Smart House.pdf

7/27/2019 Smart House.pdf

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Smart House

Julia Sciarello, Mauricio Munoz, Carla Quiller

Introduction

Our project implements a smart algorithm in order to power a house with a photovoltaic,batteries or the power grid. For this project, we worked closely with a research team whose goalis to power a home with minimal power from the power grid. In order to form this smart home,we needed to monitor the voltage and current flow from each of the sources (photovoltaic,batteries, and the grid) and the home. We implemented these current and voltage monitors. Thenext step was to come up with an algorithm that would determine what source should bepowering the house and when the battery should be charged. The final step was to send out datato a home display module so that the data can be analyzed.


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Software/Hardware Tradeoffs

Some software tradeoffs we made involved our calculations of the voltage, current, and power.When using a microcontroller, fixed-point math is the best way to go to make sure thecalculations get done as quickly as possible. However, we are more concerned with the accuracyof these numbers and chose to use floating point instead. Since we are only doing majorcalculations every ten seconds, we were not concerned with the delay this floating-pointarithmetic would have on our design. Another tradeoff that we made was to step down thevoltages using a current divider in order to measure current and voltages. This is because we areusing 12 V, and the opto-isolator we used could only handle inputs of up to 5V. We saw that a

previous project had used this opto-isolator successfully so we decided to use it and risk having asmall percent of error from scaling down. We decided to use TinyRealTime instead of interruptservice routines because we had to accurately schedule our tasks due to deadlines. Anothertradeoff involved finding the charge of the batteries. Instead of following the discharge behaviorof the batteries or integrating the current, we chose to simply measure the voltage and divide thisvoltage by the maximum voltage of the batteries to find the charge percentage. We could do thisbecause nickel-cadmium has a nearly linear relationship between charge and voltage. This is asoftware tradeoff because it saved us time in calculations and proved an approximate result.

In terms of hardware tradeoffs, we had to decide how to accurately measure current withoutlosing power to the house. We had two options: use a current divider, an opto-isolator, and an


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operational amplifier or use a current-sensing resistor and a differential opto-isolator. Weattempted to use the first option, however the opto-isolator needed 1.8V across it. This voltagedrop results in a decrease in power delivery to the house. Choosing the proper resistance valuesto get this 1.8V drop was difficult. The output of the op-amp was a voltage that we used to findthe current flowing through the house, but we could not find the appropriate scaling factorbetween the voltage and current. The voltage output was on the milli-volt range, so we did notthink the ADC converter would understand how to scale this small voltage. Therefore wedecided to use the second option, which will be described in the Program Detail section.There are many products on the market for saving energy and measuring the power consumptionof your house. There are even companies that are devoted to producing the Smart Home weare trying to create. IEEE standards for sizing nickel cadmium batteries for a photovoltaic, IEEEstandards for utility interface of residential and intermediate photovoltaic systems.

There are many products on the market for saving energy and measuring the power consumption

of your house. These products include the Kill-A-Watt and various other surge protector basedpower conservation devices. There are even companies that are devoted to producing the SmartHome we are trying to create. For this type of product, we must satisfy IEEE standards forsizing nickel cadmium batteries for a photovoltaic as well as IEEE standards for utility interfaceof residential and intermediate photovoltaic systems.

Program Details

Our code consisted of four main tasks. The first task, getInput(), is a continuous task used tomonitor the inputs from the user. In order to initiate the program, the user is required to input thestart time (hour in military time, minute, second) for accurate time keeping. This function thencontinuously watches for inputs from the user in order to signify a demand response event hasoccurred. A demand response event is one in which the power company would signal that thehome needs to reduce its power consumption. In order to represent this, the user would inputthe maximum power the house can use and the length of this power restriction.

The updateClk() task simply keeps track of time. It has a release and deadline time of 1.0 secondand increments the second of the clock. When the second reaches 60, it is reset to 0 while theminute variable is incremented. The minute variable is incremented until it reaches 60 as well,where it is reset and the hour variable is incremented. The hour is in military time, so instead ofrolling over to 24 it will reset to zero. Time keeping is very important in our project because weneed to keep track of the price of energy, which varies throughout the day. The price of energyplays a role in our decision-making algorithm.

PVmonitor() is the monitoring task that calculates the power of the PV, power demand of thehome, the battery charge, and the power being produced by or sold back to the grid. To get thepower, we need to find the voltage and current for each source. We do this by reading in theoutputs of the four opto-isolators into the ADC on the microcontroller, as well as the voltagefrom our voltage dividers. The voltage is calculated based on the voltage divider, as well as acalibration factor. In order to find the current, we calculate the actual voltage drop across our 2current sensor by using the equation from the opto-isolator and a factor from our voltagedivider.


