Microphone Amplifier for Micro-controllers

14 Mar 2020

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

Qty	Reference	Component
1	R4	1.2k resistor 08051.2k resistor 0805
1	R8	100 ohm resistor 0805100 ohm resistor 0805
1	R2	12k resistor 080512k resistor 0805
1	R7	1k resistor 08051k resistor 0805
1	R6	2.2k resistor 08052.2k resistor 0805
1	R3	220 ohm resistor 0805220 ohm resistor 0805
1	R1	4.7k resistor 08054.7k resistor 0805
1	R5	TS53 series 10k trimmerTS53 series 10k trimmer
1	C4	100nF capacitor 0603100nF capacitor 0603
1	C1	10uF capacitor 16V 4mm or 5mm dia10uF capacitor 16V 4mm or 5mm dia
1	C2	2.2uF capacitor 16V 4mm or 5mm dia2.2uF capacitor 16V 4mm or 5mm dia
2	C3, C5	220uF capacitor 16V 6mm or 7mm dia220uF capacitor 16V 6mm or 7mm dia
1	C6	3.3nF capacitor 06033.3nF capacitor 0603
1	C7	4.7nF capacitor 06034.7nF capacitor 0603
1	Q1	BC847CBC847C
1	MIC1	Any electret microphone elementAny electret microphone element

Attachments:

export-mic-preamp.zip

Top Comments

dubbie over 5 years ago

Shabaz,

Looks a good circuit. I designed a circuit using electrets many years ago now that was my introduction to these amazingly sensitive devices. I think it was some sort of volume sensor - if you shouted loud enough it activated something. I can't really remember as it was so long ago. The PCB design looks good.

Dubbie
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shabaz over 5 years ago in reply to cstanton

Hi Christopher,

You're right, maybe they have been underused because local processing was difficult in the past. In theory a mic should (say) make a far higher quality window sensor, since it can hear broken glass but could also hear knocking on the window frame if burglars are trying to pull off the pane, and alert far earlier than the old metal tape on glass method : )

Also this part looked really interesting:
STMicroelectronics introduces IIS3DWB MEMS vibration sensor optimized for industrial monitoring
It is not strictly a mic, but picks up vibrations that could include some of the purposes you may be thinking of too. It looks really neat, it has on-chip filtering capabilities, and also streams to its own memory, so that the microcontroller can retrieve many samples all in one go to save energy and timing : ) Would be nice to try it out on different household scenarios!
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DAB over 5 years ago in reply to cstanton

Agreed.

I would be interested in an ultra sonic microphone to see if I could capture bat signals locally.

I would also like to see an infrasonic microphone to capture very low frequency sounds.

DAB
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cstanton over 5 years ago

I get the impression microphones are an under-used component in people's microcontroller projects. Most people probably think of only using microphones to record human speech, but I'm sure there's an opportunity for audio outside of human hearing to filter on for a project.
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balearicdynamics over 5 years ago

Sounds very good in response. It is worth I try to use a couple of these instead the standard Adafruit boards I used for the ears of Seven of Nine
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