Let's Make Robots!

elp-hay me on my sensor network

Hair brained plans galore! I am planning on flooding my house with environmental sensors. Temperature for starters. All over the place. And of course I want my readings to be gathered on a central machine. Live. So, no data logging on separate memory chips. I need some sort of network. With wires and stuff, because I do not trust the airwaves for this sort of thing. Also, I am a cheap skate.

So my list of requirement looks a bit like this:
- thin two-wire cable(s) running through my house
- multiple sensor units hooked up to this cable (parallel)
- each sensor sends digital data over the cable to a central master unit

My wild idea so far extends to this:
- one little MCU on each remote sensor unit
- very little power consumption by sensor units
- power comes through cable from central unit
- sensor unit stores power, in super capacitor, I suppose
- central unit detects when voltage level peaks on the power bus
- then shuts off the power
- sensor units detect high power level (well, they simply boot up because of supply power coming on)
- sensor takes reading(s)
- one by one sensors dump their readings on the -now data- bus
- sensor dies of lack of supply and the cycle repeats when master wants a new reading

Challenges as I see them so far:
- making simple and cheap sensor units so I can sprinkle them around the house
- have up to, dunno, twelve of them on one bus
- all talking on the same line without colliding data packets
- talking on a data line that has a massive capacitor nearby and a dozen more all around the bus
- driving a data line with only the power from one 1F/5V charge, long enough for the transmission of a few bytes

All and any ideas are welcome. Feel free to denounce my plans. Alternatives are also welcome. Keep in mind: I don't really NEED this network. I just like to build and operate one.

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Bad ass. Thanks for feeding my mind.

That's really awesome. Thanks Geir. This pdf is very comprehensive reading material. I am just about to wrap my mind around "Schottky diode bridge rectifiers".

.... RS485 (cheaper and semi multi drop) and CAN-BUS (more complicated but multi drop)...

Both make use of two wires (serial differential transmission) and can go a very long distance, so if you can use 4 wires instead of two ... those would be the best choices. 

I don't know if this is cheap enough for you but you can but I think you can put up to 64 thermosensors on one bus with Dallas one-wire temp sensors. They're a little bit of a pain in the ass but they have a lot going for them IMO. I got four samples from Maxim to see how they are. Maybe you should check them out.

Another datasheet on my to-read-list. Will study this protocol and its "parasite power" feature. I wish I could rebuild that power thing alone, so I could use it for all my sensors: humidity, electrical power meter, gas meter, wind meters, door sensors, sun sensors, what have you.

Rik presence sensors!

I figure you're not going down this road anymore but I wanted to say that I found the PICAXE is not able to keep up with the one-wire interface. That screws up my agenda. I just wanted to stop others from making such plans.