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Vdiff=0.512/Vref(Vout-Vref/2)

The ADC is an analog-to-digital converter, which takes in an analog value and converts it to anumber between 0 and 255. We can retreive the original analog number by multiplying theADCH number by 5/255, where 5 is the reference voltage.

Then we use Ohms Law to get the current by dividing Vdiff by 2. The power can then becalculated by multiplying this current by the previous voltage we found.In order to get the AC power, we measured the current 1000 times in order to find the peakcurrent. Once we found the peak current, we could calculate the Vrms by dividing the peakcurrent by 2. When looking at the output signals of the opto-isolator and the AC power signal,we saw that there was no phase difference. This means we can simply multiply Vrms and Irmstogether to get the power. After collecting all the power data, we sent it out using the RS232 andUart.

In case the PV is not in use, we have a check that occurs every ten minutes so that we can get thepower of the PV. To do this we close the relays so that the PV can charge the battery, and wecan get a power reading for it. Otherwise we wouldnt know how much the PV can supply sincethere isnt current flowing through the resistor at all times. After the measurement has beenmade, we turn off the PV and return the circuit to its previous state. If the PV is supplyingenough power, the algorithm will decide to turn it back on.

Algorithm() was the biggest portion of the project. We sat down with the research group in orderto discuss the most efficient way to power the home under various circumstances. There are twomain conditions in this algorithm: whether or not there is a demand response signal from theutility. If there is no demand response signal, our algorithm progresses in the following order:

If the battery is at 20% of full charge or less, perform an emergency charge back up to75% and proceed through the rest of the algorithmIf PV power is greater than the power demand of the home, let the PV power the houseIf PV power and battery can provide enough power for the home, then they power thehouseIf PV power is greater than the home demand, battery charge is greater than 80%, and theprice of energy is in the upper 50%, we sell energy from the PV back to the gridIf PV power is greater than 70% of home demand, its before 12 PM, the price of energyis in the lower 50%, and the battery is less than 95% full, we power the house with thegrid and charge the batteries with the PV.

Otherwise, just power the house with the grid.

If there is a demand response signaled, then the algorithm proceeds in the following way:

Check to see if the power from the PV and the battery exceed the power demandIf so, proceed to check if the PV and battery can sustain the homes power demand forthe length of the demand responseIf so, check to see if the PV power is greater than the power demand.

o If it is, sell back power to grid.o Otherwise, use the grid to power the house and PV to charge the battery


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If not, use the grid to power the home and charge the battery with the PV.We keep track of the time, so that once the demand response is over we can return to the originalalgorithm.

The trickiest part of our project was trying to measure current. The opto-isolators we used werevery sensitive to voltage changes and for some reason our voltage dividers did not work quite asexpected. In order to calculate the correct current, we needed to measure multiple times in orderto find the correct scaling factor between the voltage output of the opto-isolator and the actualvoltage drop across the current sensing resistor. It was also frustrating that for each of the fourcurrent sensing circuits, the scaling factor did not remain the same. When measuring AC signals,we needed to make sure that we were getting the peak value for current in order to calculate theaverage power.

Hardware Details

Our project consisted of a few different circuits. Since we couldnt demo with a realphotovoltaic, it was necessary for us to build a circuit that effectively represented theexponential I-V curve of a photovoltaic. We found the circuit seen below from a researchpaper (see appendix). It can be made with a current source, which we made using an op-amp, three LEDs and a couple of resistors. As the current increases, the voltage outputdoes as well. Because the LEDs are different colors and have different on-voltages,they will turn on at different times, providing three different linear regions in the I-Vcurve. This creates a somewhat exponential effect for the current.


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Our current monitoring circuits can be seen in the pictures below. We have one design for theDC voltages of the battery and PV and another for the AC signals of the grid and the home. Theonly real difference is the resistors used for the voltage divider. For both circuits, a signal comes

through the 2 resistor, and the voltages are scaled down on both sides of the resistor. For theDC circuit, we scaled the voltages down by a factor of ten and the AC signals were scaled downby a factor of four. The scaled down voltages on both sides of the resistor get sent into the opto-isolator, which sends a voltage to the ADC of the MCU that corresponds to the voltagedifference of the two inputs. This voltage is then used to find the current through the resistor.We also send the scaled down voltage on the output of the resistor to the MCU so that we cancalculate the voltage on the output of the PV and calculate the power.

