Dual SIM project

Our net router is about two hundred metres away from the house at the top of a hill for our rural net connection. We'd like to use the connection for both net and phone calls and the router has a connection for a phone cable which we can connect to one of the spare pairs in the Ethernet cable so we can have a phone in the house. But unfortunately our telco Vivo doesn't provide any plan that has practical prices for both net data and phone calls. To get around this problem we need to have two separate SIM cards and a means of remotely switching between them from the house.

SIM card pinouts

It looks from these pin connections that all of them except for pin 7 (the I/O pin) could connect to both SIMs concurrently without any problems since only the I/O pin would ever be expecting anything coming back into the device that would cause a potential conflict between the two. So our switching mechanism could then just connect the I/O pin from one or the other SIM card to the pin 7 connection in the device leaving all the others permanently connected.

Pin	Use	Notes
C1	Vcc	+5vdc
C2	RST	Reset signal, used to reset the card's communications.
C3	CLK	Provides the card with a clock signal, from which data communications timing is derived.
C4	NC	Reserved
C5	Gnd
C6	Vpp	Non-volatile memory programming voltage input.
C7	I/O	Input or Output for serial data (half-duplex) to the integrated circuit inside the card.
C8	NC	Reserved

The physical connection

We probably would be using the SIMs only for use at the land so it's probably ok to just solder them into place, but if it's not too difficult it would be nice to create a system whereby the two SIMs can be removed or replaced easily.

The remote switching method

The switching itself may be able to be done with a couple 50 cent transistors since the I/O is in the form of TTL-level data and therefore is only zero or positive, but never negative. As long as the data rate is not too high, a basic PNP and NPN transistor pair can be used to detect the state of the switch with one being active when the switch is connected to positive and the other when it's floating. Each transistor's emitter and collector connect one of the I/O pin of one of the SIMs to the device. If the transistor doesn't work, then a solid-state relay would definitely do the trick.

In the house we would then have two switches for controlling the router; one for the power and one for determining which SIM is currently active. To change SIMs the router would be turned off, the SIM switch setting changed, and the router powered back up again.