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FM Transmitter Bug - Electret Mics

So one of our irc buddies @JohnS_AZ generously sent out FM transmitter bugs to a whole bunch of us - as a fun group project thing.  @MakerDino’s already gotten a HackaWeek (and even HackaDay) up about it and I thought I’d look into it a bit more; do some research on the components!

Here is the schematic provided by @MakerDino, but directly off of the paper that comes with the FM Transmitter Bug.  And based off of tests he’s done, it’s able to transmit about 30m to his radio, though the paper claims 300m.  Let’s go ahead and start left to right:

The Electret Microphone

Electret comes from electricity/electron/electrostatic and magnet and is a dielectric material that holds a quasi-permanent electric charge/polarization. 

  • Upon finding this out, I asked, aren’t dielectrics usually insulators?  How can they hold charge? 

Well even though the material as a whole is not a good conductor of electricity, its atoms still respond to electric manipulation.  So if you were to put a dielectric in an electric field, electrons would not go THROUGH the dielectric, but the atoms would shift away from their normal configurations into ones that worked with the electric field.  This is known as dielectric polarization and this polarization creates a subtle, but present, electric field that holds some electric potential.  And in fact, dielectric materials are those with high polarizability!

A nice example that helped me understand it is how when you place a magnet down in a field of iron filings, the iron filings naturally reorient in a pattern that works best for the magnetic field.  So too atoms and their accoutrements.

  • Alright so an electret has this permanently embedded static electric charge.  How does it get it? 

Back in the good old days…like 1885 good old days, they would take a polymer or wax that contained polar molecules, melt it, then allow it to resolidify in a powerful electrostatic field.  The polar molecules of the dielectric would align accordingly and producing a dipole electret with a permanent electrostatic ‘bias’.  These days, they use electron beams to botox-style-inject highly insulating dielectric materials so that they keep the electrons in - either on their surface or in their volume.

Back to the Microphone!

So the electret microphone is a condenser microphone.  Because of its electret material, it does not require a polarizing power supply. 

You might ask why it would require a polarizing power supply if it did not have electret material.  This requires a bit of knowledge of how condenser microphones work.

Firstly, most condenser mics have two plates: a diaphragm and a fixed metal plate. The diaphragm vibrates when struck by sound waves and this changes the distance between the diaphragm and fixed plate. Depending on the changes, current changes and the soundwaves translate into their electric counterparts.  However, unwanted harmonics may occur as a result of unusually loud signals that cause the diaphragm to hit against a plate.  This set up requires a power source that can create the electrical field that these two plates work with.

However, with electret microphones, the electret dielectric can replace a plate - whether the diaphragm or the fixed back plate.  This allows us to skip having the polarizing power supply as the polar charges on the electret can act as the current inducer.  Sweet.

So what powers the silly thing?

Well. It actually doesn’t require power, but most electret microphones come with a preamplifier circuit which does require power - could be phantom power or even something as small as a 1.5v cell!

And that’s the first component hah - more tomorrow!

The original patent for electret microphones from 1964!


Filed under What are electret microphones? electret electronics dielectric polarization fm transmitter bug dielectric polarization how do condenser mics work microphones how do electret mics work tymkrs

  1. tymkrs posted this