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I have a 240V dust collector that I manually switch on and off using a one-button wireless remote. The dust collector is connected to several machines.

I'd like to use a current sensing relay to automatically turn on the dust collector when one of the machines starts and then off when it stops.

I see many inexpensive current sensor relays, including ones with variable trigger points and delay timers, but they all have the same basic output: NO or NC and Closed or Open when current flows. What I think I need is a momentary closed circuit whenever the state changes. With that I can hack the wireless transmitter to replace the switch that the button closes with my new circuit.

I know I can do this with an Arduino or Raspberry Pi, or I could skip the wireless altogether, but I'd like to know if there is an existing relay that does this or something simple I can add after a current sensing relay to translate each state change into a momentary pulse.

Douglas Krugman
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  • How long of a pulse do you require? Have you worked this out for your wireless that you expect to modify? – jonk May 05 '21 at 04:27
  • You don't want to mimic the pushbutton at all. One machine turns one, one CT pulses, DC goes on. Another machine goes on, it's CT Pulses, DC goes off. Use steady state CT's to drive the external enable on the DC, it will almost always have one. – R Drast May 05 '21 at 11:31
  • @jonk - haven't worked out the timing, but I think it's pretty much instantaneous and the wireless is probably designed to handle a range of time that a user holds down the button so it won't cycle on and off if pressed too long. – Douglas Krugman May 06 '21 at 17:56
  • @DouglasKrugman The idea of converting a relay's ON and OFF cycling into two can be called a "frequency/clock doubler." If you [look here](https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3327.html) you will see a simple circuit for something like that. The idea I have requires two BJTs, four resistors, two capacitors, and two diodes. But it is along similar lines to what's on that web site. It would likely work, or something very close to it would. – jonk May 06 '21 at 18:03
  • @RDrast - I though about that. I'm the only user and would never want to run two machines at the same time. The bigger issue is going from machine to machine and not wanting to bother with keeping track of the remote. I don't like wearing it or keeping it in a pocket. Too often I'll think I'm just doing a quick thing and won't bother with the DC at all. – Douglas Krugman May 06 '21 at 18:04

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Follow the rules

The trick with anything that that which uses AC mains, is complying with Electrical Code rules for handling power. To name a few:

  • Equipment must be Approved, generally by UL (and that means UL Listed not RU-Recognized)... and you must use it according to labeling and instructions.
  • AC mains wires have to be entirely inside AC mains wiring methods and enclosures (e.g. Class 1), and low voltage circuits cannot be in the same cables or enclosures unless the entire low voltage system is, soup to nuts. No ins/outs, unless optical or wireless.

The problem with hacking the wireless relay is you're working way too hard to save a relay, which is a readily available commodity, and it also makes control more complicated than it needs to be.

You could have a circuit entirely inside the box with two Current Transformers -- one clamping a dust collector wire and the other clamping a bundle of wires (1 hot from each tool; CT doesn't care how many wires are in it). It would look for "disagree" (tools on CT off, or tools off CT on) and send the signal at that time. But you'd need to hack the pushbutton to achieve that wirelessly, unless you wanted to do something optical to get out of the panel to your chosen relay. See what I mean about 'too much work'?

The simpler way: Dust collector on when load is on

And for that, we can use an approved enclosure -- the bog-standard AC mains circuit breaker "sub panel" that is surely already in the wood shop (or you will really benefit from having one).

And we can use approved switching equipment - either... a) A Honeywell Aube (which supplies both a large relay and a 24 volt AC transformer to power the relay; it has 2 terminals, shunt them to make the relay pick up). Some Aube's bring out a third terminal, giving you both legs of 24VAC so you can power other small stuff. Or, b) do it with separates: using a UL-listed general-purpose contactor (not a specified-purpose contactor for A/C, etc.) and a COTS $13 24V thermostat transformer. All this lives entirely inside the subpanel enclosure.

The relay is wired to interrupt (a) hot wire to the dust collector.

So now, all we need is a current sensor that shunts (shorts) 2 wires when current is flowing.

Sensing AC current

How do we sense circuit current being active? AC electric wires throw a rather considerable Electro-magnetic field or EMF. So do DC wires, but theirs is static, like a refrigerator magnet; AC is like a refrigerator magnet that is spinning. If the 2 circuit wires are together, the EMFs cancel each other out. But separated, the EMF throw is considerable (everything between them becomes the core of a transformer). And that's part of why AC wiring rules are so complicated and counterintuitive.

Magnetic reed switch - closes on as little as 10 ampere-"turns". (a wire passing straight by it is a "turn" thanks to the magic of AC EMF, i.e. the thing that makes transformers work).

You can wrap the wire multiple times to increase effect by that many times. Passing the return wire (e.g. neutral) by in the opposite direction also counts as 1 turn.

At that point you're just running the normal, insulated wires past the reed switch. Noting the two wires powering the tool throw equal and opposite EMF; if they run together the EMFs will cancel each other out. So, use that to your advantage in placing the reed switch.

You do all this inside the subpanel, so you have a nice Code legal enclosure for all this stuff. You're not even piercing any of the circuit wires, there's simply no need thanks to the magic of AC EMF.

