Elenco 130-in-1 Electronics Playground - Manual (Page 137)

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This experiment uses the LED and an audio oscillator
alarm to indicate three different levels of water in a
container. The water is used as a conductor to
complete the circuits and show the water level.
When the water is below all three of the wire
connections, only the bottom segment (D) of the LED
is on (indicating a low water level).
When the water is at a level that touches the two long
wires connected to terminals 77 and 124 (but is below
the shorter wire), the base current turns on transistor
Q2 and the middle segment of the LED (G) turns on
(indicating a moderate water level).
If the water rises to a level high enough to touch all
three wires, the base current is supplied to transistor
Q1, and the top segment of the LED (A) lights. The
audio oscillator is also activated as a warning of a
high water level.
Of course, you can alter this wiring to make the LED
display show other letters of symbols to indicate the
different water levels. Can you think of any other
symbols? (How about L = low, C = center, and H =
high?)
Notes:
EXPERIMENT #115: WATER LEVEL WARNING
Wiring Sequence:
1-29
2-30
3-103-109
4-17-41-87
5-47-110
20-42-45-119
22-44
25-48-124-WIRE
40-76
43-78
46-104-88
75-WIRE
77-WIRE
121-122
Schematic