For some time now, I have my Logitech WRLS SPKR ADAPTER SET, a pretty low cost bluetooth audio sink in the saloon. It is pretty handy to play music from a laptop or tablet (or mobile phone) through a set of active PC speakers. OK, they are not the premium sound quality but yet much better than the tablet’s internal speakers!
A nice feature of this bluetooth audio sink implementation is the auto-reconnect feature: As soon as the tablet comes within radio range it will enable audio streaming via the bluetooth sink automatically. But the real-world convenience of all this is rather limited because the speakers/amplifier is not turned on/off automatically. Well, actually…. they are now! No need of any additional software on the computer and really full-auto: No power consumption of the amplifier at all while disconnected from bluetooth audio, auto power on on bluetooth connection!
The main idea of realization is based on the observation of the status LED of the bluetooth audio sink. A red led shows “disconnected state” a green led shows “connected state”. Basically, I branched from the LEDs wiring on the PCB and connected the signal to a NPN bipolar transistor that, in turn, drives the gate of a hi-side p-channel power MOSFET. In order to avoid using two power adapters I also added a 12V to 5V linear regulator to drive the bluetooth device from the speaker’s power adapter; as it only needs 20-30mA this ain’t causing problems.
Figure 1 shows a schematic of the two small PCBs inserted into the housing of the Logitech bluetooth audio sink and figure 2 the three solder points for the “INPUT” signal (to the very left in the schematic, on the lower side of the resistor on the PCB), +5V and GND (to the very right in the schematic, around the original power input plug of the PCB).
The red led is switched on it’s low side, i.e. the solder point features a high signal when the LED is off (the case when bluetooth audio is playing) and a low signal when the LED is lit (on standby). The BC548C NPN bipolar transistor does the signal level conversion from the low-voltage bluetooth PCB to the 12 volts of the speaker’s power supply. As it happens, the very same circuit could handle up to 30V (probably) without any problems and up to 18A of current (with thicker wires, of course). I used a high-side P-channel MOSFET because the low-side (GND) is probably steadily connected to the speaker’s amp via the audio cable, so high-side switching is a must!
The lower side of our home-made switch PCB looks like that:
Fully assembled it looks like that with coaxial power connectors for 12V input to the system and the 12V output to the speakers already added:
Luckily, the case of the Logitech bluetooth audio sink is that much over-dimensioned that the additional PCB and the coaxial power plugs fit right inside:
Now, the power adapter from the speakers plug into the box, and an outgoing auto-switched wire goes to the speakers. The auto power-switch feature works seamlessly with any bluetooth source! Just connect power-in 12V, power-out 12V (speakers) and the audio signal!
Please note: This hack will pretty certainly void your warranty! I will not be responsible for any damage to any device or whatsoever that goes south if you fail! Hack your status-LED aware bluetooth device on your own risk!