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Automatic Vacuum Toggle Review? Answered

I'm working on building an automatic vacuum toggle to use with power tools. I'm basing the design on what I found here https://www.instructables.com/id/DIY-Shop-Vac-Dust-Collector/

I have modified the base design to allow a little more flexibility and to make the switch box portable: https://wiki.jasig.org/display/~edalquist/Vacuum+Toggle

The part I'm most unsure about is the simple RC network that is acting as a time-delay for the big relay. I've done a bunch of reading but it seems like ever RC delay diagram I find is different. I have the big parts on their way and will be stopping by a local place for the small stuff. I'm planning on posting a detailed instructable once I get the thing working.


I'm not a fan of slow starting a relay like that. They don't like it, and there's a very vague chance it might not power up, since the R limits the available current to the relay. High current relays like good supplies.

You could do it with a transistor and the RC, but while better, its not a great solution - the relay doesn't get a good jolt to pull it in, but a gently increasing current that SHOULD make it close.

There are slightly more complicated but better ways yet to get the delay you need.


Thanks for the reply. I hadn't thought about the slow-close as a result of the RC circuit. I'll give the whole thing a try without the delay and see if the vacuum starting at the same time as the tool actually causes any problems.

Any pointers for things I should go read up on for those more complicated approaches?

Let me know if you want a quick diagram. You seem pretty well able to read a circuit diagram ?

A diagram would be great and in general I can read basic circuit diagrams. I'll go dig up my old EE for non EE majors book as well :)

Here are 2. The first SHOULD be reasonably snappy. The transistor is a Darlington type, so it has a very high gain. Once the voltage across the cap reaches ~0.6V the transistor should turn on. The values shown should give you around 0.7 seconds of delay.

The second is "done right" IMHO, with the CD40106 making things snap properly. Don't be scared of the chip, its pretty well bomb proof, and runs off your 12V DC rails. Connect +12 to pin 16 and ground to pin 8 !!


Awesome, I think I understand both of those. I'll add the transistor and CD40106 to my order and start having some fun on a breadboard.

Just make me a bit happier, and put a 470 Ohm resistor from the last gate to the base of the TIP32 ;-)

Ok, I think I have this figured out. Here is my circuit design based on what you drew up in the first example: https://www.circuitlab.com/circuit/eb4s47/relay-delay/

That appears to simulate correctly with about a 1 second on delay and a .3 second off delay for the vacuum.

As for the CD40106 circuit, I'm not quite as clear on it as I thought I was. I found http://www.newark.com/jsp/search/productdetail.jsp?id=60K5093&Ntt=60K5093 but I'm not clear how it wires up.

My understanding is I'd wire Vdd to my +12v rail and Vss to the ground. Would I then just wire the output of my RC circuit to A and then A` to the base of the transistor? If I'm reading the sheet right it looks like the CD40106 will trigger when A reaches about 7 volts.

All the parts are in the mail so I can try it out soon enough. Thanks again for the help!

Yes, connect pin 1 to your RC and pin 2 to the transistor. It might be the "wrong" logic - the relay stays on for a second and then goes off, in which case, connect pin 2 to pin 3 and the transistor to pin 4.