A Bit More Than A Microphone: The Electret Story

When designing a microphone assembly the other day, I reached for an electret condenser microphone capsule without thinking. To be strictly accurate I ordered a pack of them, these small cylindrical microphones are of extremely high quality for their relatively tiny price.

It was only upon submitting the order that I had a thought for the first time in my life: Just what IS an electret condenser microphone?

A condenser microphone is easy enough to explain. It’s a capacitor formed from a very thin conductive sheet that functions as the diaphragm, mounted in front of another conductor, usually a piece of mesh. Sound waves cause the diaphragm to vibrate, and these vibrations change the capacitance between diaphragm and mesh.

If that capacitance is incorporated into an RC circuit with a very high impedance and a high voltage is applied, a near constant charge is placed upon it. Since the charge stays constant, changing the capacitance causes a tiny voltage fluctuation that can be retrieved as the audio signal from the microphone. Condenser microphones built in this way can be extremely high quality, but come at the expense of needing a high voltage power supply to supply the charge and an amplifier to buffer and magnify the audio.

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Mini Spy Bug Walkthrough

What we like most about [GreatScott’s] project videos is that he not only shows making them but also the calculations for selecting parts and the modifications along the way. This time he’s made a mini spy bug that records up to nine hours of audio.

His first task was to figure out if the ATmega328p’s ADC is suitable for audio sampling, but only after he explains how sampling works by periodically checking the input voltage from the microphone. Checking the datasheet he found that the ADC’s fastest conversion time is 13 microseconds, which works out to a sampling rate of 76.923 kHz. Good enough.

He then walks through why and how he decided to go with a pre-made amplifier circuit built around the MAX9814 IC. Spoiler alert. His electret’s amplifier output voltage was too low, using an off-the-shelf circuit instead of making his own kept things simple, and the circuit has automatic gain control.

At this point, he added the MicroSD card adapter. Why not just transmit the audio over FM as so many others have done with their hacks? Perhaps he’s worried about someone detecting the transmission and finding his bug.

His final optimization involved getting a good battery life. He measured the circuit’s current draw at 20 milliamps. With a 160 mAh battery capacity, that would be 8 hours of recording time. Removing the Arduino Pro Mini’s voltage regulator and two LEDs got the current down to 18 milliamps and a recording time of 9 hours. Better.

Those are the highlights. Enjoy his full walkthrough in the video below.

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Ambilight for your Piano (Hero)

That old upright piano still sounds great, and now it can easily have its own special effects. [DangerousTim] added LED strips which change color when he tickles the ivories. The strips are applied along the perimeter of the rear side of the upright causing the light to reflect off of the wall behind the instrument. This is a familiar orientation which is often seen in ambilight clone builds and will surely give you the thrill of Guitar Hero’s brightly changing graphics while you rock the [Jerry Lee Lewis].

Key to this build is the electret microphone and opamp which feed an Arduino. This allows the sound from the piano to be processed in order to affect the color and intensity of the LED strips. These are not addressable, but use a transistor to switch power to the three colors of all pixels simultaneously.

We think there’s room for some clever derivative builds, but we’re still scratching our heads as to how we’d use addressable pixels. Does anyone know a relatively easy way to take the mic input and reliably establish which keys are being played? If so, we can’t wait to see your ambilight-piano-clone build. Don’t forget to tip us off when you finish the hack!

Getting great bootlegs with the bootlegMIC

Go to any concert, show, or basement band practice, and you’ll find someone recording a bootleg. While these live recordings are sometimes fairly high quality, bootlegs recorded with a cell phone usually sound terrible. The guys over at Open Music Labs have a great solution to these poor quality recordings that only needs a few dollars worth of parts.

The project is called bootlegMIC. It’s a simple modification of an electret microphone – the same type of mic found in cellphones and bluetooth headsets – that allows for some very high quality recording in very noisy environments. According to the open music labs wiki, the modification is as simple as cutting a few traces on the PCB in an electret mic and soldering on a cap and a few resistors.

An electret mic contains a small JFET to amplify the signal coming from the microphone diaphragm; the specific JFET is selected by the manufacturer to ensure the microphone has the right gain and response. Usually these JFETs are chosen with the expectation of a relatively quiet environment, and trying to record a concert only results in a ton of distortion. By putting a resistor between the source of the JFET and ground of the microphone, it’s possible to reduce this distortion.

The circuit is easy enough to solder deadbug style, and should work with most cellphones. The guys at Open Music Lab were able to get their mic working with an iPhone, but they’re still working on figuring out the Android mic input. There’s a great demo video showing the improvement in audio quality; you can check that out after the break.

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