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SOUND METER
UPDATED:.23:30 02 January 2019
DESCRIPTION SET UP NOTES

This circuit monitors an audio signal and displays it on a 10 LED bar display.

There are two modes the display can operate.

  • DOT mode - where a single LED in on.
  • BAR mode - where a bar of LEDs appear as the signal increases.

The sensitivity of the display is set by VR1. Turn clockwise to increase sensitivity.

MIC AMPLIFIER:
Set the gain by turning VR1.
Set close to, but not at maximum.

DIODE PUMP:
Set the speed of the changing signal by adjusting VR1 (Decay). Set midway to start with..

LED BAR DISPLAY:
Select DOT or BAR mode by moving the jumper.
Turn VR1 clockwise to increase sensitivity. Set midway to start with
BLOCK DIAGRAM
CIRCUIT BLOX
EXPLANATION

The microphone picks up sound which it converts into a small electrical signal. The Microphone Amplifier boosts the signal by about 40 times.

The Diode Pump smooths the AC signal by removing the high frequencies, leaving the changing amplitude at Vout. This rate of change can be adjusted with VR1 - "Decay".
The LED Display has a chain of comparators inside all comparing the input signal against 10 different voltage references. As the voltage rises above each of these reference voltages, the corresponding LED is activated.
TEACHING OPPORTUNITIES: DEVELOPMENT
Topics could include:
  • Sound waves
  • Operational Amplifiers as audio amplifiers
  • Operational Amplifiers as comparators - LM3914
  • Inverting and Non-inverting amplifiers.

The circuit can be modified to work directly from a signal source from an amplifier output. The Microphone Amplifier can be removed and the audio signal connected to the "Vin" input of the Diode Pump.

PROJECTS