Simple circuit transmiting voltage when powered on isn't working correctly

[mattek]

Hi there!

I would like to say hello to everyone here. I treat electronics and all this stuff as one of my hobbys, learning by my mistakes and knowoledge based from the internet. It gives me huge satisfaction, however, I'm newbie so maybe this is one of the reasons why I decided to post here? Nevermind, let's go

I Recently decided to built my first project with Arduino Uno - it's CD changer emulator, which connected to car's head unit, unlocks hidden AUX port. It works just flawlessly, however contains one disadvantage - this emulator uses constant 12V from headunit'd connector and even with radio turned off, voltage is still alive. In the same connector, there is another pin with about 8.8V which is only active when radio is turned on.

I found simple solution, just something like a gate which opens, when 8.8V appears, here's schematic:

Left, upper side with mosfet, transistor and two resistors. This solution was used with similar emulator but built using ATMega and confirmed to be ok.

Decided to built it, gathered all components and suprisingly, it doesn't want to work. I connected everything like this, hope it's proper connection.

Checked Mosfet with it's resistor and after giving 12V to G and S, it opens at D. Second transistor is "ok" too I think, from B to C or E, gives me 0.7V and in another cases, my multimeter shows gape.

Anyone sees something wrong with this simple circuit? I would be very thankful for the solution.

Best regards!

EDIT: Gosh, I forgot to say what's the main problem: when I give 12V, as schematic shows, I get 12V on D. It should open after 8.8V is given, not needed 12V.

[tpowell1830]

Looks like R3 is much too large to drive the NPN, try a 1k resistor.

[mattek]

Yeah, but you read my post before editing, I just forgot to say what is going on.

When I give 12V as my schematic shows, D is already opened, wihtout connecting 8.8V to CD_ENABLE pin.

Sorry!

[tpowell1830]

Yeah, but you read my post before editing, I just forgot to say what is going on.

When I give 12V as my schematic shows, D is already opened, wihtout connecting 8.8V to CD_ENABLE pin.

Sorry!

Mattek, I forgot to say welcome to the forum.

The IRF9530 is an interesting choice, did you look at the specification sheet?

Here is the link: https://www.vishay.com/docs/91076/sihf9530.pdf

Hope this helps...

[mattek]

I'm very sorry but I don't know how could I interpret your suggestion with this mosfet, I don't have so much knowledge about this :/
IRF9530 isn't my choice, whole schematic isn't mine, but author confirmed that this works.

[llkiwi2006]

How much current is it drawing from the 12v rail? If it's small enough just don't worry about it.

[mattek]

I can't check it now, my soldering iron got broken yesterday's evening, sorry! I need few days to get replacement.

But current is enough to run whole Arduino and other stuff connected to it.

[mattek]

Hi guys,

I found the problem. I had CD pin disconnected somewhere in my assembly and that was the reason why circuit didn't work properly. Thank you for any answers here.

Best regards