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@giebek could you describe the problem, what your setup looks like, and what you have tried so far to troubleshoot it? There are 137 posts in this thread so it is a bit hard to figure out which problem you are referring to.

@sharpy if the node is battery powered, NO or NC can make a big difference. Which one is best depends on if the door is usually open or closed.

Yes, with two NC in series you could detect that one of the doors is open. You would not know which door though, and if one of the doors was left opened you would not be able to detect if the other door is open or closed.

Welcome to the MySensors forum @talhayounis

Th best way to get started with MySensors is to first read the getting started guide. Then you can move on to https://www.mysensors.org/build/gas which has an example sketch to use with MQ135 and MQ7.

The best way is usually to get a gateway and a simple node running, and then expand from that.

@alowhum done

@tekka I see. Thanks for explaining.

@mfalkvidd The Arduino builder generates two .hex files, one of which contains the bootloader as specified in boards.txt. Depending on the flashing settings you may burn the sketch only or the sketch + bootloader, however, the bootloader is not needed for ISP programming.

I thought the bootloader is overwritten when using ISP? So "you don't need a bootlader" is slightly incorrect, you can not have a bootloader when using isp?

@encrypt Ok, this explains your issue: programming via ISP will do a chip erase and hence the EESAVE fuse setting is critical for e2p persistance. The most common use case is programming via serial bootloader (e.g. optiboot, atmegaboot, etc.) which only does page erases and leaves the rest untouched. To answer your question: If you're using an ISP programmer you do not need a bootloader.

@encrypt I may have an idea what's going on here: Are you programming a new sketch to your barebone AT328p via ArduinoISP or any other means of serial (=SPI) programming? Programming via bootloader (ATmegaboot or optiboot) requires a FTDI adapter and will only do page erases while leaving the eeprom untouched.

@anticimex @Encrypt That's a bit odd and certainly specific to the bootloader you're using (ATmegaBoot): AVRdude does (at least with optiboot) a page erase (vs. chip erase where EESAVE has an effect). I do not have the EESAVE fuse bit set and no issues with erased eeprom when loading a new sketch, also see below:

Arduino Uno with optiboot:

uno.bootloader.tool=avrdude
uno.bootloader.low_fuses=0xFF
uno.bootloader.high_fuses=0xDE
uno.bootloader.extended_fuses=0xFD
uno.bootloader.unlock_bits=0x3F
uno.bootloader.lock_bits=0x0F
uno.bootloader.file=optiboot/optiboot_atmega328.hex

High fuse (0xDE) does not enable EESAVE.