# Help measuring central Vacuum low voltage circuit

1. May 20, 2010

I need Help measuring central Vac low voltage circuit. I would like to put a relay in line if possible to trigger when the unit powers on. I measured 17.78 DC volts when off and then 0 when vacuum powers on. Seems like an odd voltage for the contacts that start the unit when you insert a hose.

2. May 20, 2010

### Staff: Mentor

Welcome to the PF. Your post is a little confusing, so I'm hoping that you can clarify it a bit with some more information.

For example, when you use the term "Vac", do you mean AC Voltage, or something to do with a vacuum (vacuum pump, or vacuum cleaner?)?

Can you show us a sketch of the setup that you have, and how you want to modify it? And have you measured both DC and AC voltages with your vacuum (pump or cleaner?) both off and on?

3. May 20, 2010

This is for a central vacuum unit. I would like to be able to tell my home automation when the central vacuum unit is running. A relay or an optic isolator or some item/device that can change states when the vacuum unit is powered on. The simplest way I think is to use the low voltage wires that go throughout the house to all the inlets and when you insert a vacuum hose into the inlet/pipe, it shorts the two little contacts and the vacuum starts. If I could intercept that low voltage line running around the house, and run it through a relay it might work? Problem is what size and type of relay. I measured 17.78 DC volts across the two low voltage wires when the unit is off and then it goes to zero volts when the unit is running. The only schematic Nutone has just shows an internal relay that when shorted, starts the vacuum.
Hope that helps a bit.

4. May 20, 2010

### dlgoff

Can you explain what you mean by "my home automation"? There are a couple of way I can think of to detect your central vacuum unit motor running, but that depends on what kind of "input" this "home automation" needs.

5. May 20, 2010

### MATLABdude

For most central vacuums I'm familiar with, the signaling is done using 12V AC, though that's probably not universal (though oddly enough, the peak voltage would be around 16.9V--fairly close to what you measured--are you sure you weren't measuring peak voltage instead?) Complete the contacts at any point (each port is in parallel) and the vacuum cleaner turns on.

Now here's the thing: by shorting out the contacts, you should have next to no voltage difference between the two contacts at the vacuum cleaner! If you put in a 12V AC NC (normally closed) relay (I'm assuming it's AC, but check your measurement again, and the ratings of the relay--they can often accept higher than their nominal value, and sometimes work on both AC and DC), you might put enough current through it to turn on the vacuum cleaner!

If you're familiar with microcontrollers, you might have better luck with a Hall Effect sensor on the cord going to the central vacuum cleaner--you'd be able to determine when (significant) current was being drawn by the vacuum. I don't know if anybody makes a simple adjustable current detecting relay, but that'd be ideal.

6. May 20, 2010

### vk6kro

You can also get switches that have two gas inlets on them and switch when the pressure on one inlet is different to the pressure on the other inlet of the switch.
So, they can be used to detect vacuum as well as positive pressure.

There is some detail on one kind of switch in #2 entry of this thread: