Manual

-138-
T
his circuit is a radio transmitter/alarm for
monitoring rising water levels such as on rivers,
dams, and spillways, and sends alarm signals to a
s
tandard 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 10mF
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 470kW
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 47kW
resistor limits the current; without it excessive
current could burn out the PNP transistor, especially
if the probes were accidentally touched directly
together.
W
hen the transistor is on, the oscillator produces an
RF signal. These probes can be formed of almost
any insulated conductor, but large surface areas
p
rovide 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
Wi
ri
ng Sequence:
o 47-11-6-ANT
o 7-93-113-41
o 8-12-97
o 40-87
o 42-119
o 46-98-94
o 48-73
o 74-114-120-WATER
o 88-WATER
Schematic
Note: The ANT is the 3 meters
long green wire. Connect the one
end of the wire to the spring and
hang the wire up vertically.
EP-130_62315RevC.qxp_EP-130_062812 6/23/15 11:18 AM Page 138