I'm planning to build a bed frame and I want to incorporate a load sensor in each of the 4 legs. That will let me know when the bed is occupied, which can be interesting. Plus maybe I can track body weight automatically.
I need to choose the load cells. Do you have any favorite models that would work well for this?
I'm not sure how heavy the bed frame + box spring + mattress will be, but I'm thinking four 200kg load cells is probably sufficient.
I find load cells come in disc, bar, and S types. It seems like the disc type would be easiest to mount, but I could be wrong.
That's a really tiny thing! Maybe I should look for a larger one?
I understand the weight needs to be on the little button at the top. That seems a little tricky. I guess the load cell would be mounted to a base, then the bed leg would have a recess for the load cell, the top of which is probably steel to make contact with the load cell protrusion. Does that seem reasonable?
Just looking at it… I don’t thing you need to depend the weight on that little protrusion. It is there no doubt as a guide, and the load can be spread over the entire top.
In principal, yes. Check the weight distribution on the legs for the unpopulated bed. Mechanisms for raising/lowering the bed, or tilt of the head section, might affect the choice of sensor. Also pay attention to the situation when a heavy person sits down at one end of the bed before laying down.
I've been repairing more then one broken bed.....
I think uneven distribution of the bed frame would be handled by zeroing it with no one on the bed (tare). I'll build the bed frame, there's no mechanisms. I'm not sure if I'll need a center post, but I'd like to avoid that. The tiny 200kg sensor shown above has max safe rating of 120% and absolute max of 150%, so is likely OK for the bed weight + a heavy person plopping on it. I'm more likely to end up using the load cells as presence sensors (distinguishing 1 or 2 people), so it won't be a total loss if it ends up inaccurate for body weight.
I'm still not sure if that is the right sensor, or if there's a better fit out there.
My fear was that one sensor could get overloaded, damaged and then giving bad data. As You describe the project it's not happening.
For old people You could use sensors like 50, maybe 100 kg.
For children You need to have respect for the effect of high jumps down into the bed. The shock, short time high load.
Hi @DaveEvans, it's-a-me, the FSR LED strip stairs guy! You're right FSR installation is a lot simpler. I'd need to completely give up on body weight measurements. I haven't decided on that yet.
Tekscan A201 drift is "< 5% per logarithmic time scale". I don't know what that means, but I need presence detection to continue working even when the weight (my ass ) has been on the sensor for 8+ hours. That way I know when everyone is out of bed in the morning.
I setup an A201 sensor just now using the inverting op amp circuit. I placed a weight on it and got a reading of 205 out of 255. After 8 minutes it reads 227, 12 minutes 229, 16 minutes 232. Going up is better than going down for presence detection, though I wonder if it will stop going up? Is it bad for the FSR to have a constant load for long periods?
I used a Tekscan FSR to help me alert the assisted living staff when my blind elderly and not-all-there mother would get out of bed in the middle of the night and start wandering around her apartment. It was in operation for several years and worked fine.
FWIW, 1 hour and it's at 241. I ended the experiment, took the weight off, then placed it back on right away: it then read 193.
Thanks for confirming it's sure to work (though I know first hand that situation is a bummer, sorry).
The bed frame will be quite heavy. FSRs can handle thousands of pounds, so I guess that's not a problem, I just need to figure out the right configuration (voltage, resistance). Did your setup have the bed's weight on the sensor all the time?
Hmm, maybe I can put it under the box spring instead of under a leg. That's hard to plan for, so if I go for FSRs then I guess I'd need to just build the bed frame and see what's best.
@DaveEvans Stair project is all ready to go, the MCU part at least. Now just need the stairs, and the house they go in! Actually the framing is done and the roof is on. I've just done an update in the stairs thread.
For the bed sensors project, I've now received four Tekscan A401 25mm/1" force sensors. I hate Tekscan so much, the bastards charge waaay too much for shipping and don't bother to ship for a week plus after placing an order.
Anyway, these are good up to 7000lbs/3175kg each, so I don't need to worry too much about cake. I need to load one up with a couple hundred pounds and see what voltage and resistance gives a reasonable result. Even then I won't know how much weight the bed frame will put on the sensors until it's built, so there's probably a lot of adjustment needed.
You're right FSR installation is a lot simpler. I'd need to completely give up on body weight measurements. I haven't decided on that yet.
) has been on the sensor for 8+ hours. That way I know when everyone is out of bed in the morning.
