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I wanted to expand some ideas about access control so the next step was to incorporate some devices that could be called with a cellphone to initiate some arbitrary action. The unit thus needed some GSM and Wifi capability which was provided by the A6 module and ESP8266 both of which are from Ai Thinker.
I bought a couple different preassembled A6 and A7 boards but as usual you get what you pay for and they were just junk. Problems on their PCB's and one version that had the RF antenna shorted to ground! I wasted a lot of time trying to figure out why these things wouldnt register on the GSM network and in the end some simple continuity testest showed they were just crap!
Here's a generic pic of one of the A6 variants I got. 2 of the 3 I got had the reset pin of the A6 module NOT broken out to a header pin. More wasted time trying to test resetting the module :)
Here's an A7 variant (GSM & GPS). Both of the ones I got of these had the GSM RF pin shorted to ground. The GPS RF pin was not shorted to ground. But since I was interested currently in the GSM functionality this was pretty much useless to me.
I decided to bite the bullet and just make my own. I had to remove an A6 module from one of those jumk boards as well as the large tantalum capacitor and a SIM card holder. Looking around online I eventually found a reference schematic from the manufacturer showing a typical application of the product. I ignored their integration of a switching regulator for DC to DC step down and instead supplied 5V with a wall wart and simply dropped it down a bit with a diode to get to ~4V and also used a linear regulator to give the 3V3 for the ESP8266. Simple enough.
Here's the A6 module I removed from one of the junk boards. I got a little close with the belt sander but it was OK :)
And heres the schematic. It's a little more complicated than it needs to be because I needed to make it in such a way that I could test turning the whole A6 off and on with a MOSFET and also resetting in the same way or just override that functionality with some jumpers.
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