Sound Sense Kit Instructions

The Sound Sense comprises of an electret microphone (MK1), a comparator (U1A), and a non-inverting amplifier (U1B).

Electret microphones integrate a small Field Effect Transistor (FET) with a gate that is capacitively coupled to a diaphragm. Sound waves that hit the diaphragm cause it to deform in shape, and this results in a change in capacitance. This change in capacitance, in turn, adjusts how much current the FET conducts, thereby converting an acoustic wave into a changing electrical signal. The electret microphone has a 10KΩ pull-up resistor to provide power to the internal FET, and the resulting combination of the resistor and microphone makes a potential divider, whereby louder audio signals result in a smaller voltage across the microphone (because its resistance falls with an increase in volume).

In the presence of no sound, the voltage across the electret microphone is approximately VCC, and this is problematic for trying to detect audio signals. To remove this DC voltage, we use a coupling capacitor (C2) to extract AC voltages only. Therefore, only voltages that change over time can pass, meaning that converted audio waves can pass through C2, but not the stead voltage from the power supply.

This varying audio voltage is then fed into both op-amps, U1A and U1B, which are configured as a comparator and a non-inverting amplifier, respectively. You may notice that there is a diode in reverse bias mode, D1. The purpose of this diode is to remove any negative voltage that may build up from the microphone because negative voltages (with respect to the op-amp negative supply pin), cause the op-amp to become confused and produce unexpected outputs.

The comparator (U1A), has its negative input connected to a potentiometer, which produces a voltage between VCC and 0V depending on its position. If the voltage across R1 is greater than the voltage from the potentiometer, the output of the comparator switches to VCC, indicating that the sound level is greater than the trigger level defined by the potentiometer. If the voltage across R1 is lower than the voltage from the potentiometer, the output of the comparator switches to 0V, indicating that the sound level is lower than the trigger level defined by the potentiometer. The output of the comparator is digital, and can be used to determine if the microphone has detected a minimum level of sound.

The non-inverting amplifier (U1B) has two feedback resistors, R2 and R3, which result in a gain of approximately 50, and this is used to amplify the weak signal produced by the microphone. This output is analogue, and can be used to measure the amount of sound, but remember that the negative component of the sound wave has been removed, so it won’t preserve the quality of the detected audio!