audio
I'm looking for a way to detect microphone/headset/headphones. How I can detect if two pins of audio jack has been shorted? This is the easiest way to check what kind of equipment has been plugged in.
In this situation I would check short circuit between pin 2 and 3. I'm using ADAU1761 and 4 pin jack.
1 2 3 4
|-----------\\\\
|GND|GND|GND|MIC
|-----------////
|-----------\\\\
|MIC|GND|RHP|LHP
|-----------////
|-----------\\\\
|GND|GND|RHP|LHP
|-----------////
Any idea?
The the problem is probably the standard 3.5mm jack. Smart phones use four pole 3.5mm jacks to add a mic to a headset. There are two configurations used but the most common (read what Apple pushes) is to have the sleeve as mic in, ground on the adjacent ring, right audio on the next ring then left on the tip.
I know that most recent Samsung phones reverse ground and mic in. I actually built a small crossover cable to get around this issue.
According to this page the mic should work if the resistance is about 2.2k ohms.
I only assume this works based on the success of using my crossover cable to use a headset/mic with button designed for iPhone on my Galaxy S.
The TRRS connectors are wired like this, in order:
That way the headphones will work plugged into a normal TRS jack, too.
The microphone input is meant to connect to an electret mic with a circuit like this:
The bias voltage, resistor, and DC-blocking capacitor are all inside the phone. So just connecting a signal source like a computer's sound card to the microphone input (with ground to the ground ring) should work.
If your sound source's DC output resistance is too low (normally a good thing), the phone could be interpreting it as a continuous button press (Answer or Play/Pause), because the headset button shorts the mic line's bias resistor to ground. Does it do anything to indicate that it thinks a button is being pressed? If this is the case, just put an electrolytic capacitor in series with your source, like 10 µF to 100 µF between your source and the microphone input. This will block the DC current from the bias resistor while allowing the AC signal to pass. Larger capacitance will have better low-frequency response.
For instance, if the input impedance of the phone is 2.2 kΩ, 10 µF will have a -3 dB cutoff frequency of 7 Hz, which is plenty.
Best Answer
It seems the ADAU1671 does not include a device detection feature. You need an analog switch to switch the microphone connection between pin 4 and pin 1 anyway, if you want to support standalone microphones as well as headsets. You can toggle the microphone bias voltage on and off. As the bias is current limited, it can easily be pulled down by a short circuit or a low-impedance headphone coil. So I would try it this way:
Couple MICBIAS via R and/or L to a comparator or Schmitt trigger, and its output to a general purpose input. Switch microphone to pin 4, enable bias. If the bias is not pulled down (so you detect high on the GPI pin), it's a headset. If not, toggle microphone to pin 1 (and remove headphone out from it), and check again. If it is pulled down, it's a headphone, if it's not pulled down, it's a standalone mic or no device at all (e.g. just an extension cord).