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*I tried to draw this on the circuit drawer, it's excellent, but I didn't know the exact pieces to put in. Considering the trouble with my first drawing I thought it worse to use the wrong components than to draw as much as I know carefully by hand.

I don't know much about electronics so I've put a lot of research and testing into making this patch cable. I'm proud I managed to make it work but I'd like to put this out there for other people and I can't just say "it works". To satisfy myself I've actually succeeded I need to understand too.

Can anyone explain this?

Removing or moving the resistor stops the circuit working. My guess is it is something to do with the signal sent back from the iphone to the mic.

Could it be the mic Tip line has a diode in it so when the phone sends out a signal it has to go through both resistors to ground? From my research there is a requirement for the iphone to see 1600 ohms resistance before it shuts off the internal mic.

enter image description here

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  • \$\begingroup\$ We'd need to know a lot lot more about your microphone. and to be honest, it's very unlikely that your mic works without sharing a ground (or at least a capacitively coupled ground) with the iPhone), so something's wrong here. TRRS has four contacts, but you only draw one! What does the 0Ω resistor do across ground? What is the striped bar at the iPhone's end? why is that one line thick and blue? I'm happy you've got it to work, but I'm afraid you'll need to do a slightly better job at defining what you've did for us to be able to help you. \$\endgroup\$ – Marcus Müller Sep 1 '18 at 7:52
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    \$\begingroup\$ A very easy way to make a clear drawing would be to use the schematic editor that's built in to this very website instead of making a hand drawing! \$\endgroup\$ – Marcus Müller Sep 1 '18 at 7:53
  • \$\begingroup\$ Start by actually adding all four connections on the iPhone end, and as many connections as your microphone has on the other. Connect with traces and components as correct. \$\endgroup\$ – Marcus Müller Sep 1 '18 at 7:55
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    \$\begingroup\$ I do believe it is working! It's just that as an electrical engineer I'd have to interpret your diagram in a way that says it can't work, so there's something wrong with the diagram, not with the circuitry. I really can't interpret your diagram correctly, no matter how hard I try :( For example, I've just learned that your 0Ω and 1478Ω are measurements that are just drawn as if they were the IEC symbols for resistors. You see, schematics speak a common language, and you're not using that language correctly, which is absolutely forgivable, but could be avoided by using a tool to draw! \$\endgroup\$ – Marcus Müller Sep 1 '18 at 9:20
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    \$\begingroup\$ That's totally OK! looking forward to your schematic. Worst case, it's very easy to re-open a question if its been closed :) Take your time! \$\endgroup\$ – Marcus Müller Sep 1 '18 at 9:33
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Your schematic reduces to

schematic

simulate this circuit – Schematic created using CircuitLab

with mic left, iPhone right, and the sleeves being ground, and Ring2 being the mic connection according to the CTIA standard. The rest of the connections (Ring on the mic, Ring1 and Tip on the iPhone side) are left unconnected.

Your microphone is an electret one, which probably means it's hard for the iPhone to detect the presence of the microphone simply by measuring the resistance across it.

If you imagine the mic away, then you just "shorted" the microphone input of the iPhone to its ground connection with about 1.7 kΩ resistance, which might just be in the right ballpark to tell the iPhone there's an external active microphone attached.

Of course, you don't just need the microphone to be there, you also need it to deliver sound. My guess is that the sensitivity (volts per sound pressure) of the microphone is relatively high, and that the sink impedance of the microphone input on the iPhone is relatively low: In that case, the array of resistors forms a current divider, which in turn brings the voltages at the iPhone to the level it can work with.

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  • \$\begingroup\$ Thanks for this great explanation and the drawing. You are correct, the iphone does need to detect in the area of 1.7 kΩ to switch to external mic. What was stumping me was that simply replacing the 1.5 kΩ with a 1.7 kΩ and removing the 200 Ω stopped the whole thing working. In short then, if I was to advise people about about building one, would I just say you may need to alter the size of the 200Ω until it works? \$\endgroup\$ – Danielh Sep 6 '18 at 12:10
  • \$\begingroup\$ are these functionally identical circuits? imgur.com/qRl0Ont \$\endgroup\$ – Danielh Sep 6 '18 at 16:39
  • \$\begingroup\$ no. How should they be? \$\endgroup\$ – Marcus Müller Sep 6 '18 at 16:57
  • \$\begingroup\$ Well, that's what I'm trying to get a grip on. My understanding was that when the trrs was connected the phone sent a signal from ring to sleeve. If it saw 1.6kΩ it would then turn off the internal mic believing that one of their headphone mics were attached. I couldn't just do that. Here's the article I followed. electronics.stackexchange.com/questions/38452/… \$\endgroup\$ – Danielh Sep 6 '18 at 17:34
  • \$\begingroup\$ try disconnecing tip from R2-R1 and connecting it to ring2 instead. \$\endgroup\$ – Jasen Dec 23 '18 at 5:45

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