Abstract
Here is a simple circuit for
liquid level alarm. It is built
around two BC547 transistors
(T1 and T2) and two timer 555 ICs (IC1
and IC2). Both IC1 and IC2 are wired
in astable multivibrator mode. Timer
IC1 produces low frequency, while
timer IC2 produces high frequency.
As a result, a beeping tone is generated
when the liquid tank is full.
Initially, when the tank is empty,
transistor T1 does not conduct. Consequently,
transistor T2 conducts and pin 4 of IC1 is low. This low voltage
disables IC1 and it does not oscillate.
The low output of IC1 disables IC2
and it does not oscillate. As a result, no
sound is heard from the speaker.
But when the tank gets filled up,
transistor T1 conducts. Consequently,
transistor T2 is cut off and pin 4 of
IC1 becomes high. This high voltage
enables IC1 and it oscillates to produce
low frequencies at pin 3. This low-frequency
output enables IC2 and it also
oscillates to produce high frequencies.
As a result, sound is produced from
the speaker. Using preset VR1 you can control the volume of the sound from
the speaker.
The circuit can be powered from a
9V battery or from mains by using a 9V
power adaptor.
Assemble the circuit on a generalpurpose
PCB and enclose in a suitable
cabinet. Install two water-level
probes using metal strips such that one
touches the bottom of the tank and the
other touches the maximum level of
the water in the tank. Interconnect the
sensor and the circuit using a flexible
wire.
