Switching SIMs in SIM908

I want to connect more than one sim to my SIM908 but I understand its not possible to directly do so.

Instead I wish to control the SIM-VDD voltage keeping everything else (data,clk,i/o) the same. when I want to shift the network, I will shut down the SIM908 and before I switch it back on using my microcontroller, I will apply the VDD to a different SIM. Would SIM908 then use the newly selected SIM? Am I missing something here?

I would really appreciate your sharing the experience with me.

---------------------------problem more elaborated below-----------------

This applications runs in a forest thus signals from a certain service provider may not be available all the time. But its probable there will be GSM signals from one of the 5 networks run in my country. thus I want to connect 5 sim modules to the SIM908 and if no signal from the PROVIDER1 then shutdown and restart with changed to SIM2 now.

How I wish to achieve this is by connecting all pins all 5 SIMs to the SIM908 EXCEPT for the VDD pin. I will chose to give power to the SIMX I want to use at that monent through my microcontroler.

Can I multiplex SIMs like this?

Yes you can do that. You can control the power to different SIM sockets (Circuits) using uC. After Changing the power to another SIM just reset the SIM908 module. SIM power pins should be controlled by transistors and transistors will be controlled by uC. You need 6 transistors, 5 to control power to 5 SIM sockets and one to drive the reset pin of SIM908 module.

Dear Jayanth,

Could you kindly advice me the VDD Voltage I should apply to the SIM Card? I'm using a LiPo thus I have 3.1 -3.9 Volts readily available. Is it okay if I apply that straightaway? or should I need to use some Current Controlling mechanism to control the current thats going into the SIM? (I hope its not necessary)

Okay I understand. 5 transistors to control the VDD and another to Reset the SIM908. (if I remember right I can do the same using AT commands by powering down the GSM module).

I have one more question, now when I try to achieve multiplexing this way, since other SIMs receive the SIM-DATA and the SIM-CLK , would the other SIMs be partially/ Fully powered through weak pull-up resisters and might start communicating data? I was asked to be aware of this fact and I was told this may cause issues in my circuit.

Could you kindly tell me if you've seen this method at work somewhere?

And thank you for your time that you spend to help me. I appreciate it a lot.

Without power to SIM it will not work. So you have different power switches to drive different SIM sockets which inturn provide power to SIM card. I don't know about the VDD of SIM. Check datasheet of any SIM908 SIM module and you will get the value of VDD for SIM. Drive 5 transistors from uC. If you use PORTB pins 0 to 4 to drive the transistors then just send data like 0b00000001 to drive 1st SIM, 0b00000010 to drive 2nd SIM, and values like that.

**broken link removed**

thank you so much for answering my doubts at such speed. I will surely try to operate the transistors through the I/O of my ucontroler.