How do I add a headphone jack?

[Benadryl]

I'm currently making a tablet from an old LCD and and an LCD controller board. The controller board takes audio from the raspberry pi and outputs it to L-, L+, R- and R+ connectors. Is there a way of having an audio jack between the audio connectors and the speakers so that when a pair of headphones is plugged in. No audio goes to the speakers.

I have attached a rough diagram so you can see what I mean.

Thanks

[drgeoff]

The simple way of switching off speakers when headphone are plugged in is to use a headphone jack containing "switches" which are opened when the plug is present. However this requires that the two speakers have one side commoned so that they are L, R and GND just like the headphones. If your controller board contains an audio amplifier and is really producing L+, L-, R+ and R-, then you CANNOT use that technique.

[Benadryl]

Where would I get a switch like that?

[Z80 Refugee]

It's a standard part. Put "switched stereo jack socket" in the eBay search box.

[Benadryl]

Is it possible to combine the r- and l- and use that as the ground and then have the l+ as the left and r+ as the right

[mooblie]

It depends on the audio circuitry in the LCD Controller, but probably NOT. And doing so will most likely damage that circuitry in the LDC Controller permanently. Sorry. (As drgeoff said.)

[Z80 Refugee]

There are two possibilities: the speaker driver is single ended but the -ve wires are provided separately for "convenience", or the driver is double ended. Simple test: measure the resistance between the two -ve wires, if it's zero you can combine them. If it's not zero, the speaker driver is probably using an H-bridge driver arrangement to increase the volume available from a low voltage supply, in which case a more sophisticated switching arrangement is needed. It's possible, but I will have to come back to this later.

[mahjongg]

I agree with the above (+1)

If L- and L+ are not combine-able, use separate switched jacks for left and right, instead of trying to combine them into a stereo jack.

[Z80 Refugee]

Maybe, but the problem with that is I know of no headphones where the -ve wires for L and R are not commoned. You could re-wire some headphones so that L and R go to separate mono plugs, with two mono switched sockets, but also note that the volume levels driving speakers will probably not be appropriate for driving headphones, so some kind of attenuation will be required (unless you want to keep turning the volume up and down).

There exist jack sockets with multi-way switching arrangements, but they are hard to get. The easiest choices are: (1) a manual push button / toggle switch to cut off the speakers; (2) a relay to do the job of the switch, engaged by a contact in the socket. Let me explain the manual switch circuit - it should be obvious how to automate it with a relay (for which you would need a "stereo with on off switch" socket, using the on/off contact to energise the relay).

The resistors I've shown with question marks are a case of trial an error, to get acceptable volume levels in the headphones at normal volume settings for the speakers. The capacitors float the DC load of the headphones commoned to 0V (which the bridged speaker drivers are not designed for).

58D4962A-1C56-4A7E-A6AD-95D25CBCD7B4.jpeg (136.9 KiB) Viewed 805 times

[drgeoff]

If the LCD controller does use H-bridge type outputs the following is a simple approach. It uses the commonly available type of switched headphone jack. The circuit does have one important limitation. The "ground" connection of the headphone socket must never be connected to actual ground. That could happen if it was mounted on a metal panel or if the socket was used as a LINE out to connect to an external amplifier. The voltage rating of the capacitor must be at least that of the supply voltage to the controller board. 1000uF is the suggested value for phones of 32 ohm impedance. Lower values will impact the bass frequencies.

[Z80 Refugee]

I guess that would work... worth a first try anyway (cheap and simple). The risk is that the volume will be too high for the 'phones, and there is nowhere to put attenuation without also creating cross-talk.

[Benadryl]

There is no resistance between the two negatives meaning I can combine them. I have a switched audio jack but I can't figure out what I connect to where. I have attached a picture...

[Z80 Refugee]

I can't tell you which contact is which on that photo of the socket - you will need to work that out for yourself (it's not rocket science).

I would have thought the rest was pretty straightforward (now you have established the negatives are common) too: common the negatives and take them to the negatives of the speaker and the common return contact on the headphone socket, then for each of the positives take them to the pin on the socket which links to each side of the headphones when plugged in. Finally, there will be contacts on the socket which have the audio feed when the headphones are out but not when they are plugged in.

Apart from not knowing which contact us which, I don't like explaining every last detail because nobody learns by being told the answer. The diagrams already given should be enough of a clue (even if not exactly right for this situation).

Note there will be nothing to moderate the headphone volume.

[kimmysawi]

I can't tell you which contact is which on that photo of the socket - you will need to work that out for yourself (it's not rocket science).

I would have thought the rest was pretty straightforward (now you have established the negatives are common) too: common the negatives and take them to the negatives of the speaker and the common return contact on the headphone socket, then for each of the positives take them to the pin on the socket which links to each side of the headphones when plugged in. Finally, there will be contacts on the socket which have the audio feed when the headphones are out but not when they are plugged in.

Apart from not knowing which contact us which, I don't like explaining every last detail because nobody learns by being told the answer. The diagrams already given should be enough of a clue (even if not exactly right for this situation).

Note there will be nothing to moderate the headphone volume.

It was the correct image and you can use it for headphone jack (Y) Cheers

[crofter]

this will not work for a bridge amp r + - and l + - should never be connected to each outher or it will blow up the amp chip

you need 4 pole plug on the headphone

and 4 switch contacts

good luck

crofter

[Z80 Refugee]

this will not work for a bridge amp r + - and l + - should never be connected to each outher or it will blow up the amp chip

If you had read the whole thread you would know we've already been there, and...

There is no resistance between the two negatives meaning I can combine them.

[drgeoff]

this will not work for a bridge amp r + - and l + - should never be connected to each outher or it will blow up the amp chip

you need 4 pole plug on the headphone

and 4 switch contacts

good luck

crofter

My circuit with the single capacitor to ground will work for a bridge amplifier. It does not connect any of the 4 amp outputs to any of the other 3.