Converting from electret to mems microphone at 5 volts

Sensors Sound & Audio
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Hi all!

I have some sound-reactive rave/festival apparel which lights up by responding to micophone output. The whole thing runs off a standard USB power brick (the type for charging phones) as I find this is the most convenient power source. This means I’m working with 5V - the WS28xx LEDs are 5V and that’s also convenient as I can run, program and debug the Teensy++ 2.0 off that same USB 5V. So, I’m sorta attached to the 5V thing.

However, the microphone is an elecrtet, and I find that in loud settings it tends to just ‘top out’ and register maximum volume all the time. I’ve tried playing the the gain, but it doesn’t seem to help much. Looking at the data sheet, I see that a typical max volume is 110dB, which would explain it, as a quick google suggests live music could easily reach 120dB.

So, I’ve decided I’d like to pick up the BOB-19389 MEMS breakout board, as it claims a max volume of over 130dB. However, I dan’t seem to find a 5V MEMS anywhere. Neither can [this guy.

So I’m looking into ways to make it compatible. I’m wondering if a simple voltage divider will work. I’ve looked up [a bunch of options, but perhaps a couple of resistors is enough. However, Dave Haynie’s answer on [this question has got me wondering.

So, given that I only really need an approximate representation of volume, and not a good quality audio signal, would a couple of resistors be enough for my purposes to take the voltage down to something that the MEMS can handle and provide me with a reading? If so, can you recommend some resistor values? I’m trying to balance power usage with making sure the MEMS has enough power to do its thing.](https://www.quora.com/How-do-I-convert-a-5-V-DC-to-3-3-V-DC-using-resistors-voltage-divider-I-have-a-couple-of-220-Ω-560-Ω-2-2-kΩ-and-10-kΩ-resistors)](Taking It To Another Level: Making 3.3V Speak With 5V | Hackaday)](5V mems mic breakout - SparkFun Electronics Forum)

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A piece of tape over the mic?

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Hi liaifat85,

Thanks so much for the quick response.

It’s great to know (and will save a bunch of time/effort) that a simple voltage divider will suffice. To give myself the maximum resolution from the 10-bit analog input, I’ll aim for the top end of around 3.3V (the data sheet says that upto 5V wouldn’t hurt, but I don’t trust my power supply not to give me a bit more than that).

I understand the basic maths of selecting the resistor values in the dividor, so assuming 5V from my input and desired 3.3V to the mems, we want the value of R2 to be about double the value of R1. Thanks for suggesting some real-world resistor values. On that topic, one last question I have is: is the choice of resistor values entirely arbitrary? I’m happy to go ahead with your suggested values, but for my own knowledge (and that of anyone else reading this), I assume if I chose, say 5 Ohm and 10 Ohm resistors, I’d get the same voltage for my mems, but I’d sink a lot of current through the resistors. On the other hand, what if I chose 5MOhm and 10MOhm? Would the microphone struggle to get enough current to operate when in series with such a high value resistor?

Update: In an attempt to answer my own question, I found [this pretty thorough explanation. So perhaps if I chose very high values for the resistors, I’d give the mems more voltage than intended. The data sheet for the mems board says it draws 265 microamps at 3.6V, which I calculate to a resistance of about 13.6K Ohms. However, there are other components on the board, so I figured I’d start with the values suggested in the above reply.

With 5.6k (I didn’ have a 5.1k) and 10k, I measured almost exactly the 3.3V drop I would expect across R2. Then I put the load in and I get 1.5V across the load. Not nearly enough - the output voltage from the divider was very weak.

Using this 1.5V figure, I plugged it into falstad.com (found via the link above) and worked out that the resistance of the mems board ought to be around 3.2k Ohms. I then played around with the other values in the falstad.com diagram to arrive at 560 Ohms for R1 and 1.5K for R2. I put this circuit together on a breadboard and it gave a nice 3.2V going across the mems, and a nice signal too. :slight_smile:

I also wondered if I could sink a bit less current, so I figured I’d try switching to a 150K resistor for R2. Falstad told me R1 was a (surprisingly low) 1.8K. Nonetheless I hooked it all up and went to measure the voltage across the mems. Well, the multimeter didn’t really know what to tell me. It would jump around quickly between about 2.2 and 3.5V. I guess I went too high on the resistances.

So I tried the original value of 10K for the R2 resistor. I paired this with a 1.2K for R1. Measuring the voltage across the mems, I got a slow cycle of about 10 seconds which would start at about 3.4V, and then drop down to about 3.07V before raising again. I wasn’t getting a usable audio signal either.

I’m not hugely fussed about the current (given that I’m also feeding 150 RGB LEDs, I suspect the mems current will be a drop in a pond), so I think I’ll go back to my 560 and 1.5K.

I hope this is useful to someone else out there.](How to choose value of resistor in voltage divider? - Electrical Engineering Stack Exchange)

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brow:
A piece of tape over the mic?

I like the pragmatic thinking! I’m not really sure how much difference tape (over the old mic) would make, but I may get curious and try it! If I do (it’s easy enough to swap out the new and old mics), I’ll try to remember to report back my findings.

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megamasha:
I also wondered if I could sink a bit less current, so I figured I’d try switching to a 150K resistor for R2. Falstad told me R1 was a (surprisingly low) 1.8K. Nonetheless I hooked it all up and went to measure the voltage across the mems. Well, the multimeter didn’t really know what to tell me. It would jump around quickly between about 2.2 and 3.5V. I guess I went too high on the resistances.

So I tried the original value of 10K for the R2 resistor. I paired this with a 1.2K for R1. Measuring the voltage across the mems, I got a slow cycle of about 10 seconds which would start at about 3.4V, and then drop down to about 3.07V before raising again. I wasn’t getting a usable audio signal either.

I’m not hugely fussed about the current (given that I’m also feeding 150 RGB LEDs, I suspect the mems current will be a drop in a pond), so I think I’ll go back to my 560 and 1.5K.

Update: I also get the weird slow voltage cycle and unusable signal with 560 and 1.5k resistors, so perhaps I just fried the mems with the 150K/1.8K. :frowning:

Maybe 1.8K and and 10K would work fine.

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megamasha:
Maybe 1.8K and and 10K would work fine.

Correction: I’m tired and I hadn’t switched back to the 1.5K and 560 Ohm resistors, so that’s why I still got the same behaviour.

I have now switched back and all is well.

So, interestingly, 1.8K and 10K don’t work as a combination - I can’t get a usable signal.