User Guide for FSR Sensor

FSR Integration Guide and Evaluation Parts Catalog Page 25
with Suggested Electrical Interfaces
FSR Schmitt Trigger Oscillator
In this circuit, an oscillator is made using the FSR device as the feedback element around a Schmitt Trigger.
In this manner, a simple force-to-frequency converter is made. At zero force, the FSR is an open circuit.
Depending on the last stage of the trigger, the output remains constant, either high or low. When the FSR is
pressed, the oscillator starts, its frequency increasing with increasing force. The 2MΩ resistor at the input of
the trigger insures that the oscillator is off when FSRs with non-infinite resistance at zero force are used.
The 47kΩ resistor and the 0.47 µF capacitor control the force-to-frequency characteristic. Changes in the
“feel” of this circuit can be made by adjusting these values. The 0.1µF capacitor controls the frequency range
of the oscillator. By implementing this circuit with CMOS or TTL, a digital process can be controlled by
counting leading and/or trailing edges of the oscillator output. Suggested Schmitt Triggers are CD40106,
CD4584 or 74C14.
Figure 18
FSR Schmitt Trigger Oscillator