I was originally working with adaFruit’s sim808 shield ($50) but am trying to get a cheaper solution, so I got one of these
on eBay for $16. Instead of the LiPo battery that adaFruit insists on using with their sim808 boards, I’m using a car battery (12.4V) and a buck dc-dc convertor.
The spec for this AND Tech breakout board (see link above) clearly states that it will take 5-10V, 5V being recommended. It actually has 2 voltage test points, one should yield 2.8V and the other 3.8V (regardless of whether you’re feeding in 5-10V I assume).
The actual sim808 chip (module) on the breakout board wants 3.4-4.4V, so the breakout should regulate down to that voltage, but does not seem to be doing this correctly, so I’m curious if others have had this issue. Here are the values I’m seeing:
Voltage in from buck dc-dc convertor, Test point 1, Test point 2
4.80 3.88 2.81
5.00 4.09 2.81
5.07 4.15 2.81
5.10 4.18 2.81
5.15 4.25 2.81
5.22 4.30 2.81 – inconsistently yields “Overvoltage alert!”
I assume column 1 should be anywhere between 5-10V and columns 2 and 3 should be constant at 3.8 and 2.8 respectively. I thought if I gave voltage in < 5V it would complain, but it’s actually functioning better around the 4.8V range :
Any thoughts/help/experience is appreciated!
I just realized that Test point 1 is what the sim808 chip is actually seeing. Voltage at Test point 1 is 3.9V and when I request battery voltage (using AT+CBC) I get 3911mV.