FlipperPhone - diy SIM800l GPIO module for FlipperZero. All documentation for the SIM 800 l is in the folder.
https://youtu.be/eFofI0v5RhA?si=0sd0RvA5URh4ZR1d
The first thing to start with is the SIM800L power supply. According to the specification, the module is powered by a battery, requires a voltage of 3.4-4.4 volts and can consume up to 2 amperes of current during network search! With normal activity, it consumes 20mA and only 1ma during sleep
FlipperZero has a 5 volt 2 ampere line, so this is enough to power SIM800L. 5 volts are lowered to the required voltage using two series-connected diodes, thus, the power supply from 5 volts is organized for the blue version of the SIM800L, you can see two diodes in the upper right corner of PCB. Also, two capacitors need to be supplied along the power supply line of the GPRS module - an electrolytic 220uF and a ceramic 100n
Lowering the voltage using two diodes is a rather unstable solution. It is very important to select diodes based on current strength and voltage drop, but even this does not guarantee a stable output voltage
In this board, I limited myself to a solution with two diodes, since it takes up minimal space, and the printed circuit board is single-sided and designed for home production. I didn’t encounter any critical problems during the module tests, but it’s still important to remember this point. Write if you know convenient alternatives, please. The advanced version of the module will have a step-down stabilizer
The PCB and the schematic are drawn in the EasyEDA. The current PCB is designed for self-made production, therefore, the board is one-sided, I used the toner transfer method to make my own. Blue is the lower layer, we transfer it to the textolite
Use the following settings for the print file
The signature "GPRS" on the board will help not to confuse the side of the picture:
On paper, "GPRS" should be mirrored
And on the PCB, the inscription should be read correctly
Put sim card in the SIM800L
Put the module in the FLipper's GPIO pins. Open "GPIO" settings and turn on "5V on GPIO". Red LED will start blinking - the module is ready
Open GPIO App "UART Terminal", choose 9600 baud rate - now you can control SIM800L with AT comands
All commands can be viewed in the manual
List of useful commands:
AT+CPAS - module status: 0 - ready to work, 2 - unknown (command execution is not guaranteed),
3 - incoming call, 4 - voice connection
AT+CSQ - signal quality: 0 -115 dBm or less, 1 - 111 dBm, 2-30 -110..-54 dBm, 31 -52 dBm or more,
99 -unknown or no signal.
AT+CCID - getting a SIM card number
ATD+phonenumber; - call to a phone number, example of a command "ATD+79876543210;". Don't forget to put ";"!
AT+CSQ
ATD+phonenumber;
The SIM800L has the ability to work in radio mode, control is performed via AT commands, and sound is output to the speaker.
Warning! The Red PCB SIM800L module does not have a pin for connecting an FM antenna! I couldn't catch a legible radio signal without an antenna. But there are videos on the Internet where people manage to catch the radio without an antenna. So you can try to catch the radio using the specified AT commands - if there is only noise, you will have to shove the antenna, if the radio is working - cool, you are very lucky!
RU article about antenna connection - photos and instructions are taken from there
AT+FMOPEN=0 - turn radio mode ON
AT+FMVOLUME=6 - volume selection from 0 to 6
AT+FMSCAN - auto scan of the FM band
AT+FMFREQ=960 - radio frequency selection, range 875-1080, 10x frequency in MHz
AT+FMSIGNAL=960 - returns the signal strength at that frequency, but dips the audio
AT+FMCLOSE - turn radio mode OFF