I'm starting to get into woodworking and I've run across some high-end dust collectors and shop vacuums that turn on automatically a second after the tool plugged into them (such as a table saw) is turned on. When the tool is turned off, the vacuum stays on for a few seconds longer, then shuts off automatically. There are also aftermarket switches like the iVac that do the same thing.

It seems like I would need a normally-open 120V relay to turn on the the shop vac, but how do I delay turning on the vacuum, then make it stay on for a few seconds longer after I turn off the tool?

  • \$\begingroup\$ leaving it on for an extra second may either make the electronics simpler or may be to suck up some extra dust or both. \$\endgroup\$ – Kellenjb Jan 20 '12 at 0:16
  • 1
    \$\begingroup\$ I'm not sure about it making the electronics simpler, but the purpose of the first delay (when turning on) is to separate the startup surges as the motors turn on, in case the tool and vac are on the same circuit. The purpose of the second delay (when tool is turned off) is to suck up some extra dust, as you suggested. \$\endgroup\$ – rob Jan 20 '12 at 21:26
  • \$\begingroup\$ I understand the value of turning them on at different times. The reason I say it might make the electronics simpler is if the timing is created from charging and discharging a capacitor, it might be simpler to let the capacitor discharge at the same rate that it charged. \$\endgroup\$ – Kellenjb Jan 20 '12 at 22:24
  • \$\begingroup\$ Okay, that makes sense! \$\endgroup\$ – rob Jan 25 '12 at 1:05

Electrically it's easy.
But, doing it for less that the $45 iVac would be hard.
Cost your labour at $0.10/hour and no problem. Parts cost of electronics is modest.

Here is a word picture of how you might do it.
I won't add a circuit diagram as I don't really think this is advisable given what is available.

This operates at say about 12 VDC. Whole thing could be done at mains voltage. Usues relays. Could be wholly sold state.

Mains to 10 VAC or so transformer. Supplied from saw power feed.

Rectify 10 VAC to DC and regulate to say 12V DC.

Provide "mains on" signal for one diode connected to 10 VAC upstream of main rectifier.

When 12 VAC appears a capacitor is charged via a diode and a resistor R1from "mains-on". Charge time ~~= R1 x C.

When "mains-on" disappears the capacitor is discharged via a second diode and a second resistor R2. Charge time ~~= R2 x C.

Adjust R1 and R2 to suit desired times.

Drive a MOSFET gate from capacitor so it is turned on ~R1C after mains-on and off R2C after mainson disappears.

Operate relay for vacuum from MOSFET.
relay is 12 VDC and 12 VDC supply must hold up long enough to keep relay held in for vacuum to post operate. There are other ways.


  • \$\begingroup\$ Your RC time delay mechanism is interesting. I would have done it with a cheap microcontroller and a current sensor. \$\endgroup\$ – Kevin Vermeer Jan 20 '12 at 22:21
  • \$\begingroup\$ @KevinVermeer - Yes, I would also personally lean towaurds a uC solution - cheaper faster better- choose any 3. BUT if someone not set up to do that then the learrning curve is immense and a hardwired solution is accessible to most people. uC's must come - but a project and life experience in their own right. [I used my 1st microcontroller 38+ years ago FWIW :-) ]. \$\endgroup\$ – Russell McMahon Jan 21 '12 at 2:34
  • \$\begingroup\$ Almost - needed 2 ICs minimum - program memory + processor. Extra RAM a good idea. Thus agh!!! :-) \$\endgroup\$ – Russell McMahon Jan 21 '12 at 2:37
  • \$\begingroup\$ Awesome! Do you have a rough idea in dollars what the parts cost would be, off the top of your head? I'm interested in building this as a learning exercise, but I'll probably either build at least 2 of them for different locations in the garage and basement, or might build one and buy an iVac too if the parts cost is within $10 or so. \$\endgroup\$ – rob Jan 25 '12 at 1:17
  • \$\begingroup\$ Rob - Probably $10 - $20 . Ask me again in a few days if I don'y expand on this. Rushing ... . Mains transformer to say 6VAC or so. (Rectify to make 8V DC (DC ~= 1.4 x VAC) Regulator to run electronics if uConroller used. Maybe none if discrete. Relay . 1 x CD41016/74XX14 IC. Few caps and resistors. Perf board. 1 transistor to drive relay. | if using uController then just IC + transistor plus transformer and rectifier and a few caps and maybe R or 2. Arduino would do this with ease. Or BASIC stamp. Or anything with processor. As @shawn suggests - you can but timer modules too. \$\endgroup\$ – Russell McMahon Jan 25 '12 at 12:01

The on and off delays are control circuit issues-do you know any electricians or technicians who work on industrial equipment that runs via more complex control circuitry-like a canning works or some "plant" that manufactures something (usually the whole system for those production lines used to run on 24 VAC) You might get some old time delay relays from them just for the asking. The tricky problem of sensing the turn on, specifically from multiple loads on the same breaker, makes me wonder if there is any current sensing signal available for hacking the problem in a Ground Fault Interrupter plug (or panel circuit breaker) that could be used to sense the current draw of the whole branch at the time of turn-on. But hacking a circuit designed to run on un-protected AC is an invitation to risk-taking.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.