Elenco 130-in-1 Electronics Playground - Manual (Page 138)

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This circuit is a radio transmitter/alarm for monitoring
rising water levels such as on rivers, dams, and
spillways, and sends alarm signals to a standard AM
radio. When the water-contact plates or wires are out
of the water, the circuit is not complete and nothing
happens. When the contacts are touching water, the
circuit is activated and transmits a radio signal that
can be received by a nearby AM radio. When the
radio receives the signal, you know that the water
level has reached the height of the contacts.
The emitter of the NPN transistor in the radio
frequency (RF) oscillator circuit is connected to the
ferrite coil center terminal through the 10
μF
capacitor. The capacitor acts as a short circuit at AM
radio frequencies. The radio signal is fed back to the
base through the 100pF capacitor. The 470k
Ω
resistor supplies the base current that turns on the
transistor.
The battery current must flow through the PNP
transistor to get to the oscillator circuit and back.
When the wires are out of the water, the PNP
transistor is turned off, and so is the oscillator circuit.
When the wires are in the water, current flows
through the water to supply base current to the PNP
transistor, turning it on. This allows current to flow
through the PNP transistor’s emitter and collector to
the oscillator circuit with little resistance. The 47k
Ω
resistor limits the current; without it excessive current
could burn out the PNP transistor, especially if the
probes were accidentally touched directly together.
When the transistor is on, the oscillator produces an
RF signal. These probes can be formed of almost
any insulated conductor, but large surface areas
provide the quickest results.
Place an AM radio receiver nearby and tune it to a
weak station. Next, adjust the oscillation frequency
with the tuning capacitor to a point where you can
hear your water alarm through the radio.
Notes:
EXPERIMENT #116: WATER LEVEL ALARM
Wiring Sequence:
47-11-6-ANT
7-93-113-41
8-12-97
40-87
42-119
46-98-94
48-73
74-114-120-WATER
88-WATER
Schematic