Stephen Kamichik, Ile Bizard, PQ, Canada; Edited by Paul Rako and Fran Granville -August 25, 2011
This Design Idea
battery-operated, portable soundlevel
meter (Figure 1
). Its portability
makes it useful for detecting the source
of noise in automotive engines and
power trains, factory machinery, and
residential HVAC (heating/ventilation/air-conditioning) systems.
biases the condenser
self-biasing common-emitter amplifier.
provides negative feedback.
Shunt capacitor C2
, in parallel with R5
increases the amplifier’s voltage gain.
The input resistance of the amplifier is
equal to (R2
In this circuit, the input resistance
is 10.8 kΩ. Capacitor C3
high-frequency response of the amplifier.
is a filter capacitor.
couples the output of the
amplifier to the load, P1
, a 5V positive-voltage regulator,
regulates the 9V battery supply to 5V for
, an LM3915 dot/bar-display driver. IC2
senses analog voltage levels and
In the sound-level-meter circuit, IC2
provides a logarithmic 30-dB display.
also contains an adjustable voltage
reference and an accurate 10-step voltage
The high-impedance input buffer
accepts signals from ground to within
1.5V of the positive supply. You set the
sound-level meter to bar mode by connecting
Pin 9 to Pin 3. Pin 3 is the V+
pin of IC2
and connects to IC1
’s Pin 3,
which is the device’s output voltage.
functions as a sensitivity
control. Resistor R6
sets the voltage
reference for IC2