dannable:
Despite the fact that the SIM800L runs off 4 volts, the maximum voltage on the signal lines is 2.8 volts.
I see! didnt realise that, so in theory I could just use another DC to DC buck converter, crack it down to 2.8v and run the output on the buck to the SIM800L?
Converters i'm looking at: https://www.amazon.co.uk/eBoot-LM2596-Converter-3-0-40V-1-5-35V/dp/B01GJ0SC2C/ref=sr_1_1_sspa?ie=UTF8&qid=1508328620&sr=8-1-spons&keywords=buck+converter&psc=1
I also think I am just going to use a 12v battery instead of going down the USB route so the voltage is always high enough for the buck converter.
I am reading on this page http://www.raviyp.com/embedded/203-sim900-sim800-not-working-possible-reasons that,
"You need to provide the proper duration ON pulse on the PWRKEY pin of SIM900/SIM800 to turn it ON. Not too long and not too short. 1 second pulse is recommended."
Not sure what this means, my thinking is that its not just as simple as running the power line from the output of the buck converter directly to the SIM800L, or is it?
Thanks,
-Anton