IntroductionHigh Level DesignHardware/Software Tradeoffs

Program DetailsHardware DetailsResults of DesignConclusionEthical ConsiderationsLegal ConsiderationsAppendix

Smart HouseJulia Sciarello, Mauricio Munoz, Carla Quiller


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Introduction

Our project implements a smart algorithm in order to power a house with a photovoltaic,batteries or the power grid. For this project, we worked closely with a research team whose goalis to power a home with minimal power from the power grid. In order to form this smart home,we needed to monitor the voltage and current flow from each of the sources (photovoltaic,batteries, and the grid) and the home. We implemented these current and voltage monitors. Thenext step was to come up with an algorithm that would determine what source should be

powering the house and when the battery should be charged. The final step was to send out datato a home display module so that the data can be analyzed.

High Level Design

Our project idea came from a research team focusing on building a smart home, since one of ourgroup members is working with the team. In order to decide how to power the house, it isnecessary to find the power demand of the house and the available power from each source. Thepower can be found by the following equations: P=IV, P=V^2/R, P=I^2R.For AC power calculations we needed to find average power instead of instantaneous power.


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Therefore we needed to find the root-mean-squared value for the current and the power, and wealso needed to find the phase difference between the voltage and the current.

I(t)=Ipeak*cos(2ft)V(t)=Vpeak*cos(2ft + )Pavg=I*V=Irms*Vrms*cos() => Vrms=Vpeak/2 and Irms=Ipeak/2

For our project we are using the HCPL-7520 Linear Opto-isolator in order to determine thevoltage drop across a resistor. These isolators can be modeled by the equation below, where Vinis the differential input.

Vin=.512/Vref*(Vout-Vref/2)

The voltages that we are measuring exceed 5V, therefore we used voltage dividers to scale downthose voltages. For calculations, we found the actual voltage on the line by using the followingequation, where the input to the microcontroller would be Vscale and the actual voltage would beVhigh.

Vscale=R1/(R1+R2)*Vhigh

The diagram below shows the layout of the power sources, relays, and the load. The solar panelgenerates DC power and when Relay 1 is closed, that power is sent to an inverter, whichtransforms the power to AC. The inverter outputs a 120AC signal, which is then stepped-down to

12VAC using a transformer for safety purposes. The 12VAC is then sent to the home. Thenickel-cadmium batteries generate DC power, which is also sent to the inverter when Relay 3 isclosed in order to power the home. These batteries can be charged by the grid when Relay 4 isclosed and by the photovoltaic when Relay 2 is closed. A DC to DC converter is needed whencharging the battery with the photovoltaic in order to charge the battery properly. When chargingthe batteries with the grid power, we need to transform the AC power into DC. We achieved thisby passing the signal through a full-wave rectifier and a DC to DC converter. In order to powerthe house with the grid, Relay 5 must be closed. In order to determine which relays are closed wemonitored the available power from the battery and the PV. Using the power measurements weran an algorithm that decided the most efficient way to power the house.


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Software/Hardware Tradeoffs

Some software tradeoffs we made involved our calculations of the voltage, current, and power.When using a microcontroller, fixed-point math is the best way to go to make sure thecalculations get done as quickly as possible. However, we are more concerned with the accuracyof these numbers and chose to use floating point instead. Since we are only doing majorcalculations every ten seconds, we were not concerned with the delay this floating-pointarithmetic would have on our design. Another tradeoff that we made was to step down thevoltages using a current divider in order to measure current and voltages. This is because we areusing 12 V, and the opto-isolator we used could only handle inputs of up to 5V. We saw that a

previous project had used this opto-isolator successfully so we decided to use it and risk having asmall percent of error from scaling down. We decided to use TinyRealTime instead of interruptservice routines because we had to accurately schedule our tasks due to deadlines. Anothertradeoff involved finding the charge of the batteries. Instead of following the discharge behaviorof the batteries or integrating the current, we chose to simply measure the voltage and divide thisvoltage by the maximum voltage of the batteries to find the charge percentage. We could do thisbecause nickel-cadmium has a nearly linear relationship between charge and voltage. This is asoftware tradeoff because it saved us time in calculations and proved an approximate result.

