Manual

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I
n 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
t
hat 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 47kW
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 (47kW) or the capacitor CA
(100mF).
N
otes:
EXPERIMENT #97: VOICE POWER METER
Wiring Sequence:
o 112-13-EARPHONE
o 119-124-116-33-88-90-80-72-14-EARPHONE
o 31-65-64-82
o 32-71
o 93-111-40
o 79-94-113-41
o 63-42-131
o 87-66-127-115
o 67-129-128
o 81-68-130
o 89-69-114
o 70-134
o 121-135
o 122-132
Schematic
EP-130_62315RevC.qxp_EP-130_062812 6/23/15 11:17 AM Page 117