The Design Process Abstraction & Synthesis Part II Solar Candle Continued… by Prof. Bitar

Abstraction & Synthesis Homework #2 Viable Options Preferred Solution Research Prior Art Brainstorm Apply Constraints HW#2 Abstraction & Synthesis Possible Solutions Perform Value Analysis

Product Reminder

The Customer Requirements Explicit Cat Safe Look Nice Different Colors Automatic Implicit Low Cost Reliable / Durable Low Maintenance

Product Specifications yet to be quantified… Safety / Durability Heavy Base Unbreakable LED Secure to Window Sill, Sash or Window Pane No Cords Low Voltage Aesthetics Traditional Look Interchangeable Color LEDs Flickering Option Low Operating Cost Long Battery Life (Efficient) Rechargeable Solar Rechargeable Photo Sensor or Timer

Current Block Diagram Photo Sensor White LED 3.2V @ 20mA (worst case) Solar Cell Charge Controller Rechargeable Battery Efficient Drive Circuit Mode Selection Flickering Control

A fundamental question to answer… How much energy is required to operate our solar candle (LED)?

How much energy? Consider worst case output requirement… 3.2V x 20mA = 64 mW How much time will the LED be on? 64 mW x 6 hrs = 384 mW•hrs Answer: 384 mW•hrs (units of ENERGY!) Assuming 70% efficiency, 384 / 0.70 ≈ 550 mW•hrs needed NOTE: On average, this minimum amount of energy needs to be supplied by the solar panel and stored in the battery during the day, if our candle is to be sustainable.

Focusing on the Battery Photo Sensor White LED 3.2V @ 20mA (worst case) Solar Cell Charge Controller Rechargeable Battery Efficient Drive Circuit Mode Selection Flickering Control

Things to consider… Types: What rechargeable technologies will work in this application? (NiMH, NiCd, Lithium Ion, Sealed Lead Acid, Super Capacitors…) Voltage: What is the minimum battery voltage? Capacity: How long does the battery need to hold a charge? (Six Hours Minimum) Shape: Needs to fit in candle stem or base. Cost: I don’t want to spend a lot on batteries!

Possible choice: NiCd / NiMH

Battery Specification Discharge Curve

Battery Feasibility Check… THREE 1.2V (nominal) NiCD or NiMH batteries wired in series (3.6V nominal). Cost: $2.37 each (single qty) / $1.30 (1000 qty) Shape: AA cells would fit in candle stem or base. Voltage Check: 1.2V x 3 = 3.6V Charge Capacity: 700 mA•hrs (units of charge) Energy Capacity: 700 mA•hrs x 1.2V = 840 mW•hrs (per battery) = 2,520 mW•hrs (for 3 batteries) NOTE: Since only 550 mW•hrs are required, one battery has enough energy capacity to do the job, BUT the voltage will need to be BOOSTED!

Question: Do we know the specs for a possible drive circuit? YES! Photo Sensor Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Efficient Drive Circuit LED 3.2V 20mA Mode Selection Flickering Control

Focusing on the Solar Cell Photo Sensor Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Efficient Drive Circuit LED 3.2V 20mA Mode Selection Flickering Control

How about the solar cell from Home Depot? After taking the Home Depot Landscape Light apart, I made the following measurements (in direct sun): ISC = 50mA , VOC = 4.3V

Solar Panel V-I Characteristic

Question: How long would it take this solar cell to recharge a completely discharged battery? Battery Capacity = 700mA•hrs Solar Cell (in full sun) = 50mA Therefore, 700mA•hrs / 50mA = 14 hrs Q: Is this realistic?

What voltage are we operating at? Is this efficient? Does it matter?

The Process Continues…