In terms of hardware tradeoffs, we had to decide how to accurately measure current withoutlosing power to the house. We had two options: use a current divider, an opto-isolator, and an


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operational amplifier or use a current-sensing resistor and a differential opto-isolator. Weattempted to use the first option, however the opto-isolator needed 1.8V across it. This voltagedrop results in a decrease in power delivery to the house. Choosing the proper resistance valuesto get this 1.8V drop was difficult. The output of the op-amp was a voltage that we used to findthe current flowing through the house, but we could not find the appropriate scaling factorbetween the voltage and current. The voltage output was on the milli-volt range, so we did notthink the ADC converter would understand how to scale this small voltage. Therefore wedecided to use the second option, which will be described in the Program Detail section.There are many products on the market for saving energy and measuring the power consumptionof your house. There are even companies that are devoted to producing the Smart Home weare trying to create. IEEE standards for sizing nickel cadmium batteries for a photovoltaic, IEEEstandards for utility interface of residential and intermediate photovoltaic systems.

There are many products on the market for saving energy and measuring the power consumption

of your house. These products include the Kill-A-Watt and various other surge protector basedpower conservation devices. There are even companies that are devoted to producing the SmartHome we are trying to create. For this type of product, we must satisfy IEEE standards forsizing nickel cadmium batteries for a photovoltaic as well as IEEE standards for utility interfaceof residential and intermediate photovoltaic systems.

Program Details

Our code consisted of four main tasks. The first task, getInput(), is a continuous task used tomonitor the inputs from the user. In order to initiate the program, the user is required to input thestart time (hour in military time, minute, second) for accurate time keeping. This function thencontinuously watches for inputs from the user in order to signify a demand response event hasoccurred. A demand response event is one in which the power company would signal that thehome needs to reduce its power consumption. In order to represent this, the user would inputthe maximum power the house can use and the length of this power restriction.

The updateClk() task simply keeps track of time. It has a release and deadline time of 1.0 secondand increments the second of the clock. When the second reaches 60, it is reset to 0 while theminute variable is incremented. The minute variable is incremented until it reaches 60 as well,where it is reset and the hour variable is incremented. The hour is in military time, so instead ofrolling over to 24 it will reset to zero. Time keeping is very important in our project because weneed to keep track of the price of energy, which varies throughout the day. The price of energyplays a role in our decision-making algorithm.

PVmonitor() is the monitoring task that calculates the power of the PV, power demand of thehome, the battery charge, and the power being produced by or sold back to the grid. To get thepower, we need to find the voltage and current for each source. We do this by reading in theoutputs of the four opto-isolators into the ADC on the microcontroller, as well as the voltagefrom our voltage dividers. The voltage is calculated based on the voltage divider, as well as acalibration factor. In order to find the current, we calculate the actual voltage drop across our 2current sensor by using the equation from the opto-isolator and a factor from our voltagedivider.


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Vdiff=0.512/Vref(Vout-Vref/2)

The ADC is an analog-to-digital converter, which takes in an analog value and converts it to anumber between 0 and 255. We can retreive the original analog number by multiplying theADCH number by 5/255, where 5 is the reference voltage.

Then we use Ohms Law to get the current by dividing Vdiff by 2. The power can then becalculated by multiplying this current by the previous voltage we found.In order to get the AC power, we measured the current 1000 times in order to find the peakcurrent. Once we found the peak current, we could calculate the Vrms by dividing the peakcurrent by 2. When looking at the output signals of the opto-isolator and the AC power signal,we saw that there was no phase difference. This means we can simply multiply Vrms and Irmstogether to get the power. After collecting all the power data, we sent it out using the RS232 andUart.

In case the PV is not in use, we have a check that occurs every ten minutes so that we can get thepower of the PV. To do this we close the relays so that the PV can charge the battery, and wecan get a power reading for it. Otherwise we wouldnt know how much the PV can supply sincethere isnt current flowing through the resistor at all times. After the measurement has beenmade, we turn off the PV and return the circuit to its previous state. If the PV is supplyingenough power, the algorithm will decide to turn it back on.

Algorithm() was the biggest portion of the project. We sat down with the research group in orderto discuss the most efficient way to power the home under various circumstances. There are twomain conditions in this algorithm: whether or not there is a demand response signal from theutility. If there is no demand response signal, our algorithm progresses in the following order:

If the battery is at 20% of full charge or less, perform an emergency charge back up to75% and proceed through the rest of the algorithmIf PV power is greater than the power demand of the home, let the PV power the houseIf PV power and battery can provide enough power for the home, then they power thehouseIf PV power is greater than the home demand, battery charge is greater than 80%, and theprice of energy is in the upper 50%, we sell energy from the PV back to the gridIf PV power is greater than 70% of home demand, its before 12 PM, the price of energyis in the lower 50%, and the battery is less than 95% full, we power the house with thegrid and charge the batteries with the PV.

