Elenco Electronics EP-130, Руководство пользователя

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This electronic circuit is a radio transmitter/alarm
device used to monitor rising water levels such as
on rivers, dams, and spillways. Its alarm signals are
received by a standard AM radio. When the water-
contact plates or wires are out of the water, the
circuit is not complete and there is no RF output.
When the contacts are connected by water, an RF
output received by the radio indicates that the water
level has reached the height of the contacts.
The emitter of the NPN transistor in the RF oscillator
circuit is connected to the ferrite coil center terminal
through the 10
μ
F capacitor. The capacitor acts as a
short circuit at these frequencies. Feedback to the
base is through the 100pF capacitor. The 470k
Ω
resistor supplies the base current that turns on the
transistor.
Notice that the battery current must flow through the
PNP transistor to get to the oscillator circuit and
back. With the wires insulated from each other, the
only current that can flow is the leakage current
from the collector to the emitter with the base open.
This weak current cannot drive the RF oscillator.
When the wires are connected by the water, some
current can flow through the water to supply base
current to the PNP transistor. This base current
turns on the PNP transistor so that oscillator current
can flow between the collector and the emitter of the
PNP transistor with little resistance. Without this
resistance, the PNP transistor could be burned out
by excessive current, especially if the probes were
accidentally touched directly together.
When the transistor is on, the RF oscillator
produces an RF signal. These probes can be made
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. Then, adjust the oscillation frequency
with the tuning capacitor to a point where you can
hear through the radio.
Notes:
EXPERIMENT #129: WATER LEVEL ALARM
-151-
Wiring 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