Voltage and current measurements.
Don't know if I'm going down the wrong path here but I though I'd check that I'm delivering enough voltage and current and it's not overly being soaked up by the relay and the 433MHz receiver.
While one of my 3 FTDI's is plugged into laptop and the SONOFF is connected to the 3v3 power pin, the SONOFF LED blinks green brightly every 2 seconds.
The measured voltage across the FTDI 3v3 and GND pins is 3.41 volts.
The current draw through the same pins is 38 mA.