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.

  • \$\begingroup\$ How long of a pulse do you require? Have you worked this out for your wireless that you expect to modify? \$\endgroup\$
    – jonk
    Commented May 5, 2021 at 4:27
  • \$\begingroup\$ 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. \$\endgroup\$
    – R Drast
    Commented May 5, 2021 at 11:31
  • \$\begingroup\$ @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. \$\endgroup\$ Commented May 6, 2021 at 17:56
  • \$\begingroup\$ @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 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. \$\endgroup\$
    – jonk
    Commented May 6, 2021 at 18:03
  • \$\begingroup\$ @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. \$\endgroup\$ Commented May 6, 2021 at 18:04

2 Answers 2


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.

  • \$\begingroup\$ 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. \$\endgroup\$
    – jonk
    Commented May 5, 2021 at 6:30
  • \$\begingroup\$ 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. \$\endgroup\$ Commented May 5, 2021 at 17:43
  • \$\begingroup\$ 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.) \$\endgroup\$
    – jonk
    Commented May 5, 2021 at 18:12
  • \$\begingroup\$ @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. \$\endgroup\$ Commented May 5, 2021 at 20:03
  • \$\begingroup\$ 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? \$\endgroup\$
    – jonk
    Commented May 5, 2021 at 21:08

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?


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