The LM1830 fluid detector IC from National Semiconduc tor is designed
to be able to detect the presence of fluids using a probe. This chip
requires a relatively high supply voltage and is not the most frugal
power consumer. It is also quite specialised so unless you are buying in
bulk the one-off price is not cheap.
An alternative circuit
show n her e uses a standard CMOS IC type 74HC14. It has the advantage
of operating with a 3 V supply and consumes less than 1 µA when the
alarm is not sounding, this makes it ideal for use with batteries.
The
74HC14 has six inverters with hysteresis on their input switching
thresholds. A capacitor (C1) and a feedback resistor (R1) is all that’s
necessary to make an inverter into a square wave signal generator.
In
the water alarm circuit the feedback resistor consists of R1 and the
water sensor in series. R1 prevents any possibility of short-circuit
between the inverter’s input and out-put. Resistor R2 defines the
inverter’s input signal level when the sensor is not in water. Any
open-circuit (floating) input can cause the inverter to oscillate and
draw more current.The remaining inverters in the package (IC1.B to
IC1.F) drive the piezo buzzer to produce an alarm signal. Capacitor C2
ensures that no DC current flows when the circuit is in monitoring mode
(with the alarm silent) this helps reduce the supply current.
A micro-switch can also be substituted for the water sensor to make the circuit a more general purpose alarm generator.
Author: Roland Heimann - Copyright: Elektor
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