My first post here, I'm relatively new to electronics and this is my first project that I'm trying to accomplish myself.
First off, some background of what I'm trying to accomplish. I am trying to build a CD Emulator for VW Golf MK5 and have it use a bluetooth module. I have got the emulator and bluetooth part sorted out, my only problem right now is the transistor switch.
Please note that U4 (JST-XH-04) connects to a 12V to 5V DC-DC Voltage regulator, I haven't marked the regulator in the schematic. VCC is 12V.
Both VCC and signal come from the radio, VCC is always on, even with car locked and key out, the signal however is at around 11V when the radio is on (it switches between 12 and 11 depending if engine is running or not) and is off when the radio is off.
Now if I plug in U4 GND and 5V to a USB (off a power supply or computer usb port) and plug in J1 Signal to another usb port (5V) everything works exactly as expected (note: the bluetooth module doesn't have enough power to play songs in this state, as the base resistor was chosen for 12-11V, however it still turns on and connects to the phone, as does arduino). So, using 2x usb cables and connecting them to the 5V, GND and Signal lines, this circuit works exactly as I would expect - when signal is low, circuit is off, when signal is at 5V, the circuit turns on.
However, when I connect it in the car, about 70% of the time the circuit does not fully turn off. At first I thought it could be a floating base, so I connected it to ground through a resistor (should have done from the beginning, but better late than never..).
Then I thought it could be a bad transistor, but I have tried 4 different ones now. I have also tried swapping collector-emitter, just in case I got it wrong, but the resistor went up in smoke fairly quickly when i tried that, so the polarity is also correct.
I have only 1 idea left - somehow the arduino grounds itself through CLK/MOSI/DATAIN, and the circuit completes that way. What should I check to confirm/dismiss this possibility?
Am I missing something? Does anyone have any better ideas?
Again, this is my first project where I'm trying to figure things out on my own, so my terminology or approach will be far from perfect, but I thank you for any and all help.
The circuit is available at https://easyeda.com/binginsin/rcd300-cdc . Anyone is welcome to add to it.
UPDATE: I have done some testing and found interesting results - on my 2x USB setup, I connected CLK / MOSI / DATAIN to ground without connecting the Signal. This ended up turning on the circuit, so the Arduino just grounds through it's digital pins... I will do some measuring in the car as soon as I have a chance - I will check if any of the CLK/MOSI/DATAIN lines have a connection to ground (car chassis) when the radio is off. I expect it will have a connection to ground with quite a high resistance. If that is the case, how would I go about ensuring that the arduino does not ground itself through the digital pins? Is that even possible? Is my arduino a dud?