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My house has a central vacuum system. It consists of a central suction motor, connected to multiple inlets throughout the house via a network of pipes.

Along with the pipes is a pair of low-voltage signal wires; when the homeowner plugs into one of the inlets, the wire pair is shorted together, and this activates the suction unit (via a relay within the unit).

I've experienced a malfunction where the signal wires get inadvertently shorted together, causing the suction unit to run indefinitely, putting a lot of wear on the motor and perhaps even presenting a fire hazard.

I thought it'd be amusing to create a little interface between the signal wire pair and the signal inputs to the suction unit, which will shut the unit down if it runs continuously for an unreasonable period of time.

Here's what I have so far. I solicit comments or suggestions for alternative circuits.

enter image description here

Here is the datasheet for the IC.

I've adapted the application shown on the top of page 23. In my circuit, switch SW (far left) represents the various contact pairs (at the various vacuum outlets) used to short the signal wire pair and activate the system. (sigH/sigL are to switch the system on and off in the simulation). Rcoil/Lcoil represent a relay that I will use to short the contacts to the power unit (that the signal wires normally connect directly to); the relay in the suction unit operates at 24 VAC, so this seems like the easiest way to interface my circuit with it. V1 is a 5 VDC wall wart.

Timing of the IC is shown here (from page 11 of the data sheet). (RSTbar is on_call and OUT is time_out). ...

enter image description here

Operation is as follows. When the signal wires are shorted, on_call goes hi, which de-asserts the timer reset (RST). Since time_out (OUT in the data sheet) is low, the PFET pulls high and the relay coil is energized. If user simply turns the thing off, the PFET remains turned-on, but on_call is low so the coil is not energized. If the thing runs too long (accidental short in signal wires), time_out eventually goes hi and turns off the PFET; so the coil is de-energized even though on_call is still high. (The additional components on page 23 are not necessary, since the source and drain of the PFET are constrained to be between GND and 5 VDC).

If something weird does happen with the low voltage wires, it's likely to be intermittent. I'm not sure how to handle that. Another timeout before the thing is allowed to turn on again could handle that. Simplest might be an RS flip-flop that traps a high-signal on time_out - keeping it high and illuminating an error LED, until the user pushes a reset button.

I realize there may be other ways to handle this problem, such as an over-temperature sensor on the motor, but I want to figure out how to do it this way.

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    \$\begingroup\$ I was surprised to see that this had no answer. It looks OK in principle. You could latch the system until manually reset. || A possible system is to have a series resistor AT the user connection socket, so that a valid connection is low esistance but above some crucial limit. A too-low short resistance indicates a fault and can be ignored and logged. \$\endgroup\$
    – Russell McMahon
    Apr 17, 2023 at 7:46
  • \$\begingroup\$ Did you build the original cct? How did it work? \$\endgroup\$
    – Russell McMahon
    Apr 17, 2023 at 7:46
  • \$\begingroup\$ No, I kinda dropped this for awhile. Did order the parts though, so some day when I'm bored ... \$\endgroup\$ Apr 18, 2023 at 15:21
  • \$\begingroup\$ Your analog solution is interesting though. Would require adding that resistor to every inlet, of course. Have to ponder what the circuit would look like. \$\endgroup\$ Apr 18, 2023 at 15:24
  • \$\begingroup\$ re " ... where the signal wires get inadvertently shorted together ..." --> This sounds like a REALLY bad design flaw. I assume (perhaps wrongly) that it's not in the wiring proper, but rather at a connection point of some sort. If so, addressing the design and/or construction flaw seems liable to be useful. \$\endgroup\$
    – Russell McMahon
    Apr 22, 2023 at 3:35

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