Electronics help needed for headunit circuit... - Page 1
Forum section: electronics / beginners

author: bobhaha
author_rank: Contributor
timestamp: April 27, 2014, 03:55:01 am
content: So I bought a new headunit for my car and it only has one input for a car steering wheel button control. My car has two separate controls, one on each side of the wheel.I made a short video explaining the problem, you can watch it here...To clarify I used a voltage divider for one side and left the other side of the wheel unchanged. Thinking that if there was a voltage difference it may think its a separate signal.So basically I need to be able to trick the headunit into thinking that the signals from each side of the wheel, are different and push them through the same input wire, to the headunit.Any and all help is appreciated.-Adrian

author: Shock
author_rank: Super Contributor
timestamp: April 27, 2014, 06:19:42 am
content: Electronics is a lot easier with schematics and wiring diagrams, brands and model/year numbers.When you can't understand what is going on look for the service manual or in this case the workshop manual for your car. Then get the schematic or at least the pin out of the radios.Most car audio installers will be able to tell you if a solution exists in about 5 mins on the phone.Look up the models even, to see if there is an adapter. You seem to be taking the painful route here if you haven't tried the above.

author: Rerouter
author_rank: Super Contributor
timestamp: April 27, 2014, 06:27:52 am
content: It will be resistive switches to ground, the head-unit has a lift-up resistor and the resultant voltage generally gets captured into an 8 bit ADC, with a 10 step keep out zone for each step (each switch has to be a certain amount away from the nexts voltage level,the data you are seeing is likely crosstalk from the can bus, you can expect CANbus to be present in the original radio harness,

author: bobhaha
author_rank: Contributor
timestamp: April 27, 2014, 06:33:13 am
content: QuoteThis is what I thought may be happening. Could it be possible that my voltage divider was lowering the voltage enough to be "out of range"?I was using 2x 2.2k ohm resistors as my voltage divider. Using something that will give a lesser division may work better. I'll give that a go now.

author: bobhaha
author_rank: Contributor
timestamp: April 27, 2014, 07:47:42 am
content: Ok, so either I'm not doing something right or the voltage is "out of range"My thoughts so far...https://dl.dropboxusercontent.com/u/14298539/Electronics/20140427_170051.mp4I used a 4.7k and 2.2k ohm resistor to get the 300mV of the highest value button down to 100mV but the radio still doesnt see it.What I have available:3.3V railRadio groundSignal groundSignal 1Signal 2Radio input (pull up to 3.3V)This is how I'm setting out my voltage divider:Ground --- 4.7k --- Radio input <--- 2.2k <--- Signal 1^|Signal 2Hope that makes sense...

author: Rerouter
author_rank: Super Contributor
timestamp: April 27, 2014, 11:10:57 am
content: Measure what all of the switches are to figure out what you need to do to make them live happily along side, then measure what voltage you get off a known pulldown on the radio input, (by knowing the internal pullup resistance you can calculate to see what ballpack your values will be in)the inputs should span between 3.3V open and GND usually for a single switch, floating down in a few hundred mV will not work for more than 2 switches, aim for atleast 500mV between switches,

author: bobhaha
author_rank: Contributor
timestamp: April 29, 2014, 01:32:23 am
content: I ended up getting it working.I used two 1N4007 diodes in series to drop the voltage.Looks nice and clean... ( disregard the cable tied antenna connection. Need to buy the right adapter )Now I'm having issues with this stupid chinese headunit remembering my buttons settings after powering off :/