The low voltage 24V wiring is entirely contained inside the subpanel, and since it's all contained within Class 1 wiring methods, it doesn't matter if it intermixes with AC mains voltage.

How is the magnetic reed switch going to pass inspection? Granted, you're kind of throwing yourself at the mercy of the inspector... but since it's entirely inside an enclosure, 24 volts, insulated with shrink tubing (right?) and nothing is happening except normal, insulated wires are being routed near it... there's not much to fuss about.

Harper - Reinstate Monica
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  • When I read the OP's question, it took it as given that the wireless unit is to be modified (probably by finding the button on it and bypassing it to mimic pressing on it.) They already have an existing dust collector that works wirelessly. I don't think there would be a code violation using a non-invasive COTS CT/relay and using that to activate the wireless. Just a couple of diodes, capacitors, resistors and a pair of transistors? Whether that's a good idea or not is a different question, perhaps. – jonk May 05 '21 at 06:30
  • Yeah @Jonk I should make clearer what's allowed/not. The wireless unit is basically unusable for that, first because of the "Approved" / "follow labeling and instructions" rules which effectively disallow hacking the AHJ wouldn't approve. Also, saving the existing relay requires taking a derivative of the desired action (act *on changes*) and that adds useless complexity. Now you need a second CT and an Arduino and that just begs AHJ problems. – Harper - Reinstate Monica May 05 '21 at 17:43
  • I wasn't considering anything that involved an MCU of any kind at all. Just FYI. (Also, at least in the US, we are pretty much free to screw with things we own. I know at anything that oscillates, even if it is just a receiver with an IF, is regulated in European areas. Just not here, so much. So I'm probably projecting that aspect.) – jonk May 05 '21 at 18:12
  • @jonk you may be thinking about FCC regs. Your state adopted a version of the National Electrical Code. All versions have NEC 110.2 "equipment must be approved" and 110.3(B) "must be used according to instructions and labeling". This unfortunately *does not* give a free hand to do what you want in normally-permanent wiring. – Harper - Reinstate Monica May 05 '21 at 20:03
  • I'm thinking about NEC code. As I understand it, certified professionals performing work must use approved equipment for any changes with respect to mains. That includes installing new equipment or modifying existing equipment that is attached to the mains. I don't disagree with that. But the OP would ***not*** be doing that. All mains equipment remains unchanged. They simply modify a wireless device. How in the world does the NEC get involved in that? – jonk May 05 '21 at 21:08
  • @jonk because they need a means to sense current use on the various tools. The usual method is a CT, and that requires clamping 1 wire and not both, so that means putting it somewhere splitting wires is allowed. – Harper - Reinstate Monica May 05 '21 at 21:27
  • But if a split unit is clamped around a wire (and a lot of equipment I have seen [my table saw, for example] have accessible wires that are visible when you bend over and look), would that be a violation of code? Seriously? Keep in mind that the CT must be well-centered. So I should be able to do this in a completely non-disturbing and non-contact way. Now here, I am very curious about the answer. – jonk May 05 '21 at 21:33
  • @jonk But that's at the saw. Now you've got to get the "saw running" signal from the saw, to the switch. Without violating the UL White Book (appliance standards). I don't see how you avoid having low voltage wires in the same enclosure with AC mains wires, so we're back to that again. Can a CT module be built that will induct enough power to transmit on WiFi? – Harper - Reinstate Monica May 05 '21 at 22:24
  • Perhaps I see better now. I had assumed the COTS CT module also meets appropriate standards. But those standards, while appropriate for the COTS CT may not be applicable when placing that device into another device that meets different standards (NEC, for example.) The act of doing so creates a new situation that ... well... isn't approved. Assuming I got that, thanks! I am only sorry I was so slow about the uptake. My apologies. – jonk May 05 '21 at 22:28
  • @Harper-ReinstateMonica You bring up a good point. I'm less worried about "the authorities" than I am about breaking any rules that are there for good reason. The way I see it is that I'm not crossing over into the mains at all. Installing the new circuit breaker, receptacle and plug was a much bigger deal code-wise. With a self-powered current-sensing relay I could put everything into a small box that hangs off an insulated AC wire, like a clamp on meter. Maybe there's some technical violation, but with the insulation intact, I don't see any real danger here. – Douglas Krugman May 06 '21 at 18:39
  • @DouglasKrugman Some imagine there's a lot of airspace between "what expert EEs agree is necessary for safety" and "arbitrary government regs". They always feel like violating Code is OK because *their* violation is in that airspace. *That airspace does not exist; it's a cognitive bias*. The problem with clamping a wire is you need to clamp *one* wire. Clamping the whole cable won't work, and splitting the cable is a "rule there for good reason". – Harper - Reinstate Monica May 06 '21 at 20:00
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Now that I've got some of the jargon down I'm finding multiple answers.

I need something to do positive (rising) and negative (falling) edge detection and provide a timed pulse, in short a "monostable multivibrator".

This answer was particularly helpful: How can I have a rising edge close a relay for a fixed time?

as well as: Falling and Rasing Edge Detector and: How to generate edge-triggered pulse and: How to turn continuous signal into a short pulse using logic gates?

Douglas Krugman
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