Specifications

CONSTRUCTION
used as protection diodes.
The under- and over-voltage cut-
off section (Fig. 1). It comprises a dual
comparator, two pnp transistors, and a
few other discrete components. This part
of the circuit is meant to stop the motor in
case of a low mains voltage (typically 180V
to 190V) or a voltage higher than a speci-
fied level (say 260V to 270V). The unregu-
lated DC is sampled by means of a poten-
tial divider network comprising resistors
R3 and R4. The sampled voltage is given
to two comparators inside IC LM319. The
reference voltages for these two compara-
tors are set by presets VR1 and VR2. The
outputs of both the comparators are ac-
tive-low (normally high, until the low or
high voltage limits are exceeded). That is,
when the AC mains goes below (or rises
above) the preset levels, the outputs of
the comparators change to logic zero. The
output of either comparator, when low,
results in lighting up of the respective
LED—D7 (for lower limit) and D8 (for
upper limit) via transistors T1 and T2
(2N2907), which are switching transistors.
The outputs from the comparators also
go to 8-input NAND gate IC7 (CD4068) to
control the motor via transistor T7. All
inputs to IC7 are high when all conditions
required for running of the pump motor
are fulfilled. When one or more conditions
are not met, the output of IC7 goes high to
de-energise relay RL1 via transistor T7.
Bipolar squarewave generation
(Fig. 1). One side of the secondary of
transformer X1 is also connected to op-
amp IC10 (µA 741), which is used here as
a comparator to provide bipolar square
wave (having positive and negative
halves). It is not advised to directly con-
nect the secondary output to the probe in
the tanks because, if due to any reason
the primary and secondary get shorted,
there is a risk of shock, as the secondary
would be directly connected to the probes
immersed in water inside the tank. But if
we use a comparator in between the sec-
ondary and probes, the IC would get open
in case primary and secondary windings
are short-circuited. For additional safety,
fuses F2 and F3, both of 1A capacity, are
connected to the output of secondary wind-
ings.
Pump motor fault-detection circuit
(Figs 1 and 2). A sensor probe detects
the flow of water. It is fixed just at the
mouth of the inlet pipe, inside the over-
head tank. When the motor is off (output
‘G’ of NAND gate IC7 is high), transistor
T9 (2N222) is ‘on’ (saturated) and, there-
fore, capacitor C15 is short-circuited. It
also pulls the clock input pin 3 of IC4(a)
flip-flop to ground. Zener D30 ensures that
transistor T9 does not conduct with logic
0 voltage (1 to 2V) at its base.
When the motor is running (all the
inputs to NAND gate IC7 are high), tran-
sistor T9 base is pulled to ground and
thus capacitor C15 starts charging via re-
sistor R16. The RC combination is selected
[using the well-known charging formula
V(t) = V
final
(1-e
-t/RC
)] such that it takes
about 15 seconds for the capacitor to reach
1/3 Vcc, i.e. about 4 volts to clock flip-flop
IC4(a) to toggle, taking its Q pin low to
stop the motor (via IC7, transistor T7,
and relay RL1). However, if water starts
flowing within 15 seconds after the start-
ing of motor, transistor T8 would start
conducting and discharge capacitor C15,
not allowing it to charge, irrespective of
the state of transistor T9. Thus capacitor
C15 remains discharged.
But if water does not flow due to any
reason, such as air lock or pump motor
failure, IC4(a) toggles after about 15 sec-
onds, which makes its Q pin 2 low. As a
result, the output of IC7 goes high and
the motor stops. Simultaneously, capaci-
tor C15 is discharged. At the instant Q
goes low, Q (pin 1) goes high and so a
clock is applied to IC5 via resistor-capaci-
tor combination of R18-C16, so that clock
input pin 14 of IC5 goes high after about
10 seconds. As a result, pin 2 of IC5 goes
Fig. 2: Part circuit of complete water level solution (contd. from Fig. 1)
Note: Identically marked points in
Figs 1 and 2 are interconnected
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