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Wouldn’t it be
great to detect a
flooding issue
before it’s
too late?
BUILD A FLOOD
DETECTION
ALARM
Well, this device can
detect when rising
water occurs. The
circuit shown in
Figure 1 is a
schematic for a flood
detection alarm.
This project consists of a box
containing circuitry and the
water probe assembly. When
water touches the probe assembly,
an alarm will sound. A relay is
provided to switch on a water
pump. As long as the probe
assembly is in contact with the
water, the pump switch will remain
on. The alarm can be shut off with
a switch once the flooding has
been detected. Optionally, a 12V
lantern battery can be used as
a back-up power source if the
power from the wall transformer is
interrupted.
Circuit Description
The circuit in Figure 1 consists
of a 700 Hz oscillator, rectifier,
threshold detector transistors, and
drive transistors. U1, R1, R2, and C1
form the 700 Hz oscillator. This
drives probe P1 on TB1. When
water touches probe P1 and P2, P2
picks up the square wave. The signal
at P2 is rectified by D1 and charges
C4. Q1 is used as a threshold detector. If the charge on C4 reaches a
threshold of approximately 600 mV,
Q1 turns on. When Q1 is on, driver
transistors Q2 and Q3 are also on.
■ FIGURE 1
42
May 2007