Pressure and Force Sensors
Pressure or Force Switch Circuits – Note #7
Honeywell 1MICRO SWITCH Sensing and Control 11-800-537-6945 USA 1F1-815-235-6847 International 11-800-737-3360 Canada
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REFERENCE AND APPLICATION DATA
INTRODUCTION
MICRO SWITCH solid state pressure or
force sensors offer significant advantag-
es over mechanical switches in such ar-
eas as stability, repeatability, long life,
and adjustability. In addition, they provide
the capability of multiple switch points.
Using MICRO SWITCH 140/160/180 and
240PC amplified pressure or force sen-
sors as inputs (Vi), pressure switch cir-
cuits can be readily constructed. If you
are using an unamplified pressure or
force sensor, 120PC or FS, then you must
construct a circuit from the application
information on Bridge Amplifiers found on
page 109. The output from that circuit
then becomes your input for (Vj).
Configuration Of The Basic Force or
Pressure Switch
An op amp with positive feedback to pro-
vide hysteresis (Figure 2) is the basic
element of the switch. Its output (Vo)
switches from zero volts to the high state
(Vs−1.5V) for Vi≥Vpot wiper.
In this circuit, RIshould be chosen to mini-
mize loading of both the sensor output,
and the potentiometer used to adjust the
switch point. If a 2K pot is used, RIJ20k
is suitable. RHis selected to provide
enough hysteresis to eliminate noise-in-
duced jitter at the switch point. The
amount of hysteresis can be calculated
using the equivalent circuit in Figure 2.
When the output (Vo) is in the low state
(Vo≅0),
Va(L) JViRH≅V1for RH>> RI
RH+RI
When the output (Vo) is in the high state.
Va(H) JVa(L) +VoRI≅Va(L) +VoRIfor RH>> RI
RI+RHRH
The hysteresis is given by DVaJVa(H) −Va(L) ≅VoRI
RH
If the LM124 op amp is operated from a
10V supply, VoJ8.5V, and RHJ5 meg-
ohms yields a hysteresis of 34 mV.
Figure 1
Figure 2 Figure 3
Figure 4
Other Configurations
1. Output switches from high to low state for:
Vi≥V pot wiper (Figure 3).
2. Output switches from low to high for switch point
1≤Vi≤switch point 2 (window detector) (Figure 4).
Reference/Index
