Sony Ericsson MMR-70 transmitter LED mod for Raspberri Pi

My Raspberry Pi based Internet Radio/Weather Station is going to get a new feature soon - an FM transmitter. You can buy a discontinued Sony Ericsson MMR-70 FM Transmitter for less than 2 EUR now and it can be connected to Raspberry Pi quite easily. Tobias Mädel has done excellent work putting together a small FMBerry daemon which can control MMR-70 connected to RPi's I2C bus. In this case Raspberri Pi is connected directly to I2C pins on Alps TSMZ1-603A FM transmitter (built on NS741 low power FM transmitter chip). Onboard Atmega 32-M is not used and forced to a sleep state to avoid any control conflicts.

MMR-70 has an LED, but it's connected to Mega32 pin 16, so a bit of high precision soldering is required :) To control the LED we need to solder wire directly to the LED or close to Mega32 pin 16:

Using high temperature isolation tape you can mask the control point close to pin 16 and then easily solder a wire to it.
As usual, to protect RPi GPIO, a transistor is used as a buffer to power on the LED:

Now we can flash he LED every other RDS refresh cycle (one cycle - 80 RDS frames):

This is how it looks with a logic analyser:

Channel A - LED flashing, Channel B - RDS interrupts requesting next frame to be transmitted. So it takes 0.5 ms to process a frame request, then MMR-70 transmits this frame ~21.5 ms.

Links:
FMBerry on GitHub
MMR-70 discussion on www.mikrocontroller.net 

Note: even without the LED lit up MMR-70 could take up to 27mA, which is a half of recommended RPi's load for 3.3V supply. With the LED turned on consumption rises to 36mA, so make sure you are not overloading your RPi if you have other devices connected to 3.3V. For my radio I'm going to solder TS2950-33 voltage regulator to MMR-70 testpoints to be able to use RPi 5V supply and connect MMR-70 to I2C bus 0 on P5 so it will not directly connect to RPI's 3.3V.