3–311
Motorola Sensor Device Data
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TEST/CALIBRATION PROCEDURE
1. Before testing the circuit, the user–defined values for
RTH, RH and Roff should be calculated for the desired
application.
The sensor offset voltage is set by
Voff
Voff
R1
Roff
VCC.
Then, the amplified sensor voltage corresponding to a given
pressure is calculated by
Vsensor = 201 x 0.0002 x APPLIED PRESSURE + Voff,
where 201 is the gain, 0.0002 is in units of V/kPa and
APPLIED PRESSURE is in kPa.
The threshold voltage, VTH, at which the output changes
state is calculated by determining Vsensor at the pressure that
causes this change of state:
RTH
R7
RTH
VCC.
VTH = Vsensor (@ pressure threshold) =
If hysteresis is desired, refer to the LM311 Used in a
Comparator
section to determine RH.
2. To test this design, connect a +5 volt supply between pins
3 and 4 of the connector CN1.
3. Connect a volt meter to pins 1 and 4 of CN1 to measure
the output voltage and amplified sensor voltage,
respectively.
4. Connect an additional volt meter to the VTH probe point
to verify the threshold voltage.
5. Turn on the supply voltage.
6. With no pressure applied, check to see that Voff is correct
by measuring the voltage at the output of the gain stage
(the volt meter connected to Pin 4 of CN1). If desired, Voff
can be fine tuned by using a potentiometer for Roff.
7. Check to see that the volt meter monitoring VTH displays
the desired voltage for the output to change states. Use a
potentiometer for RTH to fine tune VTH, if desired.
8. Apply pressure to the sensor. Monitor the sensor’s output
via the volt meter connected to pin 4 of CN1. The output
will switch from low to high when this pressure sensor
voltage reaches or exceeds the threshold voltage.
9. If hysteresis is used, with the output high (pressure
sensor voltage greater than the threshold voltage), check
to see if VTH has dropped by the amount of hysteresis
desired.
A potentiometer can be used for RH to fine tune the
amount of hysteresis.
CONCLUSION
The pressure switch design uses a comparator to create a
logic level output by comparing the pressure sensor output
voltage and a user–defined reference voltage. The flexibility
of this minimal component, high performance design makes
it compatible with many different applications. The design
presented here uses an op amp with a transistor output stage,
yielding excellent logic–level outputs and output transition
speeds for many applications. Finally, several other
comparison stage designs, including a window comparator,
are evaluated and compared for overall performance.
F
Freescale Semiconductor, Inc.
n
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