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"Remember never to connect 5 V and 3.3 V Arduinos together. It won't hurt the 5V Arduino, but it will certainly annoy its 3.3 V brother!"As long as you use pullups (usually 1.2k to 5kOhm, depending on distance/line capacity and I2C speed) connected to 3.3v side, no problem.The 5v arduino will recognise 3.3v as HIGH level.More info on calculating pullup values can be found at http://www.ti.com/lit/an/slva689/slva689.pdf

Make sure the HC-12 pin labelled "Set" is grounded at the time you send AT commands. See Tom's Step 4 above, in this very detailed tutorial ;)

Well, when I said along X, I ment flip "up-down", not "left-right"meaning the "top part" of "DC" goes bottom. Not sure I make it clear.Here, a picture is worth 1k words

John,His model has a symmetry along X-axis, so it work anyway ;)

Sorry, I didn't get you question. Can you explain?

Sorry, I didn't get you question. Can you explain your problem more clearly?

Using a Raspberry PI3 (e.g. a quad core computer) to monitor a single binary digital sensor (on/off), and having that connected thru Ethernet is a bit... well... complicated and expensive. (and powering that with a battery pack is not suitable)Think about ESP8266 (microcontroler + Wifi). It costs ~2$, it's wireless (Wifi), smaller, draws 100+ times less power and it's more than powerful enough to do the job!Google "ESP8266 PIR"

Using a Raspberry PI3 (e.g. a quad core computer) to monitor a single binary digital sensor (on/off), and having that connected thru Ethernet is a bit... well... complicated, bulky and expensive. (and it's not worth considering to power that with a battery pack)Think about ESP8266 (microcontroler + Wifi). It costs ~2$, it's wireless (no more messy Ethernet cables), smaller, draws 100+ times less power and it's yet far more than powerful enough to do the job!