Detecting if Audio in or out is connected - Page 1
Forum section: electronics / projects

author: Hast
author_rank: Newbie
timestamp: March 31, 2020, 06:59:36 am
content: HelloI am doing a product where we want to minimize the number of connectors on it.One thing we are looking at is if it is possible to combine the audio in/out connectors into one connector.The connector is an ordinary 3.5 mm audio jack connector with left/right/GND.It should be automatic detection of audio in/out.The thing is that we would need to somehow measure if it is an audio in or out that is connected and then use an analog mux to route the signal to the right place (or is there a better way?).I see some different scenarios.-Unconnected-Connected no audio-Connected audio-Connected audio outCan this be done?Can we do some kind of impedance measurement to see if it is an audio in or out?BR

author: bob91343
author_rank: Super Contributor
timestamp: March 31, 2020, 07:27:04 am
content: Why not use a 4 pin connector. The first two are audio, the third is ground, and the fourth is grounded if it's an input and open if output. The mating connector tells the receptacle which it is.

author: Hast
author_rank: Newbie
timestamp: March 31, 2020, 07:53:30 am
content: Yes, I have thought about that but the thing is that the customer should be able to use an ordinary 3 pin 3.5 mm cable.The product must be easy to use, nothing special.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 31, 2020, 08:40:04 am
content: I'm not sure, but I seem to recall that line-audio should terminate in 600 ohm. I would imagine an audio-output source would be show a high-impedance, and an audio-input would show 600 ohm.But then again, I also know that a lot of stuff like phones and the like turn off their output amplifiers and might even short/terminate their outputs for protection?

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 31, 2020, 12:49:23 pm
content: The simple way is to have the input circuit connected all the time and the output circuit connected via 1k or so resistors. Then just mute the output circuit to use the input function.

author: t1d
author_rank: Super Contributor
timestamp: March 31, 2020, 12:52:02 pm
content: Use the 3-pin connector... Audio in and out and ground... Use the ground as a switch... Make ground and the circuit powers on... Then, use any of a great many ways to determine audio in, or out. There are audio jacks that are built to mechanically make, or break, a connection, when a plug is inserted... They are not anything special at all...

author: Bassman59
author_rank: Super Contributor
timestamp: March 31, 2020, 06:25:56 pm
content: Quote from: TheUnnamedNewbie on March 31, 2020, 08:40:04 amAudio hasn't used 600-ohm terminations since forever. We're not interested in transferring power and we are not going over lines that are a mile long. 600-ohm transmission lines require a hefty driver and you lose half your signal in the voltage divider caused by the termination.It generally uses bridging interfaces. A low source impedance drives a high-ish input impedance. So think like a 50-ohm drive into a 10k-ohm load. The source resistance is usually set by a "build-out" resistor in series with the driver. That resistor has two functions. One is that it isolates the driver from the cable capacitance (important for long cables) and two is that it prevents driver destruction if the output is shorted to ground. It does not set the impedance in a transmission-line sense because signals at audio frequencies don't need transmission lines.The advantages of this bridging system is that there is low voltage loss (the divider is only 50 ohms into 10k), you don't need a ballsy driver for it, and you can hang several receivers onto one output without much signal degradation.