Water Level Indicator With Alarm

[attachment=16682]
Here is a simple, versatile circuit
which indicates the level of
water in a tank. This circuit produces
alarm when water level is below
the lowest level L1 and also when water
just touches the highest level L12. The
circuit is designed to display 12 different
levels. However, these display levels
can be increased or decreased depending
upon the level resolution required. This
can be done by increasing or decreasing
the number of level detector metal strips
(L1 through L12) and their associated
components.
In the circuit, diodes D1, D2 and D13
form half-wave rectifiers. The rectified
output is filtered using capacitors C1
through C3 respectively.
Initially, when water level is below
strip L1, the mains supply frequency
oscillations are not transferred to diode
D1. Thus its output is low and LED1 does
not glow. Also, since base voltage of transister
T1 is low, it is in cut-off state and
its collector voltage is high, which enables
melody generating IC1 (UM66) and alarm
is sounded.
When water just touches level detector
strip L1, the supply frequency oscillations
are transferred to diode D1. It rectifies the
supply voltage and a positive DC voltage
develops across capacitor C1, which lights
up LED1. At the same time base voltage
for transistor T1 becomes high, which
makes it forward biased and its collector
voltage falls to near-ground potential.
This disables IC1 (UM66) and alarm is
inhibited.
Depending upon quantity of water
present in the tank, corresponding level
indicating LEDs glow. It thus displays
intermediate water levels in the tank in
bar-graph style.
When water in the tank just touches
the highest level detector strip L12, the
DC voltage is developed across capacitor
C2. This enables melody generating IC1
(UM66) and alarm is again sounded.