I'm currently trying to reverse engineer a remote in my headphones (one of these three button things). The remote seems to work fine, but the mic is intermittent - at times it will work perfectly, at other times it will go completely silent.
In order to proceed further in trying to figure out what the mystery IC is doing, I need to figure out what microphone is being used so that I can identify the connections. All I know is it's an SMD mic with all pads underneath the chip so I can't trace them easily.
Above is a photo of the microphone (apologies for the poor quality, I'll try for a better one later). I'm pretty sure that it must be an electret one as there are no inductors/filters anywhere that could otherwise create a supply for a MEMS microphone given that the whole controller is powered via a bias resistor embedded in the phone. (*)
The markings are as follows:
- Top Line: 1005
- Bottom Line: 7406 (I think. Might be 74C6)
The overall dimensions are roughly 4mm x 2.4mm x 1.1mm give or take. The black dot is the microphone membrane itself (so entry is on top).
The output of the mic is analogue not digital and seems to be on the order of a few millivolts.
A cursory Google search returned nothing, so I'm opening the floor for suggestions.
Again, the aim of this question is simply to identify the part.
(*) This is for an Android phone typically a 2.2V ish supply with a 2.2k or 5k series resistor (depends on which spec you read).
For those that want some background:
In terms of an overall picture of the circuit, as I say it's a 3-button remote in a set of headphones for controlling a mobile phone. The remote is supposedly both for Android and iPhone, but doesn't behave correctly. Initially on connection the mic works fine, but as soon as any button is pressed the mic will subsequently go silent until the whole thing is disconnected for a while.
My best guess at this is that my phone is an edge case that doesn't work properly with the way they have designed the controls - iPhones and Android use different signalling schemes so they seem to have some extra circuitry involved to switch between them. I am trying to work out the various connections so that I can start modifications to suit the Android spec.
Below is the schematic that I have thus far deduced. Again it is not possible to complete because I cannot physically see or measure what connections are beneath the microphone. In the schematic in the red box I've drawn the routing to match the physical layout as pictured in the photo.
Of interesting note the voltage at the input is never more than 1.4V. When it is working correctly (i.e. mic can be heard) the voltage is about 1.06V. When the mic can't be heard it has jumped to 1.4V.
Pin 3 of the unknown 6-pin IC is sitting at 1.4V when the mic is not working. When it is working this voltage drops to near 0V. Beyond that it is hard to get an idea of what is going on from the voltages as the rest of the pins seem to be roughly the supply voltage with the exception of pin 1 which is sometimes 0 and other times floats up to 0.4V ish.
The voltage at the phone input when buttons are pressed are as expected - roughly 300mV for VOL-, 150mV for VOL+ and 0V for CNTRL. From what I can assume the unknown IC is probably some form of multiplexer, but there is no information marked on the chip.
But to be honest the aim is to simply identify the connections to the microphone after which I am planning to remove the mystery IC entirely and find a way to wire in the mic correctly.