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I was looking for a way to detect a line-in device transmitting audio. I got the idea to use ADC for this from this question: How to detect line level audio signal?

I already have a 4-channel ADC (the ADS1015) on my board which uses only one channel to read from a resistor network. So, I still have three available, but is this a good idea?

Power

What should I expect line-level audio to be, worst case? Less than 5V, 10mA?

From what I understand from the datasheet, the ADS1015 is tolerant of analog voltages up to 5.5V, 10mA continuous/ 100mA momentary. But it seems not to be able to handle negative voltages:

https://cdn-shop.adafruit.com/datasheets/ads1015.pdf, page 20:

Although the ADS1013/4/5 can read bipolar differential signals, they cannot accept negative voltages on either input. It may be helpful to think of the ADS1013/4/5 positive input as noninverting, and of the negative as inverting.

Now, I don't fully understand this. Will negative voltages damage the circuit? Or can't they be measured? In the latter case, for detection purposes, I guess it's ok to measure positive voltages only? In the first case, can we filter out the negative voltages?

Connection

I have a three channel passive summing box. Can I simply hook it up to the ADC from there (see schematic)? Or should I add filters, safety, and/or, ...?

enter image description here

Notes

  • The programmable gain is currently set to "2/3".
  • I read the values from a Python script, so I can be "smart" with the detection, for example filter out fluke spikes.
  • Any tips to make it robust (prevent false positives) and somewhat idiot proof (the Line-In is on my car's dashboard, who knows what stuff people plug in when i'm not looking)?
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    \$\begingroup\$ Old question but a logic level can be created easily with a diode cap amplitude detector with a transistor and hold cap with some decay time constant like 3 s. This is the trivial solution \$\endgroup\$ Jan 18 at 4:27
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75, it's been a while since I have worked on this project. Thanks for your input. I should really blow the dust off of this one soon ;-) \$\endgroup\$
    – svenema
    Jan 19 at 16:35
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schematic

simulate this circuit – Schematic created using CircuitLab

You will need to add a circuit like the one below on each input so that the ADC input will not go below ground. There are protection diodes built into the part that will clip the lower half of the signal otherwise.

enter image description here

The signal to the ADC will be centered at 1/2 of the V+ (2.5 V) and can swing +/-2.5 volts before the audio starts to get distorted.

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  • \$\begingroup\$ "You will need to add a circuit like the one below on each input", I think you mean "above"? The circuit below is the chip itself. \$\endgroup\$
    – svenema
    Oct 28 '17 at 16:43
  • \$\begingroup\$ The first circuit is not for protection. It is just there to give the input of the A/D a center reference. As far as protection, all you need is to place a resistor in series with the A/D input. The existing diodes in the chip will clamp the signal. The series resistance limits the current the diodes will need to pass, and is where the remaining voltage will be dropped. \$\endgroup\$
    – Rudy
    Jan 12 '19 at 21:19

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