I am looking for a way to hard wire this cheap eBay Bluetooth receiver to my car.

You have to press the middle power button (also pairing button and call answer button) for 3 seconds to power it up. Short pressing the same button during an incoming call answers the call. It has a built in battery so needs to be charged via USB. What I would like to do is have this powered on with the car radio 12v remote wire without having to press the power on manually each and every time I get in the car. I would also like it power off with the car stereo.

Can someone help me with a circuit using transistor to help me achieve this. Bearing in mind, I would still like the call answer feature to also work (same button) which I will wire to a button on the steering wheel remote. A capacitor and resistor across the switch may work but will short out when answering calls. I think this may be able to be achieved with a transistor. I have seen something similar somewhere but here I need it powered on when it sees power from the radio remote whilst still having a fully functioning call answer button and not short out when pressed. Any help appreciated.

 

You may be able to create a one-shot type of circuit with an RC element to it. This definitely requires more than just a transistor or a capacitor.

A few things that you'd have to worry about:

1. This device probably runs at or around 3.3V. You will have to take care in interfacing your car's 12V (which by the way can range anywhere between 15V when on and 10v when off depending on the condition of your battery)
2. Charging, again, would require you to step down that ~12V to a usable 5V USB charging level.

I put together something I could think of off the top of my head. You'd have to tune R2, R3, C3 to get the results you'd want. You'd also have to figure out how to reset your latch when the car turns off. The important part here is the use of K1 which isolates your 12V car voltage to the bluetooth device's



Hopefully that gets you started!

 

Hi thanks! I forgot to mention. I would be using a car usb charger to power up the Bluetooth Receiver. What would be the role of C1 here. I can see R2 and R3 is used as voltage divider.

 

Just a thought, can I not just put the car usb charger in the place of the RC element and use the stepped down voltage to feed in the 3.3v?

 

Yup, if your accessory port turns off with the ignition, you could use the 3.3V completely off the device itself. No need for a bulky relay!

 

Can you talk me through what the c1 does? Also the flip-flop / latch switch would need to be resetted. I am thinking there might be a way to use a transistor to switch on the device for 3 seconds using a capacitor to control the on time length. Do you not think that this is no way possible without then latch switch? This way there wouldn't be the issue of resetting a switch and eliminates any mechanical components?