Connection fault on SIM900

I’ve been working on a PCB in around 2 years now, a lot has changed through that time, starting with a PIC, went on to a mbed LPC1768, and recently I moved to the mbed LPC11U24, because is can be had in a TQFP48.

This sucker runs at 48 MHz and calculates in 32 bit. Nice.

The board I designed contains all the necessary stuff to make the SIM900 work. DC/DC converter, simcard holder, some LEDs and before I changed it, I used N-channel FETs to translate voltages to the LPC11U24. It has now changed to a simple resistor divider. Through the ARM I’m running relays controlled through sms, which also can be used as inputs, as the common pins on the relays is connected through a current limiting resistor and a zener diode to the ARM.

I have only got small C/C++ experience, but the software is about to get finished.

Right now I’m able to store commands to turn on and off relays, read temperature and read the status of the module. The only thing I’m missing is to be able to track the module with a sms, using triangulation from SIM900.

But that’s being debugged right now.

Why I’m writing is that I had a problem where SIM900 couldn’t connect to the GSM network after I had issued AT+SAPBR=1,1 command. I had to hard reset the module, due to an error. After this it wouldn’t connect at all. This was easily fixed by removing the simcard, booting the module, powering it down and refitting the simcard. Works like a charm!!


I’ll post an image when I’ve got one.

EDIT: Picture of the boards I deasigned so far:

SIM900 Board1 SIM900 Boards2






Click on it to make it larger.

Happy electronicking!!