Attila Muhi - SM4RAN says:
They used an about 8-inch loudspeaker, a bit disassembled, with a pin attached to the center of the cone. The pin was connected to a tray. On the pin, there was an index coming out, which was sensed by an infrared fork sensor. The idea was to keep the index in the same position all the time, to keep it that way you had to increase the current when anything was dropped into the tray, thus the current in the speaker coil was proportional to the weight of what was in the tray. I remember (I hope) that this solution gave a quite good precision if it was properly tuned.
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<A HREF="http://piclist.com/techref/io/sensor/weight.htm"> Weight Sensors</A>
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