Elenco 130-in-1 Electronics Playground - Manual (Page 117)

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In this experiment, you will create a voice input power
meter. The brightness of the LED in this circuit
changes according to the level of voice input that
comes from the microphone (the earphone). Since
voice levels change quickly, the brightness of the LED
should also adjust quickly. In order to show the
highest voice input levels, we use a circuit called a
peak-level hold circuit. This allows the LED to hold
certain brightness after it reaches peak strength,
rather than turning off immediately.
Build the circuit, and set the switch to position A. You
will use the earphone as a microphone. Speak loudly
or blow strongly into the earphone. You can see the
LED get brighter temporarily and then gradually grow
dimmer.
Study the schematic. You can see that the signal from
the earphone travels through the PNP transistor and
then becomes the positive (+) input for the first
operational amplifier. The output level of the first
operational amplifier is stored in the 100mF capacitor,
and slowly discharges through the 47k
Ω
k k resistor. The
LED gets dim as the voltage on the capacitor
decreases. The voltage that lights the LED is also fed
back to the negative (-) input of the first operational
amplifier, where it is compared to the signal from the
earphone. If the signal from the earphone is larger, it
charges the 100mF capacitor; otherwise there is no
output from it.
You can modify the brightness of the LED by
changing resistor RA (47k
Ω) or the capacitor CA
(100
μF).
Notes:
EXPERIMENT #97: VOICE POWER METER
Wiring Sequence:
112-13-EARPHONE
119-124-116-33-88-90-80-72-14-EARPHONE
31-65-64-82
32-71
93-111-40
79-94-113-41
63-42-131
87-66-127-115
67-129-128
81-68-130
89-69-114
70-134
121-135
122-132
Schematic