Sound Level Detector

NUST

National University of Sciences and Technology

School of Electrical Engineering and Computer Sciences

PAKISTAN

Sound Level Detector:

The project is aimed at designing a circuit which can detect the level/magnitude of sound given

at its input. The sound level detectors can be used to find the noise at a given location.

The input (sound) to the detector is provided via computer using male and female ports. The

input is compared using the comparators. The output is shown on a LEDs depending on the

output of the comparators. The LEDs glow according to the level of sound at the input.

We built a small sound level detector using following components:

UA 741 (7 used for level detector and 1 for amplifying sound signal)

Resistors 1k and 20K

Leds green red and yellow

Female audio port

Wire with male audio ports at both ends

Circuit shown in the diagram # 1 is working as follows: (simulated in Multisim)

Diagram # 1

Sound input

All the negative inputs (inverting) of operational amplifiers are shorted and sound as an input is

applied at negative inputs. All positive inputs are used for generating reference voltage to be

compared with negative inputs. At the output pins leds are connected through 1K resistors.

Operational amplifier is working as a comparator which compares negative and positive inputs

and based on these it generates output. Comparator follows equation # 1

From equation # 1, we can deduce results that if

V+ > V- then +Vcc will be the output and if

V-> V+ then - Vcc will be the output

We used 1K resistor between leds and outputs because every IC has some output offset

voltage (5mV) even if both negative and positive terminals are grounded and IC is biased.

Diagram # 2

Voltage produced by sound or song is very low as compared to voltage at positive terminals. So

we use an operational amplifier inverting configuration (Diagram # 3) to increase its voltage by

factor of 100 (see equation # 2) and then put its output at negative terminals. Operational

amplifier is shown in Diagram # 1 by shape star.

Diagram # 3

References:

http://www.datasheetcatalog.com/datasheets_pdf/U/A/7/4/UA741.shtml http://skema-elektronik.blogspot.com/2009/07/8-led-audio-level-meter.html

http://www.elexp.com/tips/invert.gif