Elenco 130-in-1 Electronics Playground - Manual (Page 146)

-146-
You can use the operational amplifier as a
comparator for detecting changes in voltage. In this
experiment, you are going to use this comparator
function to make a water buzzer that sounds when
the wire ends come into contact with water.
Slide the switch to position B, build the circuit, and
then slide the switch to position A to turn on the
circuit. You should not hear any sound from the
speaker. Now connect the two output terminals with
a wire, and you hear a sound from the speaker.
Touch the two output terminals with your fingers. If
the speaker makes a sound again, the electricity is
flowing through your body because the wire lead is
in contact with sweat.
This experiment uses two operational amplifiers. IC
1 works as a comparator. The IC’s negative (–) input
terminal has a reference voltage of about 1.6V. When
a voltage exceeding 1.6V is applied to the positive
(+) input terminal, the output of the comparator
allows IC 2 to work as an astable multivibrator.
Notes:
EXPERIMENT #124: WATER LEVEL BUZZER
Wiring Sequence:
1-29
2-30
3-114
5-83-80-94-70-110-121
13-86-63-131
14-93-69
65-89-109
66-82-84-91
64-90-92-113
67-81
68-79-85
119-124
Schematic