Tuesday 25 March 2014

Hacking the Link Bluetooth receiver for auto-on and off (with power supply)

The Link Bluetooth APTX audio
receiver (BT V4.0 with AD2P)
The Link Bluetooth is a cheap Chinese Bluetooth audio adapter providing a very decent audio quality. With this adapter you will be able to transform most stereos (or in my case the Wooden Getthoblaster I'm building) to receive Bluetooth audio.

The Internet is full of cheaper (about 5€) Bluetooth receivers but these usually come with a bad sound quality and for this reason I decided to buy the cheapest APTX Bluetooth 4.0+EDR, AD2P adapter I could find (22.99 USD):

The problem is that the engineers who designed this adapter obviously did not think about the use I was going to make of it (shame on them ;-):

The device does not turn on automatically when plugged in

I want my Bluetooth receiver to automatically turn on when powered so I can hard wire it inside my stereo and not have to press any button to make it work. I was really disappointed it wasn't the case and decided to open the 'Link' to see what I could do.

To open it you need to remove the pads underneath and use a very tiny PH screwdriver (#00) to remove the four screws:
See the size of these screws? I used
a #00 PH screwdriver to remove them
Once open you find a battery (Li-ion, probably) which is going to be problematic if you keep on reading.
The Link APTX Bluetooth Audio receiver open

My first attempt at shortening the middle button permanently was working for the automatic power on but came with other problems: every now and then my playback would randomly stop without warning (this button is also used for play/pause when short pressed)... which is strange.

I eventually solved the issue with a capacitor. I used a 220µF 16V and it worked right away (but there is no reason to think other values would not work). When powered off, the capacitor is empty and therefore behaves like a closed circuit (or button press). The capacitor then charges opening the circuit ... and no random button presses ! Hurray !

If you do this remember to use a capacitor rated for at least 5V and mind the polarity (negative is on the right on the picture below).

Testing my auto-on feature
with a capacitor across the switch

The device does not turn off automatically when power is removed

The Link receiver comes with a tiny battery. This is cool if you want to use it on the go but is a real pain otherwise. If I switch my stereo off, I'm experting my phone to disconnected and it wasn't the case as the Bluetooth audio receiver would continue running in the background until the battery dies.

I therefore tried disconnecting it:

Disconnecting the battery leads inside the Link
audio receiver
No luck: the Link randomly rebooted while being used. I guess the battery is for current regulation. I therefore added another capacitor I had at hand: a 10V 2200µF (mind the polarity and the voltage which should be at least 5V) but such a large capacity is probably overkill (you'll need to try for yourself). I don't care, it works well (no more crashes), and the capacities it empties quickly when power is removed: my Bluetooth receiver switches off as soon as power is removed.

Replacing the Link battery by a regulating capacitor

Removing the humming

When powered on and connected but without music you can hear a (quiet) background 'hummm'. You can reduce this noise by disconnecting the blue diode:
Disconnecting the blue diode (and with the capacitor
properly connected.
Oh, and by the way... the packaging says: "Bluetooth Signal Light effect make it much elegant and noble". I guess his Lordship shall lay off his minstrels now.

User manual & specifications of the Link Bluetooth Music Receiver

  • Bluetooth 4.0+EDR
  • Support CLASS 1 and CLASS 2
  • Support A2DP, APTX
  • Transmission distance is not less than 10M
  • NFC fuction is optional
  • Inner 250mA Lithium battery
  • Size: diameter 52mm x 15mm
PCB reference/version: LinK-485BTJSQ V1.2_2013_10_09

The Link Bluetooth APTX receiver manual (P1)

The Link Bluetooth APTX receiver manual (P2)

The Link Bluetooth APTX receiver packaging