Hello,

I am learning electorincs and trying to read sound from mic on Arduino. I have used KY-038 sound sensor (schema here) but since the unamplified analog ouput is not very strong (tens of mV) I've tried to build amplifier based on the common emitter tutorial but adapting that to 5V source, higher hFE (actually 280, for simplicity I've used 225), divider/base ratio of 20:1 and using 100nF capacitors (since I don't have full assortment at home). The result is attached - I have verified the quiescent current and voltage are according to my expectations. However, when I plot the voltage at output I don't see any gain. I have omitted the output capacitor since Arduino can read only values 0 - 5V so I would have to bias to 2.5V anyway. Please advise what mistake I did. According to the KY-038 mic schematics the input resistance should be 150 Ohm - do I need to adapt the circuit?

Note that I have powered the mic through 10k resistor directly to the analog ouput pin, it's probably as good as using the VCC pin and setting the pot. One thing that caught me by surprise is that measuring A0/GND pin resistance I see 3kOhms - as the mic should be a 'capacitor' I would expect infinite DC resistance.

I have found many schematics for various pre-amps, but besides that these are usually 9V/12V powered, there's rarely an explanation why it is designed that way. And I am interested in understanding the hows and whys (I have already ordered a $1 mic with well designed pre-amp built in). One thing that these have in common is high-resistance resistor between collector and base - I get that it stabilizes the output at the cost of lowering gain. Right now I have no gain to offer :-/.

Thanks!

 

Most likely, the reason you're not seeing any gain at audio frequencies is that C1 and C2 are both way too small. Try 1 μF or 10 μF for C1, and 100 μF for C2.

 

You don't use a random capacitor you happen to have for an electronic design and expect it to work.

 

And that is not a common base amplifier - it's common emitter.

 

Uh, pardon the common base/emitter typo: I shouldn't feed gremlins and write posts after midnight.

I was actually lazy to compute the reactance of that capacitor, just have seen some tutorials using that value and blindly used that. Now that I do the math I see that for 440 Hz that's 3.6 kOhm - that sounds like a high value compared to the 150 Ohm resistor on mic shield. The tutorial linked above says that the bypass capacitor on emitter should not exceed 1/10 of the resistor (220 Ohm). Let's say I want it working in 100Hz-1000Hz range, so that should be at least 72uF - that matches your suggestion to use 100uF.

How should the choice of coupling capacitor match the rest of the circuit/the microphone? I must admit that I don't understand what's the purpose of the 150 Ohm resistor build on the microphone shield.

MOD:
Corrected your Thread Title.

 

hi radim,
I assume you know that the microphone circuit module is advertised as a 'sound detector' not a audio range preamp.
It works in the same way as a 'clap' detector, ie: a loud sharp sound creates a signal output, is that your project purpose.??

E

 

@ericgibbs Now I know a bit better, but I was assuming that the analog output would give me what I needed (and I could do the pre-amp myself). In the end I am probably going to use GY-MAX4466 with built-int preamp.

 

hi,
Looking thru the link you posted, in the parts list, its says its a Electret microphone.
You could build your own preamp using that microphone.
Do you want to read a good audio analog signal using a Arduino ADC input.? or just a loud short noise ie: clap.?
E

 

@ericgibbs Good signal, I plan to run FFT (or rather FHT) in Arduino to get the main frequency. Using this just to see amplitude would be just a plan B if the former doesn't work.

 

hi,
If you decide to build your own preamp, one these in the link may be a starting point.
https://www.google.com/search?q=ele...dAhUrL8AKHdosCwcQsAR6BAgFEAE&biw=1318&bih=894

E