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Add stereo headphone jack to an exercise bike with hardwired on-board stereo speaker pair Answered

I want to add a stereo headphone jack to an exercise bike with a pair of hard-wired on-board stereo speakers.

I imagine each speaker has 2 wires going to it. Is this as easy as taking either or both ground wires and the two signal wires and wiring a 3.5 mm TRS jack? How does a connected headphone typically turn off sound in speakers?

How do I handle the two ground wired (one going to each speaker).... can I just use either? or if combining grounds, do these need to be electrically "isolated", if so how?

Thanks in advance.


Jack A Lopez

8 weeks ago

In response to your question about, "How does a connected headphone typically turn off sound in speakers?"

The way that usually works is there is a switch inside the jack. Essentially it is this little flexible piece of metal that gets pushed by the plug, when the plug is inserted into the jack, and opens a circuit that was previously closed. When the plug is removed, the metal springs back, and closes that circuit again.

The Wikipedia article for, "Phone connector (audio)" has a picture of a jack of this kind,


It is a picture of a mono phone plug and jack, and if you look closely you can see a pair of contacts pushed open, with the plug inserted into the jack.

By the way, they make headphone jacks, with and without, these built-in switch(es). So if you want to make use of that switch, for to disconnect your speakers when plug is inserted, then you kind of have to be careful when shopping for your, what you call, "3.5 mm TRS jack," because some jacks have switches built in, and some do not.

I think the expected number of pins for a mono jack with (1) switch inside, is 3 pins, and the expected number of pins for a stereo jack with (2) switches inside, is 5 pins.

The, what you call, "ground" wires, for each speaker, are usually all connected together.


2 months ago

If you have a multimeter you can check if the negative contacts of the speakers are connected - should appear as a dead short.
In that case you can add some resistors on the positive line between speaker and headphone jack.
IMHO though it is often easier to use a stereo potentiometer if the output to the speakers is unknown.
A 10k Ohm potentiometer should do the trick.
(Set to thehighest value when connecting the headphones for a test, then crank the potentiometer until the volume is sufficient.)
The headphone port on the device should be selected to have a switch built in.
The negative for the speakers goes over the switch.
Then, if you connect the headphones the connection to the speakers will open and sound only goes to the headphones.