In order to prevent the sensor electrodes from
degrading in the ionic water solution due to plating
effects, an AC (alternating current) source is used to
stimulate the sensor electrodes. Op-amp U1D is
configured as a square wave oscillator. Resistor R1, R3
form a Vdd/2 resistive divider for the + input of U1D.
Resistor R2 provides positive feedback to the U1D+ input;
R4 pulls up the output of U1D to Vdd. Resistor R5 and
capacitor C1 form a delay time constant to the - input of
U1D.
When the output of U1D is low, the + input of U1D
equals Vdd (R3//R2)/((R3//R2) + R1) = Vdd 50K/150K =
1/3*Vdd. Resistor R5 discharges C1 until the voltage at
U1D ≤ U1D+. The output of U1D switches from low to
high. The U1D+ input switches to Vdd R3/((R1//R2) +
R3) = Vdd 100K/150K = 2/3 Vdd. Resistor R5 begins
charging C1 until the voltage at U1D ≥ U1D+. This
process repeats, producing a square wave at the output of
U1D. With the values shown, Fosc = approx. (.685 /( R5 *
C1)) = 381 Hz (400 Hz was the target).
The output of U1D is fed to capacitors C2 and C5
which are connected to one side of the low and high level
electrode sensing pair, respectively. The other side of the
low and high level electrodes connects to C3 and C6,
respectively. Capacitor C3, D1, D2, C4, and R6 function as
a charge pump. When solution allows current to flow from
C2 to C3, C4 charges to indicate the solution level is at
least as high as the low level electrode pair. Similarly, C5,
C6, D3, D4, C7, and R7 detect the high level of the
solution in the vessel.
Resistors R8 and R9 form a voltage divider that is half of
Vdd - 1.5V (due to the limited output level of the LM324
op-amps). Whenever the voltage level on C4 or C7 is
above the R8, R9 reference level, op-amps U1A and U1B
(that work as comparators) switch from low to high. When
both comparators are high, this overcomes the hysteresis
of U1C and causes U1C’s output to go high.
The operation in the sump mode proceeds as follows.
The sump level is below the low electrodes; therefore, C4
and C7 are discharged, and U1A and U1B outputs are low.
This causes LED D5 to be ON indicating the sump level is
low. U1C is low; therefore, RLY1 is OFF (so is the sump
pump).
When the solution rises above the low level electrode
pair, C4 charges and U1A’s output goes high. This turns OFF
D5 and turns ON D6 (because U1B’s output is still low),
indicating the tank level is somewhere between the low
level and high level electrodes. U1C stays low due to its
own positive feedback and the low state of U1B.
The +input of U1C = (Vdd-1.5V)*
R15//R14/(R15//R14+R13) = 10. 5 50/150 = 3.5V which
is below the reference of 5.4V. When the solution rises
above the high level electrode pair, C7 charges and U1B’s
output goes high. This turns OFF D6 and turns ON D7
(because U1A’s output is still high), indicating the tank
level is above the high level electrode pair. The +input at
U1C is now (Vdd-1.5V)* (R15)/(R13//R14+R15) = 10. 5 *
100/150 = 7V which is above the reference of 5.4V. This
causes U1C’s output to go high that, in turn, activates Q1,
RLY1, turning ON the sump pump. As the pump lowers
the tank level, U1B goes low. This turns OFF D7 and turns
ON D6, indicating the mid level.
The + input at U1C = (Vdd-1.5V)* (R14)/
When the tank level is lower than the low level electrode
pair, U1A’s output goes low, D6 turns OFF, and D5 turns
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December 2016 23
■ FIGURE 1.
