Interlink Electronics FSR 400 Series - Manual (Page 25)

www.interlinkelectronics.com
23
FSR
®
Integration Guide
FSR Variable Force Threshold Switch
Figure 12: FSR Variable Force Threshold Switch
This simple circuit is ideal for applications that require on-off switching at a specified
force, such as touch-sensitive membrane, cut-off, and limit switches. For a variation of
this circuit that is designed to control relay switching, please see the next page.
The FSR device is arranged in a voltage divider with RM. An op-amp, U1, is used as a
comparator. The output of U1 is either high or low. The non-inverting input of the op-amp
is driven by the output of the divider, which is a voltage that increases with force. At zero
force, the output of the op-amp will be low. When the voltage at the non-inverting input of
the op-amp exceeds the voltage of the inverting input, the output of the op-amp will
toggle high. The triggering voltage, and therefore the force threshold, is set at the
inverting input by the pot R1. The hysteresis, R2, acts as a “debouncer,” eliminating any
multiple triggering of the output that might occur.
Suggested op-amps are LM358 and LM324. Comparators like LM393 also work quite
well. The parallel combination of R2 with RM is chosen to limit current and to maximize
the desired force sensitivity range. A typical value for this combination is about 47kΩ.
The threshold adjustment pot, R1, can be replaced by two fixed value resistors in a
voltage divider configuration.