My car as an auxiliary audio input for connecting MP3 players etc. For this I purchased a bluetooth speaker adapter on Ebay for about a tenner.
It's a great device to connecting my phone for sat-nav, music etc, but there's a small annoyance in the fact that after getting in the car, it requires me to press and hold the power button for about 5 seconds before it'll turn on and connect to my phone. It's a bit of a first-world problem, granted, but it's kinda irritating, so I've been thinking about how to solve it.
Ideally, it should function like a built-in bluetooth adapter does in many new cars - as soon as the car is powered on, the bluetooth is powered on and connects to the phone.
After some experimenting with various approaches - microcontroller delays, 555 timers, I found the most functional circuit was a simple capacitor delay circuit, combined with a transistor NOT gate & optocoupler, so that the optocoupler is active, effectively holding down the adapter's power button, for 5 seconds, then switched off until the circuit loses power.
|A rough schematic of the circuit I used|
Powering the circuit was simple enough - I already have a car 'cigarette lighter' to USB adapter with 2 USB ports - one powers the bluetooth adapter, and it powers on with the car ignition, so I added a USB plug to the timer circuit and used that.
Finally, I didn't want to leave exposed wires or circuit boards visible in the car - knowing my luck some paranoid idiot would freak and call the bomb squad or something, so I 3D printed a simple box enclosure which houses the adapter and the circuit nicely, the box itself nestling neatly in the centre console of my car.
|The finished adapter in it's 3D printed box, ready to put in the car|