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Magnetic
Proximity Switch
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There
is practically no house without an overhead tank (OHT).
People who use electrically-operated water pumps for filling
the OHT find it very inconvenient to switch off the pump
when their overhead tank starts overflowing, specially
when they are busy. So there is plenty of water wastage
as well as wastage of power (consumed by the pump). However,
there is a solution to get rid of this headache. The circuit
given here will switch off the pump and also generate
a melodious tune when the overhead tank gets filled up
to the maximum desired level. All you have to do is switch
off the power supply to the circuit when you are relatively
free. The heart of the circuit is the CMOS latch CD4001.
Usually the latch can be operated in two modes, namely,
set and reset mode, i.e. the latch output can be set to
logic 1 or reset to logic 0 by applying appropriate active
low level input signal to pins 1 and 13, respectively.
Here, in the given circuit, the set point is pin 1 and
the reset point is pin 13. The inverted output of the
latches are obtained at pins 3 and 11, respectively. When
the circuit is powered there is a voltage drop at pin
1 due to the resistor-capacitor R1-C1 combination. The
values of resistor R1 and C1 are chosen in such a way
that pin 1 is low for about two seconds which is sufficient
to energise the relay through transistor T1 and thus the
pump starts running. When sufficient water gets filled
in the overhead tank, switch S1 in the sensing unit, in
the overhead tank as shown in Fig. 2, sends an active
low signal to pin 13 which resets latch gate N1 output
to logic 0. This causes transistor T1 to stop conducting,
thereby de-energising the relay and shutting down the
pump. At the same time, the output at pin 11 of gate N2
will be logic 1. This results in conduction of transistor
T2 and melodious buzzer sounds. The green LED also lights
up while the red LED, which remains on as long as the
relay remains energised, gets switched off when water
reaches the specified level in the overhead tank. The
circuit possesses the following advantages:
- Special sensing mechanism
(easy to build) is used to sense the water level in
the overhead tank.
- One can replace CD4011
with IC 7400. In that case, a 5.1V zener may be connected
additionally between pin 14 of IC1 and the ground.
- The circuit can be
easily fabricated on a general-purpose PCB.
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