zdebel wrote:GPIO15 must be pulled down to GND, that's the only requirement. It is possible your module didn't come with firmware loaded into it. To flash it, you need to pull GPIO0 down and do a reset, it'll go into the bootloader. The tricky part is the reset isn't broken out, you need to solder a wire to a spare goldepin from a not installed resistor near the red wire. It is mentioned in another topic.
I did start to think that perhaps my module did need firmware flashing, and I also read the thread about the XPD_DCDC pin connecting to GPIO16 and optionally to RST. I was left a little unsure of the implications - although I definitely also need to go into a very low power mode so that I can make periodic wake-ups, send sensor data and then carry on saving battery life.
I'll post a question in that other thread about the implications of making the solder bridge to bring reset to GPIO16.
Thanks for the info anyway!