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Grab your multimeter, set it to measure AC voltage, and figure out which pair of wires reads 24 volts AC when your heating/cooling system is active. Since thermostat wiring conventions vary, this requires some experimentation. The first step is figuring out which wires to monitor. In order to do this, we want to sense whether the relay is allowing power to flow or not. We want to monitor whether the heating/cooling system is running. The actual switching is usually done by a relay inside the thermostat. Programmable thermostats usually either run off of batteries, or use some kind of “power-stealing” mechanism to power themselves. (Those that don’t are outside the scope of this Instructable). Many household heating/cooling systems run at 24 volts AC. Eagle (free version, optional, for designing PCB).Plotly account (optional, for graphing data).Perl (needed for HiPi, which is needed for hipi-i2c).Optionally, SMD versions of the resistors listed above to cut down on overall size.Male header pins to connect to a Pi, or female header pins to connect to an Arduino.Optionally, female header pins to accept the SOIC breakout board.If you want to design and manufacture your own circuit board, you will also need: If you also want to build the optional temperature sensor, you will also need: Basic electronic odds and ends, such as:.A length of two conductor, insulated wire, ideally with a quick disconnect of some kind.An 8 kOhm resistor (or, more likely, a 4.7 kOhm and 3.3 kOhm resistor in series).
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A 10uF, minimum 10V capacitor (electrolytic is fine).2x 3.3 kOhm resistors, rated 1/2 watt or higher (the wattage rating is important).
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