My problem with similar sensors is that they don't work very well if you're sitting still around 3m away. Maybe my sensor isn't great, but when I'm sitting still at the computer, it frequently thinks I'm not there any more, which ruins any benefit.

There are some that detect heartbeat, I guess those might work. But those are mmWave, not just cheap 2.4GHz radars or PIR.

I've got LD2410B, which are mmWave, so they should detect heartbeat (and do), but not from more than 2-3m away.

Is the LD2410B sensing range limited by the PCB antenna?

I'm not sure, it always just made sense to me that the farther away someone is, the harder it'll be to detect their small movements.

Smallness detection depends on RF: higher frequency = shorter wavelength = more resolution for imaging.

Detection range without obstacles depends on transmit power and signal reception.

There are other models in the same family which might have more power/range.

A hack is to put a sensor on the chair

I solved the issue by linking my lights to a switch that I turn on when needed. It has worked very well so far, with no false detection at all!

plz let us know how it goes after sufficient testi g

Hmmmmm that's not a bad idea, I'll have to see how to do that, though. I don't want cables restricting me.

If your fine with zigbee, I could see having one of these aqara door sensors in the cushion of the seat, triggering to closed when you sit on it: https://cloudfree.shop/product/aqara-door-and-window-sensors...

Seems like it'd be finicky though.

Zigbee is great, but my chair has a mesh :/ I'll see if I can print something below, so that the sensor is activated, but yes, it does seem that it would be pretty finicky...

You can buy the sensors they use in car seats for like $10 https://a.co/d/0dibAFxc

Ceiling sensor is another option.

For now I've put a mmWave sensor under my desk, which works fairly OK. It's not perfect, but definitely good enough.

Pairing them with PIR detection should help to also detect stationary warm objects.

It's difficult to find good PIR sensors though. The commonly found cheap modules don't include a thermistor to compensate for room temperature. Finding one with analog output or continuous detection is even more difficult, most detect changes.

I did pair them, unfortunately PIR sensors are even worse at detecting stationary objects (less sensitive to body micromovements).