I decided to try making pressure sensors for the bed, and my results have been disappointing so far. I got some generic car seat sensors and wired them across generic ZigBee contact sensor leads, and it works on the test bench!
The trouble comes with actually putting it in the bed.
Between the mattress and the foundation, it appears that the weight is distributed too well -- it doesn't trip the sensor unless I'm directly on top of it bearing down in one spot. Certainly not when I lay down to sleep.
On top of the mattress, it's too easy to feel through the sheets, especially the cord.
Between the foundation and the frame, it's triggered 100% of the time, even if the bed is unoccupied.
What have other people found to work? Is there a way to adjust the pressure sensitivity on these at all, or a different sensor I should try?
If your interest is in the build itself, this won't be helpful at all but I've found the Withings "Sleep Analyzers" work very well pretty much right out of the box. They're already calibrated to sense a person's weight if they're placed between the mattress and the box springs.
Honestly, I don't use the sleep tracker portion hardly at all, and just use them as presence sensors. I've got quite a number of rules tied to them in fact, and they're very responsive and consistent.
Interest isn't necessarily in the build itself -- I'd happily buy something off the shelf -- but ideally I have sensors for four people/ three beds. I don't know that I'm prepared to put $500 into this, which is the cost I'm seeing for two Withings two-packs.
I am still very happy with @ogiewon’s Hubduino bed presence solution using FSRs. All documented here:
I have a heavy foam Tempur-pedic mattress and it works fine there. I had to tweak the settings to get things right and also you will see a post where I added cardboard strips on either side of the FSR to prevent the mattress from always pushing down on it.
Eight sleep has a mattress topper that works very well for this and integrates with hubitat. Sometimes you can find their old topper only (no pod) on eBay for a good price. If you want heating, cooling with presence detection, go full splurge.
I may have found the simplest solution of all.... The car seat sensor folded in half under the mattress does seem to trigger when someone is in the bed. Assuming that can be secured and doesn't cause damage to the sensor in the long-term, I might have an answer. At least good enough to try in the other beds!
I realized tonight why this works, I think. The dots are acting as piezoresistors, changing resistance from high to low when compressed. They're wired in parallel, so that any one dot being squeezed will drop the resistance on the circuit, obviously.
Resistors in parallel behave like smaller resistors. So getting two dots to reduce their resistance part-way looks the same as getting one to reduce it all the way. Getting all eight dots slightly compressed at the same time is the same as one dot being compressed all the way. So the more I can get all the dots under where the pressure is, the more sensitive -- albeit smaller -- the detection area is.
That also means that you could expand the detection area by wiring a couple of these mats in parallel across a single sensor. It also wonder if you could make it less sensitive (if needed) by putting a small resistor in parallel with the mat, depending on exactly what the cutoff is for the contact sensor to declare itself tripped...?
You can use a variable resistor to figure out the exact resistance value(s) for
triggering sensor. I will not be surprised if values for on/off are different.
