Arduino Thermostat

This is just a quick post at the moment because the project isn`t done and I am distracted by other things. The thermostat for the central heater in my house became unreliable a week or two ago. My wife rang around and a new one from the manufacturer of the heater was going to be $450 AUD. So, it seemed obvious to make my own. I`ve just installed
Arduino Thermostat - schematic

the 1. 0 of it, with the board layout and manufacture being done by Doug once again. The next sensor I wanted to add to my home was a set of hygrometers. Specifically I wanted an exterior one, and a matching interior one. This would be useful as we have evaporative cooling, and if the humidity level outside is already high, then it doesn`t make a lot of sense to put extra water into the air. Worse than that, it can also damage my books and make the house really clammy. So, adding some sensors was the first step in some form of alerting. I picked up two HS1101s from ebay quite cheaply (about $4 each IIRC). These devices are capacitors whose capacitance varies proportionally with relative humidity. You also need to provide a temperature at the sensor to correct the value, although the correction is pretty minor so I guess you could skip this if you really wanted to cut costs. Given I have plenty of code for Dallas 1820s now, I just dropped one of those onto the board too. I just used the circuit from the data sheet for my design, with a few simple tweaks (like the DS1820). Here`s my surprisingly unprofessional circuit diagram: This gives us temperature on a 1-Wire pin, and an oscillator on another pin which relates to the current humidity. You`ll notice that my circuit has some extra wires, that`s because I power down the 555 / HS1101 when I`m not taking a sample. I do this because Peter H. Anderson suggested that noise would...

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