Hello,

I'm trying to make a Bluetooth adapter for a car head unit. Bluetooth is done by CSR8675 chip, that has programmable inputs, outpus and also programmable LED controller. Adapter connects to head unit trough back connectors for aux in and telephone input.

My problem is: to activate phone mode on car a pin on head unit's connector must be grounded. As led driver on CSR8675 chip is basically a ground on off switch, i tried to connect it to head units mute pin, but it's not working. If LED is connected as per datasheet, it's working as expected - lights up when phone call is active.

LED driver circuit from datasheet.

 

I presume you have tried physically grounding the mute pin to check it works to rule out the bluetooth chip as the problem?

 

Mute pin is on head unit. Yes, I tried it, it works. It works also up to 150ohm resistor in series to ground. Any higher resistance and it doesn't work.

Head unit is Audi A4 B7 Concert 2+.

 

Solved problem myself. I didn't realise at first, that on the bench both devices are on different power supplies. All I had to do is connect grounds and It worked like charm.