I am working for a while now on a small project of building an external mic for the iPhone to be hooked in parallel to headphones (via the 3.5 mm jack). Basically, I want to replace the iPhone's original headset microphone with another microphone, but still use the headphones to listen.
Below is an image of the prototype structure (based on 2 existing products, but it is actually not working; probably due to impedance issue):
Some clarifications on the image:
item #5 - split point between microphone jack and headphones jack. item #4 - iPhone plug 3.5 mm item #3 - a microphone which for now is NOT working since the iPhone seems NOT to identify it (probably impedance issues - it's impedance about is about 650 Ohm) item #2 - simple headphones jack (any headphones with 3.5 mm plug can hook there) item #1 - microphone jack hooked to the external microphone I have for now. I would like to focus my question on the electrical aspects of my project for now. Known data I gathered so far (please feel free to correct any mistake you identify):
iPhone supply's 1.5 V on the TRRS 3.5 mm jack. the TRRS plug of iPhone is built of from 4 pins: Left/Right/Ground/Mic Questions:
What's the power consumption per EACH part of the iPhone headset? Each part means that there are 2 components - headphones &
microphone and I need the separated power consumption (especially the microphone!) What's the current the iPhone drive on the headset microphone and what's the current driven on the headphones? Read some other answers on the impedance topic, that the iPhone identifies external microphone (on the headset for example) only if the impedance is ±1650 Ohm, but than, I read another answer which claims the required impedance is ±5000 Ohm. Any ideas what is correct? Should I plan that the microphone impedance (includes the wire) to be 1650 Ohm (or 5000 Ohm based on the answer I will get for question #3) OR the whole prototype (microphone + wires + regular headphones I will hook to 3.5 mm connector) should be together the 1650 Ohm / 5000 Ohm? Is this correct to say that the 1.5 V supplied by the iPhone 3.5 mm audio output, means that the right/left/mic pins are positive contacts with 1.5 V vs. the ground pin (means that we have 3 parallel circuits sourced by one 1.5 V power source).
