The ultrasonic has been tested to be working, with some caveats:

• To filter noise, three measurements are averaged.
• The ultrasonic seems to pick up the sonic bounce from below it.
  • Solution: Use some kind of suppressor (e.g. sponge, cloth, etc.) around the sensor, especially the bottom side.

Since SIM800L module is kind of hard to test and debug, let alone make it works properly, this is an effort to see things more clearly.

The connections are as follows:

• VCC to BAT+
• RESET to BAT+ (or BAT- if you want to reset in the middle of testing)
• RXD to ESP32_TX2 (17)
• TXD to ESP32 RX2 (16)
• GND to BAT

• The issue that I was facing is that the module was constantly restarting, with repeating output similar to these when #define DUMP_AT_COMMANDS are enabled/uncommented.

RDY

+CFUN: 1

+CPIN: READY

Call Ready

SMS Ready

• When that happens, it means that the module is restarting on its own.
• Occasional warning such as these:

UNDER-VOLTAGE POWER DOWN

or

UNDER-VOLTAGE WARNNING

in conjunctions with the restarting behavior of the module may also happen.

• Kindly notice the 1000 uF capacitor is used to filter 2 amperes of transient spike that the SIM800 module produce when trying to connect. This often helps, but sometimes it doesn't :(.

To make connections between ESP32 and SIM800 works, since they use UART, they have to have a common ground.

To upload code:

• Make sure ESP32 is not connected to battery
• Connect to computer with USB cable
• Upload the code

And then, to let the system run:

• Disconnect the USB cable
• Connect ESP32 VIN and GND to battery module 5V and GND respectively
• Reconnect the USB cable so that you are able to debug from serial monitor

Reference:

• https://drive.google.com/file/d/19UxkkifQDwHhEvOkkO_yIXxYYQnAupFo/view?usp=sharing
• https://drive.google.com/file/d/19Wf8XxCUDNx_fklVCsJIVnZNRKKjZC-i/view?usp=sharing

• Kindly notice that the code for testing IS NOT ON THIS BRANCH (NOT ON main BRANCH) and is located in test/sim800l-mqtt branch instead, so that this main branch is free from test codes.
• It is recommended to enable/uncomment #define DUMP_AT_COMMANDS to be able to see what the SIM800 module output.