bad, yes? An activated torch head that the welder cannot turn off sounds extremely dangerous, at least. If that is the case, you absolutely have to interpret a lack of signal (due to interference or anything else) as an OFF signal. And I imagine you would need a fast response there; that is, you can't have it wait a second to be sure it should turn off. But then you're left with a lot of potential for intermittent false OFF signals, any time there is interference, which would be frustrating at best and potentially lead to poor quality welds.

I don't think you can get a radio link with both constant transmission and accurate transmission. You can have robust and accurate transmission of data if you allow for delays and retransmission, or you can have a constant transmission of a signal if you don't need it to be accurately received at all times. It sounds like your application needs both, which probably isn't possible for any reasonable cost.
posted by whatnotever at 3:01 PM on November 7, 2018

I assume people could be injured if a fault in your system caused the wrong welding machine to turn on?

If that is the case, don’t even think about implementing this system in your shop. You would be exposing your company to serious liability issues when someone gets hurt.

A potentially injury causing radio control system needs to be designed and implemented by someone who knows exactly what they are doing, and has insurance to prove it.
posted by monotreme at 4:44 PM on November 7, 2018

Yeah, for all the reasons above (plus a few others I can think of e.g. powering the transmitter head?), wireless is not a good solution to this problem. The best solution would be "better cable". Ideally you'd be able to find an existing welding cable with high-flexibility control wiring, but maybe not - I suspect it's one of those things where "normal" is good enough 90% of the time, and so the cost jumps rapidly as soon as you need better than that.

Looking at your sort of usage (e.g. 3/month x 50' x 12 months = 1800'/year), just good-quality high-flex 16 AWG 2-wire control cable alone will cost you $5~$10/foot, or about $375 per 50' cable - just a little bit more than your sub-$50 requirement… 😳

But it might be worth checking with the sorts of places that specialise in manufacturing &/or assembling custom cables (& not just welding cables, but all sorts) - they might have some solutions, experience, or ideas that work better for you.
posted by Pinback at 6:08 PM on November 7, 2018

To prevent the failure mode of stuck-on, you can have a watchdog timer that turns the thing off every 1 millisecond unless it gets a packet from the other end. You can put each system on its own network secured by unique security keys. Look into Xbee and/or ZigBee.

Check this thread:
https://weldingweb.com/showthread.php?523291-DIY-wireless-TIG-foot-pedal/page5
posted by at at 11:39 PM on November 7, 2018

Response by poster: Thanks to everyone who responded. After reading the replies it's clear that a better wire solution is what I should pursue, and so I have. It's still irritating though. It seems like the technology should be here by now!
posted by jwhite1979 at 3:30 AM on November 8, 2018

Just out of interest: when the existing setup fails, does it fail consistently in one way?
posted by flabdablet at 4:37 AM on November 8, 2018

Response by poster: Most of the time, yes. It's a weak spot about 10" down from the torch head where there's a lot of flexion. I had the purchasing department order 1000' of an 18/2 olflex control cable, which is supposedly better at handling this kind of wear. Another less frequent issue is with the guys connecting and disconnecting the 14-pin plug. They tend to unthread it in the wrong place, causing the soldered connections inside to loosen. But I'm hoping that issue will be solved with loctite.
posted by jwhite1979 at 9:32 AM on November 8, 2018

Would some strain relief help here? If the wire were, for example, left slack or spiraled around the welder at that 10" point, so that the bend radius were increased?
posted by meaty shoe puppet at 8:32 PM on November 8, 2018

Another option would be to repair any broken cable by installing something like an inline Molex connector two feet back from the torch head, and replacing the last two feet of 18/2 with something like 12/2 instead. That will contain many more strands, so it will take much longer to break as well as being a tiny bit stiffer and therefore somewhat more resistant to flexing injury in the first place.
posted by flabdablet at 9:46 PM on November 8, 2018 [1 favorite]