I have been trying for some time to create a custom board based on LilyGo ESP32. I found the schematic too complicated for me and that is why it took so long.

Now I came through a "tiny" SIM800C board Board was found here

Why in LilyGo board they use a so complicated, expensive way with so many extra components to do the "same" thing? Schematic is here on page 3.

I am not talking about Power which obviously is not included on the power board, neither speaker and mic.

But look at the Sim card, PWERKEY, GSM_RXD, GSM_TXD, S_DTR, S_RI The tiny board can be perfectly connected to the ESP32 using the TXD, RXD pins and work like a charm. Example is here without all those extra components.

What is the point? What am I missing?

Simcard is so complicated on the LilyGo while on the tiny board it looks so simply connected.

EDIT: I do understand that the small board cannot work alone and it needs the ESP32.. I am only wondering why it has so complicated schematic the sim800 part of the lilygo


1 Answer 1


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 "one in a thousand times" 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 "VEXT" 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 "danger zone" 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 "work" for a quick breadboard experiment, it is by no means good practice and the level translator should definitely be included.

  • \$\begingroup\$ So the tiny board is just for experiments and not for continues use.. right? \$\endgroup\$
    – Kris
    May 15, 2021 at 11:48
  • \$\begingroup\$ @Kris You can use the tiny board for permanent use as well but it doesn't contain the ESP32 that the LilyGo board has. You have to add the ESP32 externally (and the level translators too). You shouldn't just connect it directly to an ESP32 as done in the example you linked. \$\endgroup\$ May 15, 2021 at 11:55
  • \$\begingroup\$ yes I understand that(the example I have on the post is esp32+the small board). I am just asking because you said that without the extra components If I touch the board if might get destroyed or without the resistors on the sim data pins short circuit might still destroy the board.. \$\endgroup\$
    – Kris
    May 15, 2021 at 11:58
  • \$\begingroup\$ @Kris You shouldn't have to worry about ESD if you put the SIM800C board into another project as you won't be handling it all the time. Chips today also aren't as sensitive to ESD anymore as they used to be a few decades ago. And as long as you don't insert or remove the SIM card while the board is in operation, a short-circuit is harmless. The LilyGo board just goes to great lengths to make sure that the board never gets damaged. \$\endgroup\$ May 15, 2021 at 12:08
  • \$\begingroup\$ Thank you I got it now. Probably since the lilygo board is without a plastic box etc its very easy to get damaged without those extra protecting components \$\endgroup\$
    – Kris
    May 15, 2021 at 12:13

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