home-automationmotion-sensorsafetySecurity
I bought a 4 level house that had an ADT security system with a motion sensor in every room. I never used it since moving in.
But I have a fun project in mind: I'd like to drive my kids nuts when playing hide-and-seek.
Is there a relatively easy and inexpensive solution to reusing the motion sensors. It would be sufficient to have an app (or a webapp) that gives me a flat list of the most recent motion-detector "hits". Any suggestions?
You can probably replace the switches with those sensors, but you're going to have to do some creative wiring to work around the fact that they're not 3-way switches, but the switches you're replacing with them are. You'll likely end up with the light being on whenever either of the switches is on. You also might run into trouble if they ran the 3-way switches as a switch loop (no neutral in box), because "smart switches" like these require a neutral so they can use power for themselves to decide when to switch on and off.
Here's what the difference between these wiring schemes looks like if you draw it out:
Also note that those occupancy sensors still have a hard-on switch, so your tenants can still just leave the lights on. You might want to look instead at vacancy sensors. (An example vacancy sensor is Cooper Wiring Devices K01-400RW-L, but note that it does not handle CFLs, just incandescents.) These, you tap to turn them on, and they turn themselves off automatically when everyone's gone. That sounds more like what you want.
The how for switch #1 depends again on how they wired your #1 switches and the light they control together. If you diagram out the connections, it becomes straightforward. Until you have enough info to diagram it out, you don't have enough info to rewire it, either.
As an aside, all the exterior stairwell lights I've seen in apartment complexes were on timers. They'd come on around dark and flip off in the morning. I knew they were timers rather than photosensors because a couple times a year they'd end up slightly out of sync with actual daylight hours, but if these are in fact exterior lights, putting them on a photosensor and disabling the switches entirely might be an option.
A 2-gang box simply holds 2 single-gang devices. You need the single-gang device, the hard part will be finding the correct faceplate to use.
Best Answer
First we must determine whether your motion sensors are wired or wireless.
Wired
The wired PIR motion sensors used in security systems since roughly forever are pretty simple devices. You feed them 12 volts dc to power the electronics; they contain a relay which opens or closes to indicate motion. Some will have a SPDT relay so that the installer can choose whether to use the normally open or normally closed contact. "Normally" means the inactive or resting state of the device, ie no motion detected.
If your ADT system is wired there'll be a control panel buried in a closet or utility room somewhere in the house. Open it up and you'd most likely find a 4-conductor cable for each motion sensor. If the motion sensors are grouped into zones then it's possible you'd have one cable for each zone; each sensor might not have its own "home run" cable back to the panel.
All these wired sensors could be fed into some kind of multi-input interface. That might be built around an Arduino, a Raspberry Pi, or some other experimenter/hobbyist platform.
Wireless
If your motion sensors are wireless then things aren't necessarily harder, just different. First you'll have to figure out what RF frequency and protocol they use. At very least they should be labelled with an FCC ID number which reveals the manufacturer and model of the device, and from there the rest can be discovered. They might also simply be labelled such that a person familiar with the industry would recognize them. GE/Interlogix is one particular make that's quite prolific. Each sensor has a unique serial number which it periodically transmits along with its battery state, contact state, and other info. Receivers are available that can listen for these sensor transmissions and send it over a serial bus like RS-232.
Pulling it together
Identifying what sort of sensors you're working with is the easy part. From here you're probably going to be looking for some kind of open source home automation platform such as openHAB, Home Assistant, etc. These may provide a software framework for receiving the security sensor inputs and putting them into a web or mobile app front end you can use. This all goes beyond the scope of the DIY stack exchange, but hopefully I've given enough ideas and key words to help your research get started.