If you build a microcontroller sound sensor, you can install it on an LM386 based amplifier. It’s cheap, easily available and does not require too many external parts.
A nice feature of the LM386 is that it can drive a small speaker. This allows you to design and test the amplifier easily, using a portable music player and a desktop speaker. Once the amplifier works fine, adding a microphone to complete the sound sensor should be easy.
LM386 is a power amplifier designed for use in low voltage applications. The gain is set internally to 20 to keep the number of external components low, but adding an external resistor and a capacitor between pins 1 and 8 will increase the gain to any value between 20 and 200.
The inputs refer to ground, while the output automatically tilts to half the supply voltage. Standby power is only 24 milliwatt when operating from a 6 volt source, making the LM386 ideal for battery operation.
Let’s take a look at the pinout and see what the role of each part is in the diagram above.
Pins 1 and 8 get control. When not connected (NC), the amplifier gain is 20. The addition of a 10uF capacitor between them gives a gain of 200. Intermediate values are also possible as described in the data sheet.
Pin 2 is the negative input – GND in our case.
Pin 3 is the positive input, which is the true signal to be amplified. There is a 10K potentiometer in front of it, which adjusts the input signal level, acting as a volume control.
Pin 4 (GND) and 6 (Vs) provide the supply voltage for amplification. For this configuration, a AA NiMh AA 4x battery is used, which gives ~ 5V.
Pin 5 is the output. It is biased to 1/2 of the supply voltage Vs. In simple terms, this means that the signal has two components: an AC component, which is the amplified input signal, plus a DC component of 1/2 V = 2.5 V. This bias voltage can not be fed directly to a speaker, but this is exactly what we need for a μC audio sensor. The 250uF electrolyte conductor filters the DC component and the rest of the AC is in the loudspeaker.
The 0.05uF capacitor and the pair of 10 ohm resistors from pin 5 to ground have proved to be called “Boucherot cell” or “Zobel network” and are used to prevent high frequency oscillations.
Pin 7 is called bypass only, but the datasheet does not provide additional details about it or its use.