Strain gauges are simple sensors that can be used to measure forces. They consist peice of conducting material that changes resistance as it is stretched in a given direction. The diagram below shows this:
Typically, the change in resistance of the strain gauge is very small. In order to accurately measure this small change, special circuitry is needed. For this, a wheatstone bridge configuration is usually employed. There are variants on how this circuit can be arranged; two are presented below.
The first configuration is the simplest method. The wheatstone bridge measures small imbalances in the resistances. Here it is comparing the strain gauge resistance to R3, which has a resistance equal to the resistance of the unstretched strain gauge. The other two resistors should have similar values.
Next is a more advanced circuit used for measuring strain in both directions. Two strain gauges are used, and must be positioned carefully, as shown in the second figure.
The FX1901 is an inexpensive 1% strain gauge that can be used for a variety of applications. It is available for about $30 and comes in 10, 25, 50, and 100 lbf models
Here is an example set up circuit with amplifier amp and tuning potentiometer:
Ensure that the power lines are very stable, it is suggested that in noisy environments, a DC-DC converter, power regulator(L78L05) and large power capacitors are used to ensure clean signal. In more ideal environments, a simple capacitor across the power lines should be sufficient.