> From: Russell McMahon[SMTP:apptech@PARADISE.NET.NZ] > Sent: Tuesday, January 06, 2004 10:16 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: [EE]: Re: [AR] data acquisition- instrument amplifier help >> It seems that a "naked" strain gauge or pressure transducer will run $50 - >> $100 when purchased as such, out of an electronics catalog. Meanwhile, I'm >> looking at my digital bathroom scale that I paid $25 for, and thinking it probably >> has a strain gauge in it (and hopefully accessible leads or traces where I >> can siphon the raw voltage signal). Also, possessing a usable "midsize motor" >> load range right out of the box. >> Any of you folks ever played with this el cheapo >> consumer-technology-crossover possibility? ... > Bathroom scales that I am aware of include > > - Single strain gauge on each of 4 points, each on a short stubby beam. > Between them they may make a full bridge of sorts > - Single strain gauge on two long metal arms running front-back on each > side. An inferior solution that may make a half bridge > - Some scales still use a mechanical mechanism with a rotating optical > encoder disk. Essentially useless. _______ > > The two kitchen scales that I have dismantled both use a true load cell and > are much closer to the sort of starting point one would want. Nicely enough > built single point parallelogram unit in each case. Both out of China with > no manufacturers name on them. The cheaper of the two (a 2 kg unit) had a > 5th strain gauge at right angles on a thicker part of the web to allow > temperature compensation. Very impressive for a unit that I bought complete > for about $US15 on special (usually about $US30 ish). Very nasty electronics > construction and assembly though. > The other is a 5 kg unit ($US40 usual price) with a full 4 strain-gauge load > cell and a single PCB for all electronics. Much better designed and > assembled. Temperature compensation probably provided by processor doing its > own local temperature measurements. > Both scales are able to resolve steps of 2g in a 2kg or 5 kg range. I have > plotted accuracy and linearity but haven't got them immediately to hand. > Accuracies of say 0.1% should be easily achieved. > Both scales use a single LM324 quad opamp to make a strain gauge amplifier > and voltage to frequency converter for the processor to read. Very cheap and > nasty approach that is evidently good enough for the target market. > I'd say that for most rocketry applications that one of these load cells > with some form of force divider would provide a very acceptable solution. > Russell McMahon I have a bathroom scale that doesn't use a load cell at all. It transfers the weight of the platform into tension in a vibrating wire. The chip drives the wire and measures its resonant frequency. You can hear a low frequency tone from the scale, which changes in frequency when you step on the platform. Conversion of the frequency to a digital output is trivial. The manufacturer is General Electric. John Power -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads