Hvac control and Feedback System 0 Group 2 Steve Jones Mathew Arcuri Elroy Ashtian, Jr

4.3 Main Control Unit

The design of the main control unit remains as it was done by the previous team where the unit will be powered by the 24 V AC common wire that is installed in the building during the initial construction project. The advantage of this design is that the consumer doesn’t have the hassle of having to recharge batteries for the thermostat or have to put in any additional wires. Of course, a future upgrade may be to add in batteries as a back-up power source in case there is power outage in the house. Having batteries to power the device would call for a bit more conservative approach since the parts would have to be low power in order to sustain a long product working time on one charge.

The main control unit houses several components of the overall design. The components located in the main control unit are the SHT21 Sensor, the dsPIC33FJ256GP710A main microcontroller, the Xbee 802.15 Transceiver, and the MRF24WB0MA 802.11b wireless chip. The main control unit of the HVAC system will also include the LCD screen and the gumstix board which will be the main hub for all of our operating system needs. Based on our design, we will be mounting Linux onto the gumstix board which will allow us to create the sophisticated GUI. On this board, we will have a WIFI module which will connect to the internet for various needs such as live weather report, online configuration and so forth. The GUI will enable the user to input a schedule of what temperatures they would like to set and also the times of scent modes. Having this GUI effective will be integral in our entire system as it is the attraction device to the consumer.

All of our components are within a reasonable operating voltage range which would require us to convert the 24 V AC from the wall down to voltage of 3.3 V to 12 Volts. Our design includes a full wave rectifier followed by voltage regulators. Making a decision on the voltage regulators included choosing the right regulator that will be able to handle the 32 V max and give us the sufficient output current the load is going to require. The 802.11b wireless chip has an operating voltage range of 2.7 V – 3.6 V with a typical voltage of 3.3 V. The Xbee chip has an operating voltage range of 2.4 V – 3.6 V with a typical voltage of 3.3 V. The main microcontroller has an operating voltage range of 3 V - 3.6 V. The temperature and humidity sensor has an operating voltage range of 2.1 V – 3.6 V with a typical voltage of 3 V. These voltages are show here in table 14 below.

In order to transform the input voltage from 24 V AC to 3.3 V DC, several steps needs to be taken in order to achieve this. First the signal needs to be converted from AC to DC by the method of a full wave rectifier. This method is also called bridge rectification and is accomplished through the implementation of four diodes. Two diodes will be used at each time accounting the positive side of the input sinusoidal voltage signal and the negative side of the input sinusoidal voltage signal. The bridge is an arrangement of four diodes in a diamond configuration that no matter the polarity of the input, the polarity of output is the same. In essence, when the AC signal goes below zero, the output from the bridge rectifier would be that same pattern but reflected in the positive domain. Therefore, upon advancement through the bridge rectifier, we will have a signal all in the positive y plane.

At this very moment, the signal is of one polarity but still is in a periodic form which doesn’t make it a DC signal. The signal is continuously varying or “pulsating” in magnitude from zero to its magnitude. This type of variance is not good for our parts as they require a constant voltage with a value of their threshold. If we are to put this signal in, the parts will transition through the on and off stage possibly causing damage. In order to smooth out this variance, we have to include a capacitor, known as reservoir or smoothing capacitor. The smoothing capacitor is dependent upon the voltage regulator implemented. We use a capacitor simply because the voltage of a capacitor cannot change instantaneously which gives us a signal that is a lot smoother. Of course, the signal after the capacitor is not perfect since ripples exist, but the variance is not as detrimental as before since now the signal will ripple around its maximum.

Once this process is done, the signal is considered close enough to true DC signal to be sent though the voltage regulator. The voltage regulator is designed to take in a threshold input voltage and produce a desired voltage. The regulator also has specifications as to the amount of current that can be drawn from it. This is critical to our design since we realized the previous groups’ mistake where they chose a part that didn’t supply enough current causing their circuit to be faulty. For the main control unit, we have a need for 3.3 V, 5 V and 12 V, therefore, requiring us to purchase three different voltage regulators to power our components on the main control unit. All of this is essential to get out components working such as the PIC microcontroller, the temperature and humidity sensor, Xbee wireless chip, LCD and ARM board.