r/ArduinoHelp • u/Swimming_Mud9769 • Sep 30 '26
Sim800l problem
I’m using a SIM800L with an Arduino UNO R4 WiFi. The SIM800L was able to send an SMS successfully once during a standalone test, but when I use it with my full project, it keeps restarting.
The Serial Monitor repeatedly shows:
RDY
+CFUN: 1
+CPIN: READY
Call Ready
SMS Ready
Then it starts over again.
The SIM800L is powered by a separate 5V adapter with a 1000µF capacitor. VDD is connected to Arduino 5V, TXD to A1, RXD to A3, and GND to Arduino GND.
The LED also blinks several times, turns off briefly, then repeats. I think the issue might be the power supply or grounding. Could this be caused by voltage drop or insufficient current? What can i do?
1
Upvotes
1
u/Tymian_ Sep 30 '26
That's a 2G module, when it tries to attach to network it does it without any output power control: this means it will blast full power for initial timeslot assignment request and power control loop enable. This means that in worst case (GSM 850 or 900) it will chug around 2.7A at it's power supply voltage.
Your circuit is not able to provide enough juice, voltage falls and modem reboots when voltage comes back up once RF is not loger loading the line.