I would like to add a second garage door motion shuft-off sensor for the safety of one of my cats that often finds himself crawling very very slowly under the half-open garage door to go in or out.

There is a sensor pair inside that should interrupt the garage door if he happens to be trying to go outside, but I'd like to add a second one outside.

I imagined the sensor would act as a simple switch that would interrupt power if the line of sight between the two sensors was interrupted, similar to a float switch, but after testing I realized this would not make sense since thee sensor needs continued power to be able to emit the light and detect the line of sight (LOS).

After testing, I noticed that the voltage is normally about 11.30V DC, but is stepped up to 12.60V when the line of sight is interrupted.

I tried testing wiring a second sensor pair in parallel, however, the "normal" voltage appears to be around 9V DC and the "interrupted LOS" voltage jumps to about 11V, more importantly only one of the sensor pairs appears to affect the voltage.

How can I wire these two sensors such that the garage door is interrupted if either sensor's line of sight is interrupted?

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    Rather than design your own, 1. get a schematic for the existing circuit. 2. See if placing an identical sensor in parallel is feasible, i.e., low emitter current (LED rather than incandescent) and whether sensor side opens or closes on beam break. If close on beam break, just parallel sensors. If open on break, can sensors work in series because voltage is split? Commented Jul 5 at 3:47
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    I interpret the problem being the sensor is such that it only detects the cat moving from the inside to the outside. If the cat is outside and strolling inside while the door is coming down then the beam might not be broken until it is too late. With a safety beam on both the outside and inside then the chances of a closing door harming a cat is much reduced. There's likely a pressure sensor on the door to reverse the motion if the closing door meets too much resistance, such as a car parked where the beam is beneath the body and between tires, but a cat would not produce enough resistance.
    – MacGuffin
    Commented Jul 5 at 13:12
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    Yes, that is correct, if the cat is coming in from outside, his body may be partly under the garage door and outside but nor yet inside enough to trigger the sensor inside, so I'd like to add another sensor pair outside that would detect this.
    – user84207
    Commented Jul 5 at 15:03
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    Your cat shouldn't be roaming the world freely anyway. They are not in a natural, competitive environment, and as a result they kill billions of songbirds annually, and many times that in other animals. Contain your pet, please.
    – isherwood
    Commented Jul 5 at 16:12
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    Another problem is that the state doesn't help. When I called animal control about my neighbors feeding cats leading to litter after litter of feral cat population explosion, trying to at least get help get them fixed, the agent on the phone simply told me that "in Florida cats area allowed to roam free", so there was nothing they could do, they would not even help with TNR. It's very limiting what I as an individual can do when the entire neighborhood and the state do not care.
    – user84207
    Commented Jul 5 at 17:35

1 Answer 1


Annoyingly the standard garage door sensors aren't anything simple like a voltage level. Instead they use a pulse train to signal that they are present and the beam is clear. I suspect this is done to keep people from just adding a jumper wire and not installing the sensors, because if it was that simple to bypass them, idiots would do it.

You do get a bonus from this, however. If you short out one of the existing sensors, or disconnect both sensors, the garage door should reverse as it will no longer have sight to the sensors. You can take advantage of this to add a second sensor, so long as it's a sensor which drives a relay rather than one which is designed for use with garage doors.

This comes up occasionally because people don't want the garage door moving if the hatch is open on their van or SUV. If you can add a second sensor for that you reduce the risk. Of course it doesn't work to just add another pair in series or in parallel.

  • I imagine that something like this would probably work? "Photoelectric Switch Sensor,1 Pair M18 Through-Beam Reflection Optical Photoelectric Beam Sensor,NPN NO (Normally Open) 6-36VDC Photoelectric Switch Sensor" amazon.com/…
    – user84207
    Commented Jul 5 at 15:00
  • Possibly. Beam sensors like this outside are not always reliable. Better would probably be to keep your cat as inside-only if you are worried about them getting hurt - there's lots of reasons cats can get hurt outdoors besides just garage doors.
    – KMJ
    Commented Jul 5 at 16:59
  • Unfortunately the cat is adopted and has spent all of his life outdoors and it's probably not possible to re-wire him to stay indoors only, added more details in response to comments in the original question. I'm mostly worried about not being me the one to hurt the cat especially since I have automations that may close the garage door without a human being present.
    – user84207
    Commented Jul 5 at 17:44
  • If I were in that circumstance, where I had the health of the animal as a primary concern without the ability to eliminate the risky behavior, I would cease use of the automation.
    – KMJ
    Commented Jul 5 at 23:03
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    No witch-hunt intended; I think you have the pieces to give you a chance to making some version of this happen. The comment was made with the hope that it lets you consider this as a possible XY problem.
    – KMJ
    Commented Jul 5 at 23:31

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