Otherwise, just power the house with the grid.

If there is a demand response signaled, then the algorithm proceeds in the following way:

Check to see if the power from the PV and the battery exceed the power demandIf so, proceed to check if the PV and battery can sustain the homes power demand forthe length of the demand responseIf so, check to see if the PV power is greater than the power demand.

o If it is, sell back power to grid.o Otherwise, use the grid to power the house and PV to charge the battery


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If not, use the grid to power the home and charge the battery with the PV.We keep track of the time, so that once the demand response is over we can return to the originalalgorithm.

The trickiest part of our project was trying to measure current. The opto-isolators we used werevery sensitive to voltage changes and for some reason our voltage dividers did not work quite asexpected. In order to calculate the correct current, we needed to measure multiple times in orderto find the correct scaling factor between the voltage output of the opto-isolator and the actualvoltage drop across the current sensing resistor. It was also frustrating that for each of the fourcurrent sensing circuits, the scaling factor did not remain the same. When measuring AC signals,we needed to make sure that we were getting the peak value for current in order to calculate theaverage power.

Hardware Details

Our project consisted of a few different circuits. Since we couldnt demo with a realphotovoltaic, it was necessary for us to build a circuit that effectively represented theexponential I-V curve of a photovoltaic. We found the circuit seen below from a researchpaper (see appendix). It can be made with a current source, which we made using an op-amp, three LEDs and a couple of resistors. As the current increases, the voltage outputdoes as well. Because the LEDs are different colors and have different on-voltages,they will turn on at different times, providing three different linear regions in the I-Vcurve. This creates a somewhat exponential effect for the current.


14/19

Our current monitoring circuits can be seen in the pictures below. We have one design for theDC voltages of the battery and PV and another for the AC signals of the grid and the home. Theonly real difference is the resistors used for the voltage divider. For both circuits, a signal comes

through the 2 resistor, and the voltages are scaled down on both sides of the resistor. For theDC circuit, we scaled the voltages down by a factor of ten and the AC signals were scaled downby a factor of four. The scaled down voltages on both sides of the resistor get sent into the opto-isolator, which sends a voltage to the ADC of the MCU that corresponds to the voltagedifference of the two inputs. This voltage is then used to find the current through the resistor.We also send the scaled down voltage on the output of the resistor to the MCU so that we cancalculate the voltage on the output of the PV and calculate the power.

The other part of our circuit includes the five relays. Each of these relays has four connections.Two are for the MCU ground as well as a signal from a pin on the MCU and the other two arefor the two connecting or disconnecting the power source to the rest of the circuit. When theMCU sends out a 5V signal to the relay, the switch will close allowing current to flow throughthe circuit.


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In doing research for this project, weve been closely referencing the ECE 4760 PowerBoxproject from a couple years ago. They used the same opto-isolators we are using and a similarpower averaging technique. We did not copy their design or code, but did use it as a referencefor our circuit and code.Over the course of this project, we have tried several different ways of measuring current andsending data. We had originally tried to use an H11F1 opto-isolator along with a current dividerand an LM 358 operational amplifier as described earlier, however we were not able toeffectively determine the I-V curve. We did some research and found the PowerBox project anddecided to attempt their method of measuring current. As for sending data, we had set up the SPI

port so that we could send data via pin B.5, but we realized that for the amount of data we had tosend it would take a large amount of time to transmit. The receiver would also have to decodeour message in a specific way. We did all of our calculations in floating-point so they were moreaccurate, but in order to send them over this SPI port we would have had to convert them tofixed-point. This may have caused some inaccuracy in the readings. After talking to the otherteam that is working with our research group, we decided to just print the data out through pinsD0 and D1. That made it easier for us to send data and for them to receive it.


