LilyGo Sim800C vs Tiny Sim800C Module, Why so many components on LilyGo?
The circuitry around the SIM card itself is mostly for ESD and short-circuit protection. There are TVS diodes (ED7..ED10) and an ESD protection array (U11) to protect the circuitry from someone touching the SIM tray while being electrostatically charged. Without these components, the board might get damaged if you touch it. While this might be a &quot;one in a thousand times&quot; chance, it can still happen without the protection circuitry. The resistors in-line with the SIM's data pins limit the current in case of a short-circuit, which is very likely to happen during SIM card insertion and removal.
The circuitry containing the NPN transistors Q3 and Q4 is a level translator for the UART lines. The ESP32 runs off 3.3V while the SIM800C uses 2.8V for its &quot;VEXT&quot; supply. This means that the logic levels of the ESP32 are slightly incompatible with the SIM800C's. While the GSM_TXD line going from the SIM800C to the ESP32 most likely didn't need a level translator (2.8V is well within the input range of 3.3V CMOS), the same isn't true for the GSM_RXD line. Pushing 3.3V into an input pin expecting at most 2.8V is well within the &quot;danger zone&quot; where the high external voltage will activate the SIM800C's internal ESD protection diodes and might potentially cause damage to the chip. While this might &quot;work&quot; for a quick breadboard experiment, it is by no means good practice and the level translator should definitely be included.