Multiple function buttons for car radio

[swordz60]

Hello all,
I have a rather simple question which i can't seem to figure out on my own. I want to add extra functions to existing car radio push buttons.

I am fiddling with my car radio to retrofit a usb port to it. I already managed to do so by using an mp3 module from ebay, but ran into this button problem so to speak. The mp3 module has 4 buttons of its own (volume control, next/prev song and a playback mode switch), but I want to control them using car radio buttons. I managed to do that, but I had to disconnect them from the radio board, so the functionality was lost on the radio, and worked only for the module.
I also tried to put a switch that disconnects ground from the selected radio buttons so that they only control the module. However all other radio buttons lost functionality, so I figured that's not the way to do this.

I thought that I might be able to accomplish my task by employing transistors, but I can't think of a way to do that.
I'm open to any suggestions you have for solving this little problem of mine.
Sorry if I didn't make myself clear (since I'm not a native speaker).
Thanks in advance.

 

[BradtheRad]

Sounds like a job for a 2-input AND gate.
Case 1: When the radio is playing and you press a button, send the command to the radio.
Case 2: When the mp3 plays and you press a button, send the command to the mp3 module.

You'll need 8 gates (4 going to the radio, 4 going to the mp3 module). You'll need to be careful to match voltage levels, commands active high or active low, etc.

It is possible to make an AND gate from transistors and diodes. Or even from diodes and resistors.

 

[swordz60]

Great idea. Totally forgot those multiplexer/comparator chips, I guess I got spoiled by Arduino doing all the work. I think I'll try applying 74HCT08 for the job, since I had some of them lying around. I'll keep you posted if anything good turns out or if I'll figure out another way of doing this.