DIY Wired Contact Hack Using Visonic MCT-340 E

After looking, unsuccessfully, for an off-the-shelf Zigbee device that would sense and report the state of mechanical contacts---relay contacts, simple pushbuttons, etc. I decided to make my own from a Visonic MCT-340 E. It's a "supported,'" relatively low cost magnet sensor used for doors, windows and the like. 'Turns out it is also quite easy to hack. (See photos.)

  1. Remove the battery cover.
  2. Pry off the rest of the rear cover. (Use a small screwdriver or razor blade.)
  3. Remove the PCB, noting it's original orientation so that you can reassemble it.
  4. Notch out or drill the top and bottom covers so that wires can exit.
  5. Carefully solder really small wires (awg 22 or smaller) to the contacts for each end of the reed relay.
  6. Route the wires thru the hole you made and button things up.
  7. Power-up and add the sensor to your device list as usual if you haven't already done that.
  8. It will report "Open" until you connect the two external wires, at which point the status will change to "Closed."

The magnet can still be used if you leave the reed switch in place. You should be able to remove the reed switch at Step 5 if you like, tho I have not tried that.


Thanks for this! I had to bastardize my zigbee network with an Ecolink zwave in order to hack the Mighty Mule driveway sensor.

You can also do some other mods as well. I am linking a post from ST because there's a driver in there you can easily port over if you need other capability.

Yep, it's a good hack. This can be done with several others too, regardless of protocol. I'm using a Xiaomi Aqara with a standard doorbell button to give me remote notification, Alexa announcements, and event capture from my front door Wyze cam when the door bell is pressed. This gives me an extra 12 seconds of event capture when someone pushes the button, instead of just the 12 seconds when they walk up to the door.

Cool. That link (which mentions using these as water (flood) sensors, inspired the rain sensor below. Hazah!


Bingo! 10 cheap sensor plates on order.

But you will have to wipe the sensor plates after each rain shower, otherwise water will just sit on the plate, raining or not until it dries. You might try mounting two face to face with plastic washers separating them to channel the water between the plates, but allow enough room for the water to drain at a optimal rate.

I've used these little PCBs with some success before. Mounting them at an angle lets the water run off and/or evaporate pretty quickly. But yeah. The circuit might be closed for awhile after the rain stops, just like the "sponge" sensors used in sprinkler systems.

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They'll work OK in my house. I have a couple of always-supposed-to-be-dry drain pans I want to monitor.

BTW, we mostly talk about pressurized water leaks as the problem. I'm going with the LeakSmart valve for that. But for me, hvac condensate overflow also requires monitoring. I have a float switch to kill power to the tstat but I eventually want HE to intervene if any moisture is detected in the pan.

Does anyone know where the temperature sensor is located on this board? Im wanting to see if I can connect an external temperature probe so I can register the temperature inside a freezer.

Did you try it in the freezer? Might still transmit. If it does, then just add wires to move the battery outside the freezer or power it with a 3 volt adapter, or use a 5v to 3v step-down buck and a phone charger.

That's a different way to look at it! I'll try moving the battery out. Thanks for the tip!

I've successfully used three in my fridge for months. I positioned them (and their magnets) so that I get fridge temps and warning if a door is not shut tightly or has been open too long. If the temp in the refrigerator or freezer get too warm, Alexa lets us know. The hub sees the sensors reliably and is about 30' away, in another room.

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