A few months ago, I realised a meter reading system following this french guide: http://www.magdiblog.fr/gpio/teleinfo-edf-suivi-conso-de-votre-compteur-electrique/
I use the following circuit:
Which is connected to my Raspberry Pi B+, it works really well but now i'm trying to improve the system to read two electric meters using the same circuit.
I was told that an NPN transistor would allow me to "switch" between the two electric meters. But I don't know how to choose the transistor and where should I place this transistor in the current circuit.