K.P. VISWANATHAN
Here is a simple, automatic waterlevel
controller for overhead tanks
that switches on/off the pump motor
when water in the tank goes below/
above the minimum/maximum level. The
water level is sensed by two floats to operate
the switches for controlling the pump
motor.
Each sensors float is suspended from
above using an aluminium rod. This arrangement
is encased in a PVC pipe and
fixed vertically on the inside wall of the
water tank. Such sensors are more reliable
than induction-type sensors. Sensor
1 senses the minimum water level, while
sensor 2 senses the maximum water level
(see the figure).
Leaf switches S1 and S2 (used in tape
recorders) are fixed at the top of the sensor
units such that when the floats are
lifted, the attached 5mm dia. (approx.) aluminium
rods push the moving contacts
(P1 and P2) of leaf switches S1 and S2
from normally closed (N/C) position to
normally open (N/O) position. Similarly,
when the water level goes down, the moving
contacts revert back to their original
positions.
Normally, N/C contact of switch S1 is
connected to ground and N/C contact of
switch S2 is connected to 12V power supply.
IC 555 is wired such that when its
trigger pin 2 is grounded it gets triggered,
and when reset pin 4 is grounded it gets
reset. Threshold pin 6 and discharge pin 7
are not used in the circuit.
When water in the tank goes below
the minimum level, moving contacts (P1
and P2) of both leaf switches will be in
N/C position. That means trigger pin 2
and reset pin 4 of IC1 are connected to
ground and 12V, respectively. This triggers
IC1 and its output goes high to
energise relay RL1 through driver transistor
SL100 (T1). The pump motor is
switched on and it starts pumping water
into the overhead tank if switch S3 is ‘on.’
As the water level in the tank rises,
the float of sensor 1 goes up. This shifts
the moving contact of switch S1 to N/O
position and trigger pin 2 of IC1 gets connected
to 12V. This doesn’t have any impact
on IC1 and its output remains high
to keep the pump motor running.
As the water level rises further to reach
the maximum level, the float of sensor 2
pushes the moving contact of switch S2
to N/O position and it gets connected to
ground. Now IC1 is reset and its output
goes low to switch the pump off.
As water is consumed, its level in the
overhead tank goes down. Accordingly,
the float of sensor 2 also goes down. This
causes the moving contact of switch S2 to
shift back to NC position and reset pin 4
of IC1 is again connected to 12V. But IC1
doesn’t get triggered because its trigger
pin 2 is still clamped to 12V by switch S1.
So the pump remains switched off.
When water level further goes down
to reach the minimum level, the moving
contact of switch S1 shifts back to N/C
position to connect trigger pin 2 of IC1 to
ground. This triggers IC1 and the pump is
switched on.
The float sensor units can be assembled
at home. Both the units are identical, except
that their length is different. The depth
of the water tank from top to the outlet
water pipe can be taken as the length of the
minimum-level sensing unit. The depth of
the water tank from top to the level you
want the tank to be filled up to is taken as
the length of the maximum-level sensing
unit. The leaf switches are fixed at the top
of the tank as shown in the figure.
Each pipe is closed at both the ends by
using two caps. A 5mm dia. hole is drilled
at the centre of the top cap so that the
aluminium rod can pass through it easily to
select the contact of leaf switches. Similarly,
a hole is to be drilled at the bottom
cap of the pipe so that water can enter the
pipe to lift the float.
When water reaches the maximum
level, the floats should not go up more
than the required distance for pushing
the moving contact of the leaf switch to
N/O position. Otherwise, the pressure on
the float may break the leaf switch itself.
The length of the aluminium rod is to be
selected accordingly. It should be affixed
on the metal/thermocole float using some
glue (such as Araldite).
