Elenco Electronics EP-130 - Water Level Alarm

-
138
-
This circuit is a radio transmitter
/
alarm for monitorin
g
r
isin
g
water levels such as on rivers, dams, and
sp
ill
ways, an
d
sen
d
s a
l
arm s
i
gna
l
s to a stan
d
ar
d
AM
r
adio. When the water-contact plates or wires are ou
t
o
f the water, the circuit is not complete and nothin
g
h
appens. When the contacts are touching water, th
e
c
ircuit is activated and transmits a radio si
g
nal tha
t
c
an
b
e rece
i
ve
d
b
y a near
b
y
AM
ra
di
o.
Wh
en t
h
e
r
a
di
o rece
i
ves t
h
e s
i
gna
l
, you
k
now t
h
at t
h
e wate
r
level has reached the hei
g
ht o
f
the contacts.
The emitter o
f
the NPN transistor in the radi
o
frequency (RF) oscillator circuit is connected to the
f
errite coil center terminal through the 10
μ
F
c
apacitor. The capacitor acts as a short circuit at AM
r
adio
f
requencies. The radio si
g
nal is
f
ed back to the
b
ase throu
g
h the 100pF capacitor. The 470kΩ
r
es
i
stor supp
li
es t
h
e
b
ase current t
h
at turns on t
h
e
transistor
.
The battery current must
f
low through the PN
P
transistor to
g
et to the oscillator circuit and back
.
W
hen the wires are out o
f
the water, the PNP
transistor is turned o
ff
, and so is the oscillator circuit.
W
hen the wires are in the water, current
f
lows
t
h
roug
h
t
h
e water to supp
l
y
b
ase current to t
h
e
PNP
transistor, turning it on. This allows current to
f
lo
w
t
h
roug
h
t
h
e
PNP
trans
i
stor
’
s em
i
tter an
d
co
ll
ector t
o
t
he
o
scilla
tor
ci
r
cui
t w
i
t
h
li
tt
le
r
esis
t
a
n
ce
.
The
47
k
Ω
r
es
i
stor
li
m
i
ts t
h
e current; w
i
t
h
out
i
t excess
i
ve curren
t
c
ould burn out the PNP transistor, especially i
f
the
p
robes were accidentally touched directly together.
W
hen the transistor is on, the oscillator produces a
n
R
F si
g
nal. These probes can be
f
ormed o
f
almost
any insulated conductor, but large sur
f
ace areas
prov
id
e t
h
e qu
i
c
k
est resu
l
ts.
P
lace an AM radio receiver nearby and tune it to a
weak station. Next, adjust the oscillation frequenc
y
with the tunin
g
capacitor to a point where you ca
n
h
ear your water a
l
arm t
h
roug
h
t
h
e ra
di
o
.
Notes:
EXPERIMENT #11
6
: 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
S
chemati
c