Connecting the USB2250 Data Acquisition System to a String Pot

A string pot is a displacement measuring sensor that can conveniently measure displacements of a few inches to a few feet. It’s something like an ordinary tape measure but instead of a ruler there is a cable attached to a spring-wound potentiometer so that the distance the cable moves changes the position of the pot wiper. Simply mount the string pot securely and connect the cable to the moving part to be measured. When the potentiometer is powered, the output is a voltage proportional to the displacement of the cable.

String pot construction is shown below along with a typical enclosed unit.

Electrically, the string pot is a simple potentiometer and it can be powered by the USB2250 and the signal received as a single-ended DC voltage. The connection diagram is shown below:

The +5 V power is supplied by the USB2250 terminal block. The position of the wiper changes the voltage at the A-In terminal from 0 to +5 Vdc, depending on the position of the wiper.

A simple scale factor is all that is needed to convert the voltage into a reading in inches. For example if a string pot has a 7 inch displacement, the signal will be 0 to +5 V from 0 to 7 inches. The scale factor for Easy Sense to convert the voltage signal into a reading in inches is 7 inches/5 Vdc = 1.4