Interlink Electronics FSR 400 Series - Manual (Page 26)

www.interlinkelectronics.com
24
FSR
®
Integration Guide
FSR Variable Force Threshold Relay Switch
Figure 13: FSR Variable Force Threshold Relay Switch
This circuit is a derivative of the simple FSR Variable Force Threshold Switch on the
previous page. It has uses where the element to be switched requires higher current, like
automotive and industrial control relays.
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
sees 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 transistor Q1 is chosen to match the required current specification for
the relay. Any medium power NPN transistor should suffice. For example, an NTE272
can sink 2 amps, and an NTE291 can sink 4 amps. The resistor R3 limits the base
current (a suggested value is 4.7kΩ). The hysteresis resistor, 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 and LM339 also
work quite well, but must be used in conjunction with a pull-up resistor. 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.
Two fixed value resistors in a voltage divider configuration can replace the threshold
adjustment pot, R1. The diode D1 is included to prevent fly back, which could harm the
relay and the circuitry.