Elenco Electronics EP-130, Manual, Page: 114 / 160

You can use a microphone to detect sound. Here we
will make a circuit that lights the LED when the
microphone detects sound, using the speaker as a
microphone.
Slide the switch to position B and assemble the
circuit. When you finish the wiring, turn on the power
by sliding the switch to position A. Then speak into
the microphone - the speaker - or give it a light tap.
The LED flashes in response.
Look at the schematic. IC 1 acts as a microphone
amplifier - a non-inverting amplifier with a gain of
about 100. IC 2 works as a comparator. Its positive
(+) input terminal receives a reference voltage from
the battery. The output of the microphone amplifier
goes to the comparator’s negative (–) input terminal.
When this input voltage is higher than the reference
voltage, the comparator’s output level becomes low
and the LED lights.
Notes:
EXPERIMENT #95: VOICE ACTIVATED LED
Schematic
-114-
Wiring Sequence:
o
1-29
o
2-30
o
3-110
o
5-76-74-80-70-121
o
85-31-63-132
o
33-64
o
79-65-112
o
73-86-66
o
90-67-111
o
89-68-115
o
69-109
o
75-116
o
119-124
o
122-131