Hacking a Z-Wave Door Sensor Into a Mailbox Sensor for Home Assistant

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My mailbox - yes, my physical mailbox where I receive actual mail - is one of the things that has stubbornly resisted my attempts to automate it. I’ve tried a few different solutions. Third party proprietary chimes. A Z-Wave tilt sensor on the door. But nothing has worked long-term.

The two big problems are:

  • It’s a metal mailbox, so it’s a giant faraday cage when it comes to radio waves. You can’t actually have anything inside it.

  • Anything on the outside has to be subtle enough to not attract unwanted attention from neighborhood Karens with too much free time.

I first tried using a tilt sensor with a wire attached to it for the antenna. This sort of worked, but was very temperamental. Any storm would result in me receiving hundreds of push notifications from the tilt contacting and, eventually, it stopped working entirely.

I also thought about trying to use a standard door sensor. These have a magnetic reed switch inside them that contacts when a magnet gets near them. But they either have to be inside the mailbox (again, faraday cage) or outside where they are far too noticeable.

But then I thought, what if I modify an existing small Z-Wave door sensor to use an external contact switch, like the type you usually see in home security systems? That way the contact switch could be inside the mailbox, and the sensor outside. And it’s small enough to be unnoticeable unless you are specifically looking for it.

Hacking a Door Sensor

The sensor I picked was one I had floating around in a box. It’s a ZD2102-5 sensor sold by Vision Security. There are tons of these type sensors available online, so these same principles should apply to any of them. Once you take the cover off, you are presented with a battery and a very small PCB. The reed switch is easily found: it usually looks like a 1cm long glass tube with a couple of pieces of metal inside it.

The magnetic reed switch after it has been removed.
The magnetic reed switch after it has been removed.

Magnetic reed switches come in two varieties, normally open and normally closed. The adhere, obviously, to whether the circuit is open and closed when the switch isn’t triggered. For these Z-Wave door sensors they are normally open, so you will want to buy the normally open contact sensors. These are the ones I used.

Start by removing the battery and the board from the enclosure. Be careful, on mine, there was a small, barely noticeable screw that needed to be removed to free the board. Then, drill a small hole through the top of the sensor enclosure. This will allow our wires to leave the enclosure, so go ahead and feed the wires through the hole.

Using a soldering iron, I carefully desoldered the reed switch from the PCB. Using a pair of tweezers and helping hands is very helpful here. I used the hands to hold the PCB and very gently applied the soldering iron to the back of the board, just until the solder loosened enough to gently tug the reed switch off. I did it one leg at a time.

Then, very gently, I heated the remaining solder on the back of the PCB and applied the wire from the sensor until it popped through the hole. Removing the iron will cause the solder to almost instantly harden, so this takes a little finesse. I did this for both sides of the contact. It would also be a good idea to put a small drop or caulk on the hole to keep any water from getting inside the enclosure.

Then, I reassembled the board back into the enclosure and put the battery back in. The light came on, which I interpreted to be good sign that I hadn’t killed it! From there, I paired it with Home Assistant like any other sensor and, what would you know, it actually worked!

One thing I noticed is that the effective sensor distance that would trigger the magnetic sensor was on the matter of a few millimeters. Not more than one centimeter. But using the stock magnet that came with the sensor resulted in a much larger effective distance, on the order of 3-4 centimeters. I decided to use that instead.

Finally, I used some double-sided tape to tape the sensors inside the mailbox, and the enclosure outside the mailbox. I put the enclosure on the underside of the mailbox, where it is shielded from both most of the weather and from prying eyes.

The invisible mailbox sensor.
The invisible mailbox sensor.
Inside.
Inside.
Underneath, showing the sensor enclosure nestled up under the mailbox.
Underneath, showing the sensor enclosure nestled up under the mailbox.

So far this has worked brilliantly. From here you can use Home Assistant to make any kind of automation you want. I scared the hell out of my wife by having it play the old AOL “You’ve Got Mail!” sound over Sonos, but she didn’t like that so right now it just sends a push notification to our phones. This way we don’t have to walk outside to the mailbox to check it.

Push notification sent by arriving mail.
Push notification sent by arriving mail.
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