The blog post here is just a place to put files and diagrams related to an electret microphone element amplifier circuit that is intended for basic sound or speech applications. The circuit is based on requirements from  BigG  and analysis from michaelkellett.

It was created as part of a discussion in the comments here:

https://www.element14.com/community/message/289305/l/trying-to-understand-how-r-c-affect-an-audio-signal-when-used-in-a-…

That discussion can be referred to, for the detail!

 

Example output when placed several meters away from a battery-powered smoke alarm which has just started to sound:

Zoomed-in:

 

Prototype:

 

Here is the general circuit. It accepts an electret mic element input along with a 3.3V supply, and outputs on J1 pin 2 a signal suitable for listening via an audio amplifier, or alternatively J1 pin 3 can be used to directly connect to an analog-to-digital converter (ADC) input on a microcontroller.

For alarm sounds, if it is desired to reduce the frequency response for normal speech, then C1 and C3 can be reduced in value, for instance C1 can be 100nF and C3 can be 10uF.

The printed circuit board (PCB) layouts are below. The attached zip file below can be sent to a PCB manufacturer. The board size is 37 x 15 mm.

All the parts are large and easy to hand-solder.

 

Top view:

Solder-side view:

 

 

Component List

 

QtyReferenceComponent
1R41.2k resistor 08051.2k resistor 0805
1R8100 ohm resistor 0805100 ohm resistor 0805
1R212k resistor 080512k resistor 0805
1R71k resistor 08051k resistor 0805
1R62.2k resistor 08052.2k resistor 0805
1R3220 ohm resistor 0805220 ohm resistor 0805
1R14.7k resistor 08054.7k resistor 0805
1R5TS53 series 10k trimmerTS53 series 10k trimmer
1C4100nF capacitor 0603100nF capacitor 0603
1C110uF capacitor 16V 4mm or 5mm dia10uF capacitor 16V 4mm or 5mm dia
1C22.2uF capacitor 16V 4mm or 5mm dia2.2uF capacitor 16V 4mm or 5mm dia
2C3, C5220uF capacitor 16V 6mm or 7mm dia220uF capacitor 16V 6mm or 7mm dia
1C63.3nF capacitor 06033.3nF capacitor 0603
1C74.7nF capacitor 06034.7nF capacitor 0603
1Q1BC847CBC847C
1MIC1Any electret microphone elementAny electret microphone element