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Titlecircuit 4 water
TagsRelay Transistor Electrical Network Power Supply
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E L E C T R O N I C S F O R Y O U • D E C E M B E R 2 0 0 5 • 9 5W W W . E F Y M A G . C O M





This simple, economical and ver-satile circuit switches on themotor pump when water in the
overhead tank falls below the lowest
level and turns it ‘off’ when the tank
is full. Moreover, if the pump is run-
ning dry due to low voltage, it sounds
an alarm to alert you to switch off the
controller circuit (and hence the mo-

tor pump) to avoid coil burn and
power wastage.

The water-level controller circuit
(see Fig. 1) is built around IC 555 (IC2)
to monitor the water level in the over-
head tank and ‘on’/‘off’ status of the
motor through the inverter and driver
circuits. The transistor switch circuitry
monitors the flow of water and raises
an alarm if the pump runs dry.

Power supply is obtained through
step-down transformer X1, diodes D1
through D4, capacitor C1, series current-
limiting resistor R1, regulator IC1, and
noise-filtering capacitors C2 and C3.

The set-up for the water-level sens-
ing electrodes is shown in Fig. 2. Elec-

trodes are suspended into the tank
such that they don’t touch each other.
Points B, L and U of the water-level
controller circuit are connected to the
respective points of the sensor elec-
trodes assembly.

When water in the tank is below
the lowest level L1, all the electrodes
are electrically separated and hence
points L and U (pins 6 and 2 of IC2,
respectively) are pulled up to the sup-

ply voltage through resistors R2 and
R3, respectively. Therefore, to reset IC2
the output of IC2 at pin 3 goes low.
As a result, transistor T1 stops con-
ducting to drive transistor T2 and re-
lay RL1 energises. The motor pump
now starts running to fill the tank with
water. Freewheeling diode D5 prevents
chattering of the relay due to the back
emf produced by the relay coil.

When the water level rises to
bridge the electrodes, because of the
conductivity of water, pin 6 (E1) is
pulled down to ground (E2). This does
not alter the output state of IC2, which
maintains its previous state, and the
motor keeps running.

When water rises to the overflow
level L2 and touches electrode E3,
point U (pin 2 of IC2) is connected to
already sunken ground electrode E2,
thereby triggering it. IC2 resets to give
a high output at pin 3. This is inverted
by transistor T1 to cut off transistor
T2 and de-energise relay RL1. The mo-
tor pump now stops to prevent water

As water is consumed, the water

level comes down leaving electrode E3
isolated from ground electrode E2.
Now point U (pin 2 of IC2) is pulled
up to the supply voltage. This does
not change the output state of IC2 and
the motor remains switched off.

When water level again falls be-
low electrode E2, IC2 resets to cut off
transistor T1. Transistor T2 conducts
to energise relay RL1 and the motor is
powered to run. This is how the pro-
cess continues. LED1 glows whenever
the relay energises, indicating that the
motor pump is running.

As the values of resistors R2 and R3
are very high, corrosion of electrodes is
very little. Capacitors C2 through C7

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