Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!uunet!peregrine!ccicpg!cci632!rit!tropix!ur-valhalla!uhura.cc.rochester.edu!rochester!udel!gatech!bloom-beacon!apple!agate!ucbvax!hplabs!hpl-opus!hpnmdla!hpsad!frankb From: frankb@hpsad.HP.COM (Frank Ball) Newsgroups: sci.electronics Subject: Re: How to convert weight to volts? Message-ID: <1840007@hpsad.HP.COM> Date: 15 Aug 89 15:22:44 GMT References: <7903@etana.tut.fi> Organization: HP Signal Analysis Division - Rohnert Park, CA Lines: 36 *One of the more imaginative solutions I have seen is a project in *Elektor where they used a loudspeaker. The perimeter of the cone was *cut away. A platform was then glued onto the cone. A photoelectric *sensor was attached. The circuit was an op-amp which pumped enough *current through the coil to clear the photoelectric sensor (feedback *loop). The current was then a measure of the weight on the platform. *A DVM chip and display completed the project. Unfortunately this *balance only did up to 2 kg. Maybe you can get a monster loudspeaker *from a rock group and increase the range. :-) In school I had to build an electronic scale for a project. I suspended the weight from a solinoid. There was a light and a photo transistor to sense the position of the solinoid. The photo transistor was part of a feedback loop with an op amp and drive transistor that varied the current in the solinoid to hold the weight at the same height every time. A volt meter was attached to the solinoid to measure the voltage across it. A lookup table was emperically derived to convert from voltage to weight. It wasn't linear, but it worked and it cost <$20. Most of the class used strain gauges. I don't know where they got them, they aren't cheap. A friend had an interesting scale: The weight was placed on the end of a horizontal bar about 3" long and 1/2" square. The bar was firmly attached to a base at the other end. A phonogragh cartrige was mounted under the bar, near the middle, with the stylus just touching the bar. The output of the cartridge was connected to a FET input op amp and measured the deflection of the bar when the weight was put onto it. I don't remember the electronics side of it, I guess he must have integrated the output to get a voltage proportional to the weight. Frank Ball 2LR-O frankb@hpsad.HP.COM Hewlett Packard (707) 794-4168 1212 Valley House Drive fax: (707) 794-4452 Rohnert Park CA 94928-4999 I'm the NRA.