SCX Series
Precision compensated pressure sensors
8/10
August 2005 / 053
www.sensortechnics.com
PRESSURE SWITCH
The circuit shown in Figure F is an example of using the SCX01D
to make an accurate 0.5 psi switch. This design can be easily
adapted to other pressure ranges by using higher pressure range
SCX sensors.
Circuit description
Operating the SCX01D from 5V the sensor will have 5/
12 the
sensitivity given on the SCX data sheet. The output at 0.5 psi will
be 3.75 mV. Resistor R
4 sets the amplifier gain to 269 V/V. Zener
D
1 biases the amplifier output voltage to 1.2 V with zero input
pressure and amplifier A
2 swings from 1.2 V to 3.2 V for an input
pressure from 0 to 1 psi. Resistors R
7 and R8 provide 5 mV of
hysteresis to comparator A
3 to ensure clean transitions for slow
movins input signals. R
11 sets the switch point for the comparator.
Care must be taken when laying out the comparator circuit; lead
lengths should be as short as possible.
Adjustment procedure
Apply 0.5 psi and adjust R
11 until the output just switches from a
high state (4.3 V) to a low state (0.0 V).
Figure E. Timing diagram
PORTABLE PRESSURE METER
The circuit shown in Figure G is a 0 to 200 millibar portable pressure
meter. The 3 1/
2 digit display will read 199.9 millibar full-scale. This
circuit operates from a single 9 V battery and draws approximately
4.5 mA supply current. This will provide a typical battery life in excess
of 100 hours. The minimum battery voltage is approximately 6.5 V.
This meter provides resolution to 0.1 millibars. The same circuit
can also be used for other pressure ranges simply by changing the
sensor and gain.
Circuit description
The LM10CN (A
1) is used to generate a regulated 5 V supply to
power the SCX05DN, amplifier A
2, and the ICL7106 Amplifier A2 is
a high input impedance diff-in,
diff-out amplifier. The sensor output is amplified to 200 mV full-
scale for the A/D input. R
5 sets the gain to 14V/V for the values
shown. The gain equation is:
A
V = 2 (1 +
R1/
R5)
R
4 is the zero adjust pot and R8 provides the full-scale span
adjustment. The voltage from pin 35 to pin 36 is approximately 100
mV when the display reads 199.9 millibar.
Component selection
The value of the components R
6-R10 and C1-C5 have been optimized
for 200 mV full-scale (see the Intersil ICL7106 datasheet). R
4 and
R
8 should be 15 turn cermet pots, R6-R10 are metal film 1% resistors.
C
3 the integrating capacitor should be polypropylene, the reference
and auto zero capacitors can be polystrene or mylar, the clock
capacitor, C
4, is mica.
Adjustment procedure
Apply 195 millibar to the meter and adjust R
8 until the display reads
195.0. Apply 0 psig and adjust R
4 until the display reads 000.0.
Repeat if required.
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