A simple, efficient interface circuit for piezoresistive pressure sensors

Piezoresistive pressure sensors are currently available that could lead to a vast number of applications if they could be interfaced to microcontrollers using inexpensive circuits. Basic pressure sensors integrate four strain gages in a four-arm resistor-bridge configuration. Microcontrollers with embedded time counters can measure resistance by determining the time needed to charge or discharge a given capacitor. We have applied this method to interface two low-cost pressure sensors (SX15AD2 from SenSym and MPXV53GC7U from Motorola-Freescale) to a microcontroller PIC16F873 without using any active analogue component between the bridge and the microcontroller. The bridge is considered to be a resistive network with three input nodes and one output node rather than a resistor network with two terminals for excitation and two terminals for detection. The resistance between each of the three input nodes and the output node depends on the measurand. Using each input in turn to discharge a capacitor connected to the bridge output yields three different time intervals, and a difference of ratios between those time intervals is proportional to the applied pressure. The linearity is better than 0.9% for the SX15AD2 and 1.5% for the MPXV53GC7U. This performance suits many non-demanding low-cost applications.