Cheaply monitoring 20+ doors

I am setting up refrigerator and freezer monitoring for a food pantry. They have more than 10 refrigerators and freezers and people sometimes leave a door open or the power goes out leading to a huge amount of spoiled food. I already added temperature monitoring using ESPHome (on a Olimex ESP32-POE-ISO board) and a whole bunch of Inkbird IBS-TH2 sensors - seems to be working very well.

I am now looking to add door monitoring to this setup. The fridges/freezers could have 1, 2, or 3 doors each, totaling more than 20 doors. I am looking for a cost effective and unobtrusive way to monitor the door position (without drilling holes that would void the warranties on the newest units).

For example, I found Aqara sensors, but at $14-$18 each, that gets expensive very quickly (plus needing a Zigbee2MQTT interface). I also considered adding old-fashioned burglar alarm-style reed switches, but that means getting wires to each door.

My Home Assistant installation itself is a virtual machine and is very far from the food area, so I can’t connect any hardware (i.e. ZigBee radio) directly to it. I also prefer PoE powered devices as much as possible rather than separate power supplies since they have a robust PoE infrastructure and not many outlets.

Any recommendations?

You will want a ZigStar LAN Coordinator or SLZB-06M to extend via POE to the areas where you need to setup new zigbee based devices where your main network cannot reach if you don’t use router devices like smart plugs to extend it, else you will be investing in wifi based sensors.

The simplest solution once you setup your coordinators/router devices for each area where you need them to be is using the door/window sensors, they all come with doublesided tape already attached so you can stick them to what you need to monitor.

Can’t these just have a reed switch also?

I also considered adding old-fashioned burglar alarm-style reed switches, but that means getting wires to each door.

They don’t want to run the extra wires to each door they want to monitor.

How are the doors situated?

Asking because one of the methods might be not using physical sensors, but … video. One camera may be able to cover more than one door, or all.

Depending on how the doors are situated, inexpensive video camera(s) feeding Frigate could be trained to alarm on open doors after certain amount of time.

They are in two different rooms, some against different walls and some (chest freezers) as islands/work tables in the middle. Some of the doors are swinging and some are sliding. The sliding doors are sometimes only open half an inch, but that could be enough to cause problems.
The food pantry patrons are also uncomfortable with there being a camera in the room - many of them don’t want people to know they are receiving charity.
Putting that all together, I don’t think cameras are a good solution for this specific issue.

Not just that we don’t want to run wire - it would be a major pain and would probably be really ugly since we don’t want to drill any holes.

Hence my initial reply suggestion, will be the most straight forward setup for you to get going until you need something more.

If the doors on each fridge are closed enough to each other when shut (I’m assuming so since this is a fridge), then a single carefully placed door sensor would be enough to cover 2 doors.

You put the sensor itself on one door, and its magnet on the other. With some experimenting, you’ll be able to find the right distance between magnet and sensor so that it just registers as closed when the door is open. Opening the doors even a little bit would reliably mark the sensor as opened.

Depends where you’re looking for the sensors. Amazon tends to be more expensive. A 2-minute search unearthed a 6-pack for around $10 each or a 10-pack for less than $9. You could potentially get them for less if you contact the seller and ask for bulk discounts.

I have 2 * 600ltr upright freezers in my garage and have tried a number of solutions to monitor them.

I tried reading temperature off the front of them with a camera, feeding the image into Seven Segments OCR. It worked-ish but not reliably enough to be confident with it.

Each door has a Samsung Multi-sensor on it to detect if it is open however they are too expensive to scale to your needs. I would suggest looking at the Aqara contact sensors but buy them from the Aqara store on Ali-Express as they will cost ~£4 (5usd) each from there.

I also have each freezer on a power monitoring socket which can give you a good indicator if something is wrong. It pointed me to a failed fan on the heat exchanger on one of the freezers.

Lastly I have a Yolink Temp / Humidity sensor in each freezer. These reliably will broadcast out of the freezer due to using LoRa. I initially used Govee 5075 Temp / Humidity monitors but the BLE only broadcast ~ 10ft from the freezers.

Just a few ideas.


If there’s already an ESP32 board at each unit, the cheapest method is to get wired magnetic reed sensors hot-glued or command-strip’d in place; these are under $1 each on aliexpress.

If you want wireless sensors, consider using 433Mhz “transmit-only” sensors which can be found for under $2.50 each in bulk. You would need to add a receiver to your network within a few hundred feet of the sensors, such as the Sonoff RF bridge (which can be flashed with ESPhome).

Xiaomi makes a bluetooth door sensor that I see on aliexpress at 2-for-$7 but I’m not sure whether these work with HA as easily as their hygrometers; more research would be required. But at least your ESP32 boards could serve as BLE relays.

Personally I use Ring Z-wave sensors which I nab when they go on sale for 2-for-$17 refurbished, but I don’t see any on sale right now. I prefer Z-wave to Zigbee any day of the week for reliability.

PS - @ShadowFist I’m seeing that Aqara 10-pack on aliexpress is $80. They like to pull tricks with their images not matching the selected quantity.

Meh, I left out the word “each” from the last part. Obviously no one in their right mind would sell 10 for $9 in total. You’ll barely be able to buy the included batteries at that price.

I will try this. I am already using ESPHome to listen to the BLE updates broadcast from the Inkbird sensors - I will see if there is a receiver I can add to the Olimex board for 433.

Any reason not to use a single ESP32 for both BLE and 433? Since they are all broadcast devices, I don’t see any documentation about limits.