NPN Transistor choice

Question

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:
_{(source: magdiblog.fr)}

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.

Answer 1

The meter sends data continuously on the serial port. If you use two meters but have only one serial port available on the pi, combining both RX lines will lead to collisions and the data frames will get corrupted.

But maybe listening to one meter at a given time is acceptable, if the meters repeats the same info regularily. For example if you listen to one meter during 0.5 seconds, then the other meter for 0.5 seconds, you'll miss some information, but you may still get the relevant info you need at the frequency you require.

If this is the case, then it is easy: just use the same circuit as above on the second meter, and use a 2-to-1 MUX (e.g. SN74LVC2G157) to switch the RX line from one meter to the second. You'll need an additional GPIO configured as an output on the pi to select one meter or the other. You could use transistors for this, but a single multiplexer chip is simpler.

On the other hand, if you really need to listen to all the frames of both meters continuously, and you can't afford missing frames, you'll absolutely need two serial ports on the pi.