The Design ProcessThe Design Process

Abstraction & SynthesisAbstraction & SynthesisPart IIPart II

Solar Candle Continued…Solar Candle Continued…

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Prof. BitarProf. Bitar

Abstraction & SynthesisAbstraction & SynthesisHomework #2Homework #2

ViableOptions

PreferredSolution

Research Prior ArtBrainstorm

ApplyConstraints

HW#2

Abstraction&

Synthesis

Possible Solutions

PerformValue Analysis

Product ReminderProduct Reminder

The Customer RequirementsThe Customer RequirementsExplicitExplicit– Cat SafeCat Safe– Look NiceLook Nice– Different ColorsDifferent Colors– AutomaticAutomatic

ImplicitImplicit– Low CostLow Cost– Reliable / DurableReliable / Durable– Low MaintenanceLow Maintenance

Product SpecificationsProduct Specifications yet to be quantified… yet to be quantified…

Safety / DurabilitySafety / Durability– Heavy BaseHeavy Base– Unbreakable LEDUnbreakable LED– Secure to Window Sill, Sash or Window Secure to Window Sill, Sash or Window

PanePane– No CordsNo Cords– Low VoltageLow Voltage

AestheticsAesthetics– Traditional LookTraditional Look– Interchangeable Color LEDsInterchangeable Color LEDs– Flickering OptionFlickering Option

Low Operating CostLow Operating Cost– Long Battery Life (Efficient) Long Battery Life (Efficient) – RechargeableRechargeable– Solar RechargeableSolar Rechargeable– Photo Sensor or TimerPhoto Sensor or Timer

Current Block DiagramCurrent Block Diagram

Solar CellCharge

ControllerRechargeable

Battery

EfficientDriveCircuit

White LED3.2V @ 20mA(worst case)

FlickeringControl

ModeSelection

PhotoSensor

A fundamental question to answer…A fundamental question to answer…

How much energy is How much energy is required to operate our required to operate our

solar candle (LED)?solar candle (LED)?

How much energy?How much energy?Consider worst case output requirement…Consider worst case output requirement…

3.2V x 20mA = 64 mW3.2V x 20mA = 64 mW

How much time will the LED be on?How much time will the LED be on?64 mW x 6 hrs = 384 mW•hrs64 mW x 6 hrs = 384 mW•hrs

Answer: 384 mW•hrs (units of ENERGY!)Answer: 384 mW•hrs (units of ENERGY!)

Assuming 70% efficiency, Assuming 70% efficiency,

384 / 0.70 ≈ 384 / 0.70 ≈ 550 mW•hrs needed550 mW•hrs neededNOTE: On average, this minimum amount of energy needs to be NOTE: On average, this minimum amount of energy needs to be

supplied by the solar panel and stored in the battery during the supplied by the solar panel and stored in the battery during the day, if our candle is to be sustainable.day, if our candle is to be sustainable.

Focusing on the BatteryFocusing on the Battery

Solar CellCharge

ControllerRechargeable

Battery

EfficientDriveCircuit

White LED3.2V @ 20mA(worst case)

FlickeringControl

ModeSelection

PhotoSensor

Things to consider…Things to consider…Types: What rechargeable technologies will Types: What rechargeable technologies will work in this application? (NiMH, NiCd, Lithium work in this application? (NiMH, NiCd, Lithium Ion, Sealed Lead Acid, Super Capacitors…)Ion, Sealed Lead Acid, Super Capacitors…)

Voltage: What is the minimum battery voltage? Voltage: What is the minimum battery voltage?

Capacity: How long does the battery need to Capacity: How long does the battery need to hold a charge? (Six Hours Minimum)hold a charge? (Six Hours Minimum)

Shape: Needs to fit in candle stem or base.Shape: Needs to fit in candle stem or base.

Cost: I don’t want to spend a lot on batteries!Cost: I don’t want to spend a lot on batteries!

Possible choice: NiCd / NiMHPossible choice: NiCd / NiMH

Battery SpecificationBattery SpecificationDischarge CurveDischarge Curve

Battery Feasibility Check…Battery Feasibility Check…THREE 1.2V (nominal) NiCD or NiMH batteries wired in THREE 1.2V (nominal) NiCD or NiMH batteries wired in series (3.6V nominal).series (3.6V nominal).

Cost: $2.37 each (single qty) / $1.30 (1000 qty)Cost: $2.37 each (single qty) / $1.30 (1000 qty)

Shape: AA cells would fit in candle stem or base.Shape: AA cells would fit in candle stem or base.

Voltage Check: Voltage Check: 1.2V x 3 = 3.6V 1.2V x 3 = 3.6V

Charge Capacity:Charge Capacity: 700 mA•hrs (units of charge) 700 mA•hrs (units of charge)

Energy Capacity:Energy Capacity: 700 mA•hrs x 1.2V 700 mA•hrs x 1.2V

= 840 mW•hrs (per battery)= 840 mW•hrs (per battery)

= 2,520 mW•hrs (for 3 batteries)= 2,520 mW•hrs (for 3 batteries)

NOTE: Since only 550 mW•hrs are required, one battery NOTE: Since only 550 mW•hrs are required, one battery has enough energy capacity to do the job, BUT the voltage has enough energy capacity to do the job, BUT the voltage will need to be BOOSTED!will need to be BOOSTED!

Question: Do we know the specs for a Question: Do we know the specs for a possible drive circuit? YES!possible drive circuit? YES!

Solar CellCharge

Controller

RechargeableBattery

1.2V NiCd

EfficientDriveCircuit

LED3.2V

20mA

FlickeringControl

ModeSelection

PhotoSensor

Solar CellCharge

Controller

RechargeableBattery

1.2V NiCd

EfficientDriveCircuit

LED3.2V

20mA

FlickeringControl

ModeSelection

PhotoSensor

Focusing on the Solar CellFocusing on the Solar Cell

How about the solar cell from Home Depot?How about the solar cell from Home Depot?

After taking the Home Depot Landscape Light apart, I After taking the Home Depot Landscape Light apart, I made the following measurements (in direct sun): made the following measurements (in direct sun): IISCSC

= 50mA , V = 50mA , VOCOC = 4.3V = 4.3V

Solar Panel V-I CharacteristicSolar Panel V-I Characteristic

T

Solar Cell Voltage (V)0.00 1.00 2.00 3.00 4.00 5.00

So

lar

Ce

ll C

urr

en

t (A

)

0

10m

20m

30m

40m

50m

Question: How long would it take this Question: How long would it take this solar cell to recharge a completely solar cell to recharge a completely

discharged battery?discharged battery?

Battery Capacity = 700mA•hrsBattery Capacity = 700mA•hrs

Solar Cell (in full sun) = 50mASolar Cell (in full sun) = 50mA

Therefore, 700mA•hrs / 50mA = 10 hrsTherefore, 700mA•hrs / 50mA = 10 hrs

Q: Is this realistic?Q: Is this realistic?

What voltage are we operating at?What voltage are we operating at?T

Solar Cell Voltage (V)0.00 1.00 2.00 3.00 4.00 5.00

So

lar

Ce

ll C

urr

en

t (A

)

0

10m

20m

30m

40m

50m

Is this efficient? Does it matter?Is this efficient? Does it matter?

The Process Continues…The Process Continues…