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Results of Design

For our design, we were not concerned with the speed of the execution as long as all of the taskswere executed by the deadlines and as long as all the calculations could get done by thedeadline. Because our main tasks execute every 10 seconds, we were able to use floating-pointcalculations without running into timing issues. We were not able to notice any sort of hesitationor flickering within our program. It seems to work as desired.In terms of accuracy, we used voltage and current values from the multi-meter to compare withthe calculated voltages and currents. We then applied an appropriate calibration factor to achievethe right voltage and current. However, these scaling factors varied by device, workstation, andrandomly; therefore our calculations will always have some error associated with them.Initially, the research wanted us to scale the power to the full 120VAC. However we decided that

this voltage was too dangerous to be handling in lab. Therefore we used 120V to 18Vtransformers. We also used isolators to separate the power sources to ensure that themicrocontroller is protected from voltages exceeding 5V. Our design was safe and did notinterfere with other design projects.We aimed to make this device as user-friendly as possible. Therefore the product only needs tobe installed and initialized, and then the program will run by itself. In the real world, the utilitycompany, not the user, will issue any demand response signal directly to the device. The onlyinput needed from the user is the time of day at the initial set-up.

Voltage

Actual(V)

CodeMeasurement(V)

Error(%)

10.38 10.364 .154

14.07 13.968 .73

4.34 4.281 1.38

Current

Actual(mA)

Code

Measurement(mA) Error(%)4.45 5.11 14.9

7.8 7.14 8.42

8.5 9.17 7.30

Conclusions


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We changed our design several times. We wanted to use SPI, however that was too timeconsuming. Instead the RS232 proved to be more simple and efficient. If we were to do thisproject again, we would have used opto-isolators that could handle higher voltages, so that wewould not have had to use voltage dividers. This would have decreased the error in our currentand voltage calculations. Another feature we would have liked to implement is weatherforecasting. If our algorithm function could receive the daily weather updates, we would be ableto more accurately predict the power from the PV, which would be useful in demand responsesituations. The behavior of the nickel cadmium batteries did not meet our expectations. Thedischarge rate was faster than we anticipated and the charging rate was slower than expected.Therefore we could not incorporate the batteries into our demo. Instead we used another powersupply to simulate the battery voltage.We did not use any other code. However we did use a similar design for calculating current andvoltage as in the SmartBox design. We also referenced code written for the ECE 4760 lecturesfor using SPI, ADC, and TRT. This project was not aimed at reverse-engineering a patented or

trademarked design, but there are many people who are working on solving the same problem.Power conservation is a very significant issue in America. Our project utilizes environmentally-friendly power sources, which decreases the dependence on the power grid. Therefore ourproject has the possibility of getting a patent or being published.

We did our best to follow the IEEE standards for sizing Nickel Cadmium Batteries for use with aPhotovoltaic as well as IEEEs recommended practice for the Interface of Residential andIntermediate Photovoltaic Systems. Our research group had already picked out the batteries andthe photovoltaic for us, so they had looked into this. In making our circuits we had to make sureour PV wouldnt have excessive voltage flickers and we also made sure that the power from thePV was converted to 60 Hz AC, to match the grids AC frequency. For the batteries, our code

made sure that if the battery supply dipped below a certain value, we would have an emergencycharge to bring the battery power back up to a certain percentage.

Ethical Considerations

While working on this project, we followed the IEEE Code of Ethics. While we referenced otherprojects and informational sites, we wrote our own code and gave all credit where it was due.Our group has always tried to help other students in the lab, especially the other group who wastrying to use the same opto-isolator as us. Our ECE 4760 class consists of students of all races,

genders, and religions, and we have treated each and every one of them fairly.On our report we have accurately portrayed our design and our results, as to give an honestreview of our project. We also sought out help for circuit design because there were many waysto implement current and voltage measurers. We respected the property of the other students bynot touching things that werent ours, by following safety rules, and by avoiding horseplay in thelab. Safety was our highest priority, which is why we used transformers to step down the ACsignal from 120V to 18V. We had to solder the opto-isolators, so we used goggles to protect oureyes.Throughout this project we learned how to properly opto-isolate, use SPI, measure current and


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voltage, rectify AC signals, operate relays, and make current sources. Therefore we broadenedour knowledge of circuit design and the capabilities of microcontrollers.

Legal Considerations

Our project is just for research purposes in order to simulate a home, so there are no legalconsiderations.

Appendix: Schematics


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Appendix: Cost

Product Quantity Cost Each TotalAvago Technologies HCPL 7520 4 $6.02 $24.08

Inverter from research group

2 Transformers from ResearchGroup

Power Supply 1 $5.00 $5.00

White Board 5 $6.00 $30.00

STK500 1 $15.00 $15.00

Ni-Cad Batteries

2 Ohm Resistors from Lab

LM358 Op-Amp from Lab

Various Capacitors and Resistorsfrom lab

Total $74.08

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