Sim800L works until I add to final circuit

Well - I am at wits end again.

Working condition:
I have a Sim800L EVB hooked to a Nano through softwareserial. I am powering the Nano and the Sim800L through a 3A boost converter hooked to a 3.7v 3000mAh Lipo. I am boosting to 5v since this particular SIM800L takes a 5v in and its logic is safe with 5v. With this setup I can consistently connect to the network and send an SMS text.

Non-working condition:
I added a RTC circuit that turns the power on once per minute (at this point - in the future, once per day). This circuit works when hooked to the Nano and it works when connected to the Nano and the Sim800L once again hooked through the boost converter, but the Sim800L only powers on, does not find a network and does not send a text. Attached is the circuit for the RTC. I would think based on the MOSFET IRLZ44N, it would be able to handle the 2A current needed by the SIM800L.

Any thoughts would be appreciated.

Code (I have some items commented out for testing):

#include <DS3231.h>
#include <Wire.h>
#include <SoftwareSerial.h>

//Create software serial object to communicate with A6
SoftwareSerial mySerial(3, 2); //A6 Tx & Rx is connected to Arduino #3 & #2

DS3231 Clock;

// constants won't change. They're used here to set pin numbers:
const int FloatPin1 = 6;     // the number of the pushbutton pin

// variables will change:
int FloatState1 = 0;         // variable for reading the pushbutton status

void setup() {

  // Start the serial port
  //Begin serial communication with Arduino and A6
  // Start the I2C interface

  // initialize the pushbutton pin as an input:
  pinMode(FloatPin1, INPUT);
  FloatState1 = digitalRead(FloatPin1);
  // Alarm is not enabled! Should set alarm
    // 0b1111 // each second
    // 0b1110 // Once per minute (when second matches)
    // 0b1100 // Once per hour (when minute and second matches)
    // 0b1000 // Once per day (when hour, minute and second matches)
    // 0b0000 // Once per month when date, hour, minute and second matches. Once per week if day of the week and A1Dy=true
    // Set alarm to happen every minute (change to your wanted interval)
    Clock.setA1Time(1, 1, 1, 0, 0b1110, false, false, false);



  mySerial.println("AT"); //Once the handshake test is successful, it will back to OK
  mySerial.println("AT+CSQ"); //Signal quality test, value range is 0-31 , 31 is the best
  mySerial.println("AT+CCID"); //Read SIM information to confirm whether the SIM is plugged
  mySerial.println("AT+CREG?"); //Check whether it has registered in the network

  // read the state of the float switch value:
  FloatState1 = digitalRead(FloatPin1);

  mySerial.println("AT+CMGF=1"); // Configuring TEXT mode
  mySerial.println("AT+CMGS=\"+17175755931\"");//change ZZ with country code and xxxxxxxxxxx with phone number to sms

  // check if the pushbutton is pressed. If it is, the buttonState is HIGH:
//  if (FloatState1 == HIGH) {
    // Send text below 1/3:
//  mySerial.print("Tote 1 - National Drive, Garland, Below 1/3"); //text content
//  updateSerial();
//  mySerial.write(26);
//  Serial.println("");
//  Serial.println("FloatState1 High");
//  delay(10000);
//  } else {
    // Send text above 1/3:
  mySerial.print("Tote 1 - National Drive, Garland, Above 1/3"); //text content
  Serial.println("FloatState1 Low");
//  }

  // Reset alarm to turn off the device


void loop()
//  Serial.println("Hello");
  // Nothing to loop

void updateSerial()
  while (Serial.available()) 
    mySerial.write(;//Forward what Serial received to Software Serial Port
    Serial.write(;//Forward what Software Serial received to Serial Port

This is the SIM800L EVB I am using:

If it is a current issue, should I put a solid state relay between the battery and the Sim800 and use the rtc circuit to trigger the relay?

The point of all this is to get real long battery life and more stuff means more energy drain.


Make sure the SIM800 is powered up for long enough. It takes some time to connect to the network, possibly a couple of minutes if I'm remembering right.

Those modules seem to be very finicky about power. It's possible the mosfet is introducing sufficient resistance to disrupt it. Narrow the problem down by using the same sketch but remove the mosfet and power the Sim directly. You could also try adding a capacitor across the sim's power lines. Say 2000uF.

your boost converter drains your batt that's one, get rid of it by having sim800L of this version


that version takes all batt voltage in ( no dropping )

the version you have drops about 0.6 V ( because of that big black diode you see, some version has two diodes ) from your precious battery, that accounts a lot for terrible voltage drop when connecting to network , sending SM or making http request)

wires between battery and sim800L should be thick and short,

you don't need a transistor to turn on/off the module, use AT commands to put it int sleep mode( or wake it up)

you can either use AT+CFUN=0 to deep-sleep ( flight mode) and AT+CFUN=1 to wake it up

you will not receive any call or SMS when in flight (airplane) mode

or you can use AT+CLSK=2 to put sim800L into sleep , wakes up only if there is 'something' at it's serial terminal , then goes back to sleep after 2ms when idle ( this saves current from 13mA down to 1mA , i measured it with network LED stripped off )