Vacuum Dust Collection With Self-Powered Relays

Like many people with multiple woodworking tools, [Will Stone] wanted to create a centralized dust collection system. But he quickly found that the devil was in the details, as he struggled to find an economic way to automatically kick on the vacuum when one of the tools started up. His final solution might be one of the most elegant, and surely the cheapest, we’ve ever come across.

As with other DIY systems we’ve seen over the years, [Will] is using a simple inductive current sensor to detect when AC power is being drawn by one of his tools. But where the similarity stops is that there’s nothing so pedestrian as a microcontroller reading the output of the sensor. He realized that when the coils in the sensor were energized they were putting out about 7 volts AC, which should be more than enough to trigger a relay.

So he threw together a rectifier circuit on a piece of perfboard, using four LEDs in true hacker style. With the addition of a capacitor to smooth out the voltage, this little circuit is able to trip the 40 amp solid state relay controlling power to the vacuum using nothing more than the energy harvested from the sensor’s coil.

Using a current sensor is great when the tools are close enough to all be plugged into the same line, but that doesn’t help the folks with cordless tools or supersized shops. In that case, you might need to look into a sound-activated system.

22 thoughts on “Vacuum Dust Collection With Self-Powered Relays

    1. Under seven volts, though? They aren’t getting very bright, I doubt the current is very high. But maybe you’re right. A solenoid coil can make some serious spikes over time. I guess he’ll just have to cannibalize something else once they do!

      Also, this is one of those things where I’m glad they didn’t use a sledgehammer to smash a mosquito so to speak and use a microcontroller or.. eugh.. a pi. I think some people just enjoy the coding part and I get that, but certainly a lot of things like this need no electronic brains, only carbon-based brains.

        1. What LEDs have such low breakdown voltages? I have never seen one. Every one I have seen, used or tested has no significant reverse current (<20 uA) below 24V, and most are <10 uA at over 30V (these tested in the context of a Mims-inspired LED as color sensitive light detector). And they all survive that reverse voltage just fine. IR, red, green, blue doesn't matter. They're still really crappy as rectifiers, but they work.

  1. Years ago I did something similar with current sensing feeding a PLC input. Way before cheap microcontrollers were available. The logic controller did have one advantage over simply turning the dust collector on. I added a delay so that the dust collector would continue running for 10 seconds after the power tool was shut off to clear the sawdust from the lines.

  2. “which should be more than enough to trigger a relay.”. Not necessarily. Voltage is not everything. SSR’s do not require much input current due to the buffered input stage, but the coils on EMR’s usually do.

      1. Ummm a micro could come in handy if say you wanted to stop the shop cooler or toaster triggering the vac…. via differentiating the power drawn by the various tools (learning mode) other than that! I agree, simple is best.

  3. Or, if UL or CSA certification is important to you, you could just purchase an “auto-switching power strip”. You can have one in a day or two from your favorite Prime retailer for $20 or so.

    I’ve had one on my desk computers since 1990. Was more important when a 24″ CRT made a pretty good room heater, but still handy now. They were a dear $20 back then too, so I built my own, very similar to this one. It’s still working after continuous use for 30 years.

    1. An auto switching power strip would be okay for devices with fairly low current draw but a vacuum cleaner and something like a table saw would need to be on separate circuits as together they would draw more than 20 amps and trip a breaker

      1. If you’ve got severely limited power in the shop, like one 15A period, you could do the reverse of this and flip on an air compressor to a big tank when nothing else is running, then make a venturi vacuum setup (Or compressed air blower motor, what’s more efficient?) to extract when the tools are running.

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