Introduction: Auto-switching Bluetooth Amplifier

About: I enjoy making things - both hardware and software. I run a small company that makes the Open Source Espruino JavaScript interpreter ( as well as the R4 and Morphyre Music Visualisers (…

In my front room, I have some big speakers and an amplifier connected to my TV. However sometimes, I don't want the TV on, and don't want the big clunky amplifier - I just want some background music, played off my phone, that I can turn on and control wirelessly.

That's a problem - because the amplifier expects to be connected directly to the speakers. The only option would be to leave the amplifier on, or to somehow control mains voltages from signals on a small Bluetooth receiver.

In this instructable you'll modify a SANWU Bluetooth Audio Amplifier so that it'll control a set of 4 relays. Whenever the Bluetooth amplifier needs to play music, it'll switch them over. When it doesn't need to play anything (including when it's connected via Bluetooth but no music is playing), the relays are in the default position which leaves the amplifier connected.

The relays are rated 10A at 250V, so should happily work in line with most amplifier/speaker combinations that'll be used as sensible volumes in a home.

You'll need:

  • A Bluetooth amplifier (to be sure the pins are the same use a SANWU 50W+50W TDA7492 CSR8635)
  • An LP395Z transistor (a FET or any other transistor with built-in protection resistors will do)
  • Two, 2x Relay boards
  • Some wire
  • A board to mount everything on

Step 1: Getting the Signal

The SANWU amplifier I used contains a CSR8635 bluetooth receiver. While the board doesn't have an output we need, the CSR module does have a pin that does what we want. It goes high when something is playing, and is low when it isn't.

After checking each pin, I discovered it is Pin 8 (PIO9) - the 8th pin down from the gold dot by the aerial.

However this pin probably acts as a power signal to the on-board amplifier. We don't want to just connect it to our relay modules (which also expect a signal of the opposite polarity). Instead, I wired the LP395Z transistor base between to PIO9 (Pin 8) and the emitter to GND (Pin 17) - this then creates an output on the collector that is not connected when no sound is playing, but is shorted to ground when it is.

Step 2: Wiring the Relays

So now all you need to do is wire the relays up.

  • Connect GND of the Relays up. I used the big tab on the voltage regulator (see the picture).
  • Connect VCC on the relays to the 5V output of the voltage regulator (leave the jumper on the relays between JD-VCC and VCC).
  • Connect IN1 and IN2 of both sets of relays to the LP395Z's collector as in the picture).
  • Connect the speaker outputs from the SANWU board to each of the relay's NO (Normally Open) pins

Step 3: Fitting/Finishing

To keep everything in place I screwed everything down to a perspex sheet with 3mm screws and 3mm thick washers. I then have another sheet of perspex that can be screwed to the top when everything is wired up.

To wire this to your speakers/amplifier, just:

  • Connect speaker left/right to the COM pins on each of the 4 relays (matching where you connected the SANWU board to)
  • Connect the amplifier outputs to the NC (Normally Closed) pins in each of the 4 relays
  • Connect a power supply (8 ~ 25V) to the SANWU board

And you're sorted! Normally the amplifier will be connected to your speakers, but as soon as you connect to the SANWU board with Bluetooth and play something, the relays will switch over to the SANWU board.