Can sensor show a strong signal but still be blocked?

My mailbox door contact sensor is a long way from the house and through a wall but the app still shows 3 bars of signal strength for the sensor. However, the sensor always shows the door is closed…whether it is open or closed. It never sends any notification when the door is tested. Can the signal show up as strong (3 bars) yet still be blocked in some other way or can I rule out the signal as my potential culprit?

In theory your signal could interfere with itself if there is something reflecting the radio signal. This is a pretty deep explanation of the effect multipath fading

What I would do is remove the sensor and magnet and then experiment with them beside the mailbox, then closer to the bridge. Also you could experiment with moving the cam that has the bridge by a meter or two side-to-side. It is possible that the bridge is in a spot where it gets the direct signal and a reflection of the signal that cancels it. That phenomena will be very localized - imagine a photograph of choppy waves in the ocean - the camera could be at a crest or a trough of wave interference. The wavelength of the signal from these sensors is about 35 centimeters if that helps you.

The root cause could be something much more stupid and simple. Make some experiments to divide and conquer. It sounds like you will be able to solve this.

It is possible that there is something else that is interfering at the 915MHz ISM frequency band used by the sensors. Read about ISM frequency use


Thanks PlinkerCraig, I’ll do some experimenting using your suggestions.

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Update: I moved the camera/bridge to a better spot with direct line of site to the mailbox but through a window. This did not improve things. So I tried replacing the original sensor with a spare. Interestingly, the spare sensor only shows one bar of signal strength BUT it works and is sending good notifications when the door is opened or closed. So I just swapped the two sensor heads. One side note, I did discover that when I unpackaged the spare sensor it would not send the 3-blinks when poked with the pin. When I took it apart I found the battery was not firmly locked into position…possibly it got jarred during shipping. Once the bat was pushed firmly in place everything worked to pair it up. Now I’m going to open up the sensor that wouldn’t work to see if I can discover anything amiss. So problem solved for the moment.


Nice work @PlinkerCraig!