How do I connect/use a 3 Wire Digital Scale Strain Gauge Weight Sensor?
I took it from an old weight meant for people. :)
The wires are red, black and white.
I found this, http://www.nerdkits.com/videos/weighscale/ ,but they use a 4 wire.
Does it look like this sensor? If so, put the power (maybe try 3 to 5V for starters) across black and white; signal is on red. Even if you apply the maximum force (50kg), you can expect the signal to change by an incredibly small amount. Make sure your voltmeter is up to the task; the sensor sold by SparkFun will vary 1mV per 1V of input, when the max force is applied. Additionally, it'll handle up to 10V of input, so you'll want to get as close to that as you can.
I'd bet that the wires are:
But it's hard to guess what the "power" voltage is. If you knew the power voltage, you could connect that voltage across the red and black wires, and check to see what signal comes out of the white wire.
If you can't figure out a way to determine the proper voltage, you could use an adjustable power supply. Start with a low voltage (3 V is probably safe), and then slowly raise the voltage while looking for a signal with an oscilloscope. There's a decent chance you'll go too high and burn the sensor, but it may be your only option.
Are there any identifying marks on the sensor? Did it come from a battery powered scale?
If the scale had a battery in it, the voltage for that type of battery is probably what you want to put across the red(+) and black(-) leads. A digital scale that I opened up had one of those 2032 'coin' cells in it, which is 3V. The signal will come back on the white wire, referenced to ground (i.e., the black lead), but the protocol may be tricky to sort out.
I am working on the same 3-wire-strain gauge. I am using 2 sets of the sensors to get a better results because I was experiencing same problem with you. Try to reverse the power polarity if you still cant get any values.