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Will you need pull up resistors to get the DS18B20 working reliably with the pi?
Also, why not do it wirelessly? You can get a decent WiFi Dev board for less than a fiver now days. I know that's another £50 down but surely worth it for no cables?
Stonehedge-
I always wire them like this and that works a treat. I believe you can power OneWire devices off of the data line, but I never really got it to work and it wasn't worth the hassle for me.
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Yes, pull up resistors required.
I've already got 4-core alarm cable running to most rooms.
I also don't want a whole bunch of wifi devices to have to manage.
Tijmen
Greenbank
Ordered 10 x DS18B20 from China (due in a month or so by slow post) but cheap (under £5 inc P&P).
Also ordered three DHT22's to check how accurate they are for temp/humidity monitoring.
Plan is to have a DHT22 in each room (2 bedrooms, study, lounge, hall, kitchen, toilet, bathroom, communal hall, outside) and a DS18B20 ziptied to a radiator pipe in each room to monitor heating.
Also looking for a 1-wire barometric pressure sensor, CO2 sensors for the bedrooms and a CO sensor for the kitchen.
50m reel of alarm cable (and I can use the existing alarm cable in the flat to get to some of the rooms already) to bring it all back to a Raspberry Pi to do the data capture and logging.
Will also look to get a photodiode sensor on the electricity meter blinkenlight (which apparently blinks as every thousandth of a unit is used). Wonder if gas meter does the same...?
All for shits and giggles with total spend probably under £50 if I can keep it that way.