The above figure shows an early Advent Controls Alpha GSM Gate Opener and Alarm Auto-dialer circuit schematic diagram. There have been a few updates to the design over time but the basic elements of the design remain.
The top left hand section shows the DC to DC converter (based on the AP5004 300kHz switching regulator) which takes the 12-24VDC supply and converts it to a lower voltage suitable for running the GSM chipset and base-band output drivers. The relay is also powered directly from this supply. For further details see this article here.
We then move on to the opto-isolated input circuit (based on the KB817 part) which connects the outside world to the low voltage digital section on the microcontroller. Please see this article for further details.
In the centre of the diagram we have the relay output section which is how the board outputs to the real world. For further information please view this article.
On the right hand side of the diagram we have a very interesting circuit. This section converts the audio output from the GSM chipset – the voices and tones heard through the phone – into a form which the simple microcontroller can understand. For further details on how this section works please see this post.
The bottom left of the schematic shows the heart of the system – the GSM engine built around the MediaTek MT6252DA built into a seperate module by our commercial partners Quectel. There is more information of this subsection here.
If the GSM engine is the heart of the system the PIC16F1825 microcontroller which runs our firmware is the brain of the unit. This component is an entire computer in one chip featuring RAM, a CPU and flash EEPROM program memory (effectively the internal hard drive). It handles all of the GSM commands, connects the input and output to the real world, performs Digital Signal Processing (DSP) on the all the of the audio received by the board and stores all of the user telephone numbers and setting. This device is what allows our board to be so compact.
We were asked what the two circles with dots in the centre were. These are fiducial markers which appear on the PCB. They help the pick and place machine which assembles the board accurately position the module in to place. The placement of this component is critical to ensure the antenna matches the GSM output.