Oops.

I fucked up. One of my latest projects did not immediately work and it’s for the dumbest reason. This also has implications on past projects too, so buckle in.

This project will be featured next, but the important thing right now is that it uses a headphone jack. I have a preferred headphone jack that I like to use, a standard footprint that lets me use both salvaged and new parts. This also has some cool features.

This is the footprint I mean. You have probably seen it around but you may never have wondered what those 5 pins are doing there. The plug that goes in here only has 3 wires so why 5? Well, I’ll show you

This is what they do, there’s a left and right channel and also pass-through pins that are connected when the plug is not inserted. You can see from this picture that it is frequently used to disable the internal speaker when an external one is plugged in. When my device didn’t do what I suspected I measured the pins on the board and I did not see those pins connected when no plug was inserted. I opened up one of these jacks and this is what I was.

Uhhh…. not good. I think I know what that is, but let’s examine the datasheet real quick.

Yeah, that is an electrically isolated switch that connects when a plug is inserted. Useful, but unexpected. Why did I get nothing out of the speaker?

Well, it’s because I fed the audio out the right pins, but the speaker was just connected to the switch which I thought was a pass-through. This one doesn’t get the ‘mute speaker if jack plugged in’ feature, but I jumpered over the pins so now it does both. The jacks pictured at the top are ones I bought from digikey that do act how I expect. The ones from LCSC have this strange configuration that fits the standard footprint, but behaves entirely differently. What other implications does this have?

This is not the first time I’ve used those headphone jacks. And I even shipped them out to people after I tested them (working I thought!). What are the implications of using those on this design?

The microphone input is not grounded when not in use, it may have some static on it. Not the end of the world. Same with the speaker in connection, noisy but OK. The line out and speaker out though, We have an issue! In both cases when you plug a cable in it closes the switch and makes the audio output mono. That’s not good. Can it be fixed?

Yeah, but it’s not pretty. I’m really not happy about this turn of events and I’ll be using these parts up on things that don’t use the pass through function and never ordering them again. The datasheets were right, they were just unexpectedly laid out.