Instead of a drinking glass, a PVC (plastic), stainless
steel, or glass tube is used to trap the air and is
connected to the pressure sensor by a flexible tube.
(Again, jump ahead and check out Figure 8 that shows
the sense tube, flexible connecting tube, and the
controller unit.) The type of sense tube material is
determined by the type of fluid that is being measured.
As the pressure rises, the pressure sensor — which
is based on a piezo-resistive bridge — puts out a small
differential voltage that linearly corresponds to the
pressure it senses. The math works out as follows.
The sensor has a full scale range of 50 kPa. Since
1.0 kPa (kilo Pascal) equals 0.145 PSI, the full scale
range is 7.25 PSI. The MPXM2051G is ratiometric with
the supply voltage. It is rated at 40 mV full scale with
In this design, to accommodate the MCP6022 dual
op-amp (used as a zero referenced, differential
instrumentation amplifier) five volts of excitation voltage
provided by a 78L05, U4 is used. This means that the
full scale output of the sensor is 20 mV at 7.25 PSI.
Therefore, since 1 PSI = 27.68 in of water, the full scale
value of 7.25 PSI = 27.68 in 7.25 = 200.7 in of water for a
20 mV output; 20 mV / 200.7 in H2O = .09965 mV / in
H2O. Thus, about 15 feet of water would produce about
18 mV of sensor output.
To get a full scale output from the differential amplifier,
a gain of roughly 250 is needed. The gain is (R4/R3 + 1) =
(1M/3.9K + 1) = 257.4, so the full scale output is 4.62V.
This is fine because the MCP6022 is an RRIO (rail-to-rail,
input and output) op-amp. The output of U2B provides a
common input level to the U3A and U3B window
comparators. Trim pot resistor R8 provides the low level
reference to U3A, and trim pot resistor R9 provides the
high level reference to U3B.
For a differential level of six inches of water, the
differentialvoltage from U2B is about .09965 6 257.4 =
154 mV. To prevent noise from causing erratic
operation, capacitor C3 in parallel with R4 form
a low pass filter with a pole at 6.2832 .1 µF *
1 Mohm = 1.6 Hz, or a time constant of .1 sec.
Resistor R5 is included to adjust the offset of
U2B output to around .3V to .5V, to increase
the span of level adjustment.
The intention here was to use inexpensive
readily-available parts to build the controllers. All
of the resistors are carbon film 5%. I chose a
SERPAC model 032 plastic case which I have
to use the mounting bosses in the case. For the
ionic level controller, three wire terminals are mounted on
the PCB (printed circuit board) and have slots in the side
of the enclosure to allow for the insertion of the sense
wires and the pump/solenoid load wires. The power jack
(a 2.1 x 5. 5 mm barrel jack) was mounted to the side walls
of the case. A 12V/1A switching power adapter is used to
provide the needed power for the controller.
The Gerber files for a two-sided PCB layout (refer to
Figure 3, ionic; Figure 4, pressure) are available at the
article link. For the build, point-to-point wiring was used
on a prototype board from RadioShack and cut to the
desired size to fit in the enclosure (refer to Figure 5, ionic;
Figure 6, pressure). The components were laid out and
positioned the same as on the PCB layout.
After wiring and testing the circuit board, the AC out
and input terminals were each connected to their sensor
electrodes and load (pump). The power jack was installed
December 2016 25
■ FIGURE 5.
■ FIGURE 6.