I have a circuit where I am using a SIM900 GSM/GPRS module. As per the datasheet it needs 3.4 V - 4.8 V and upto 2 A of current. I am using LM2576. The input to the regulator is 12 V & 4.2 A and output (which is going to SIM900) is 4.175 V. I have connected leds for NETLIGHT and STATUS LIGHT. I have connected a switch between PWRKEY and GND to start the SIM900. Following is the schematic of regulator:

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The problem I am facing is as soon as I press the switch S1 to turn on the SIM900, its netlight led starts blinking and status light led is always off. What does this means? Is the module faulty? How can I debug it?


  • \$\begingroup\$ Did you tried to use the debug port? \$\endgroup\$ – Adam Calvet Bohl Dec 6 '16 at 6:32
  • \$\begingroup\$ Is it a brand new modul or was it previously used? As @Adam suggested try to send a simple command to it and see if it answers. If the netlight is blinking then the module should be working on some level. Also you have missed to connect one GND pin, number 29. \$\endgroup\$ – Bence Kaulics Dec 6 '16 at 9:22
  • \$\begingroup\$ @BenceKaulics Thanks for pointing out that 29 GND pin. Is it possible that the module is not turning on due to this unconnected GND pin.? \$\endgroup\$ – S Andrew Dec 6 '16 at 10:38
  • \$\begingroup\$ @BenceKaulics I connected the pin 29 to GND and tried but still the problem is same. I would also like to add that net light blinks whenever switch is pressed but as soon as I release the switch, it stops. \$\endgroup\$ – S Andrew Dec 6 '16 at 10:56
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    \$\begingroup\$ @MHD Thank you but the problem has now been resolved. I changed the inductor and used a 330uH inductor. I guess inductor was blocking the current needed by the SIM900. \$\endgroup\$ – S Andrew Jan 18 '17 at 11:12

From the document "SIM900 hardware design:"

enter image description here

Seems that you might be up and running!

Time to connect the serial port...

If all else fails, some users report reflashing the firmware worked.

  • \$\begingroup\$ Yes, but according to Figure 11. the Status pin should be logic high after power on. But its not for the OP. \$\endgroup\$ – Bence Kaulics Dec 6 '16 at 9:26
  • \$\begingroup\$ Yes, as soon as I press the switch to make PWRKEY low, status led should be high but its not, which means the SIM900 is not ON. Instead the netlight starts blinking at very fast rate and keeps on blinking. \$\endgroup\$ – S Andrew Dec 6 '16 at 10:44
  • \$\begingroup\$ @S_Andrew check the added firmware reflash reference. I assume that you've carefully checked the supply voltage to always remain within the specified range \$\endgroup\$ – neonzeon Dec 6 '16 at 19:03
  • \$\begingroup\$ Also, please add the timing of the led flashes to your question. \$\endgroup\$ – neonzeon Dec 6 '16 at 19:06
  • \$\begingroup\$ @neonzeon net led is blinking twice a second. \$\endgroup\$ – S Andrew Dec 7 '16 at 12:47

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