Elenco 130-in-1 Electronics Playground - Manual (Page 144)

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This circuit works as a rain detector. This circuit stays
off and draws no current if the resistance between the
long wires is more than about 250k
Ω, whether the
key is open or closed. The speaker produces a tone
when the key is closed and water (or anything else
that has a resistance of less than about 250k
Ω) is
connected to both of the test wires.
Connect the wires to other wires or metallic plates
laid out on an insulated surface. The alarm turns on
when water completes the circuit by spanning the two
wires or plates.
This oscillator is the basic pulse-type that we’ve used
several times in this experiment kit. The 22k
Ω
k k resistor
protects the circuit against excess base current, in
case the wires are shorted together. The 100k
Ω
resistor keeps any transistor leakage current from
turning on the oscillator.
Notes:
EXPERIMENT #122: AUDIO RAIN DETECTOR
Schematic
Wiring Sequence:
1-29
2-30
3-104-110
124-4-WIRE
5-41-109
86-89-103-40
42-90-138
85-WIRE
119-137
121-122