Monday, May 31, 2021

Filtering out distant Bluetooth signals

 TL/DR: nearby Bluetooth devices have a RawSignalStrengthInDbm in the 50s and 60s.

I love playing with Bluetooth devices and writing little apps to control them (including the very special Gopher of Things). One of the hassles with developing, though, is that we're in a sea of Bluetooth devices. Any "watcher" code you write will be inundated with events from everyone else's device (notably their Apple devices which helpfully send lots of Bluetooth advertisements)

So how to filter them out? Step 1 is to look at the RawSignalStrengthInDbm in your Bluetooth watcher's BluetoothLEAdvertisementReceivedEventArgs argument. I did a little experiment: all of the devices I was interested in coding for had a signal strength in the 50's and 60's. Everything in the 80's and higher was noise from the rest of the house.

Note, though, that the strength is in decibels. A strong signal is -50 and a weak signal is 89. To quickly return when the signal strength is too low, do this:

    const int filterLevel = -75;
    if (args.RawSignalStrengthInDBm < filterLevel)

In my test, this filters out most of the undesired signals.