Modification of Aqara Contact Sensor for external contact input

[JohnRob]

I wanted to have a simple contact sensor to monitor my furnace burner. I first looked at the Sage Doorbell sensor but it had no heartbeat or battery capability. I know there are other contact sensors available with external contact ability but I had the Aqara.

Its Zigbee
Its small ~1.5+ X 7/8 inches
Uses a CR1632 battery

Modifications:

My first concern was the possibility of EMC or static coming in on the external wires. However looking at the board the Reed Relay has a capacitor to ground then a 500K resistor into the processor. I figured the input is not bulletproof but should provide enough safety margin to not be an issue.

The wire I used is a #28 stranded with silicone insulation. this wire is extremely flexible and the insulation doesn't melt back when soldering.

The steps: (any questions please let me know).

[SmartHomePrimer]

That looks a little easier than both sides of the reed switch like I did. If you need to do more of this type in the future, the Mijia sensor are much easier to get open and put back together. No fiddly little floating reset button like the Aqara has.

[JohnRob]

The reason I chose the location near the "top" of the reed switch is the capacitor is located right there making it less likely to be troubled by EMI.

I still have another 1 or 2 Aqara's. I think I bought them for $11 from Banggood some time ago.

Thanks, I'll look at the Mijia if I find I need more.

UPDATE I just looked at the Mijia. It looks like the exact same board layout as the Aqara. If you chose to modify one I would solder to the same points as shown in the above modification. Also the #28 silicone is awesome, so limp it doesn't pull on your solder joints.

John

[JohnRob]

Important Update!

I would not recommend this modification.

After completing the modifications and checking basic functioning I attempted to deploy it. I found the signal was unreliable and would show closed then the wires were opened.
I performed more in depth testing, with the wires looped tightly around the sensor (to limit the antenna effect). It still was unreliable.

I removed the wires and preliminary testing shows it to be properly functioning.

I believe the issue is with the ultra low current supplied to the sensor. This sensor is normally closed in many door applications so the keep the battery life reasonable the current drawn by the inputs are very low and the impedance is very high. It seems almost any pickup from the wires will make an open contact look closed.

[scottgu3]

@JohnRob did you ever find a solution for this?

I'm looking for something that can sit in my Septic Alarm panel and alert me when the alarm (high level) goes off. As near as I can tell from the system wiring diagram, there are 3 floats, all connected to what appear to me to be NO contacts. I've got one of those Ecolink Window & Door sensors with the terminal in it for a contact connection, and I think that will work, but I saw this old thread and wondered if you had a better solution?

Scott

[JohnRob]

No, I made the modification on that device because I had one and figured I could find a use but the unit was not happy with external wires and I didn't want to spend the effort to filter them.

The real deal breaker was this unit had not heartbeat function and no battery or the like so it could loose connection and I would never know.

[scottgu3]

Bummer. Thanks!

Scott

[Plano_Harry]

@JohnRob if you can use z-wave, the EcoLink contact sensor already has screw terminals for an external switch.