Specifications

CIRCUIT IDEAS
U
sually rain-alarms employ a
single sensor. A serious draw-
back of this type of sensor is
that even if a single drop of water falls
on the sensor, the alarm would sound.
There is a probability that the alarm
may be false. To overcome this draw-
back, here we make use of four sensors,
each placed well away from the other at
suitable spots on the roof. The rain alarm
would sound only if all the four sensors
get wet. This reduces the probability of
false alarm to a very great extent.
put of gate N7 is high if any one or more
of the rain-sensor plates SR1 through
SR4 remain dry. The output of gate N7 is
coupled to inverter gates N5 and N6. The
output from gate N5 (logic 1 when rain is
sensed) is brought to ‘EXT’ output con-
nector, which may be used to control
other external devices. The output from
the other inverter gate N6 is used as
enable input for NAND gate N8, which is
configured as a low-frequency oscillator
to drive/modulate the piezo buzzer via
transistor T1. The frequency of the oscil-
lator/modulator stage is variable between
10 Hz and 200 Hz with the help of preset
VR1. The buzzer is of piezo-electric type
having a continuous tone that is inter-
rupted by the low-frequency output of
N8. The buzzer will sound whenever rain
is sensed (by all four sensors).
6V power supply (100mA) is used
here to enable proper interfacing of
the CMOS and TTL ICs used in the
circuit. The power supply require-
ment is quite low and a 6-volt battery
pack can be easily used. During
quiscent-state, only a negligible cur-
rent is consumed by the circuit. Even
during active state, not more than
20mA current is needed for driving a
good-quality piezo-buzzer. Please
note that IC2, being of TTL type,
needs a 5V regulated supply. There-
fore zener D1, along with capacitor
C2 and resistor R5, are used for this
purpose.
A parallel-track, general-purpose
PCB or a veroboard is enough to hold
all the components. The rain-sensors
SR1 to SR4 can be fabricated as shown
in the construction guide in Fig. 2.
They can be made simply by connect-
ing alternate parallel tracks using
jumpers on the component side. Use
some epoxy cement on and around
the wire joints at A and B to avoid
corrosion. Also, the sensors can be
cemented in place with epoxy cement.
If the number of sensors is to be
increased, just add another set of
CD40106 and 7413 ICs along with the
associated discrete components.
Another good utility of the rain-
alarm is in agriculture. When drip-irri-
gation is employed, fix the four sensors
at four corners of the tree-pits, at a suit-
able height from the ground. Then, as
soon as the water rises to the sensor’s
level, the circuit can be used to switch
off the water pump.
CLEVER RAIN-ALARM
RUPANJANA
M.K. CHANDRA MOULEESWARAN
The four rain-sensors SR1 to SR4,
along with pull-up resistors R1 to R4
(connected to positive rails) and invert-
ers N1 to N4, form the rain-sensor-moni-
tor stage. The sensor wires are brought
to the PCB input points E1 to E5 using a
5-core cable. The four outputs of Schmitt
inverter gates N1 to N4 go to the four
inputs of Schmitt NAND gate N7, that
makes the alarm driver stage.
When all four sensors sense the rain,
all four inputs to gates N1 through N4 go
low and their outputs go high. Thus all
four in-
puts to
NAND
gate N7
also go
high and
its output
at pin 6
goes to
logic 0.
The out-
Fig. 2
Fig. 1
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