Transcript Part II
The Design Process Abstraction & Synthesis Part II Solar Candle Continued… by Prof. Bitar Abstraction & Synthesis Homework #2 Research Prior Art Brainstorm Possible Solutions HW#2 Perform Value Analysis Viable Options Apply Constraints Preferred Solution Abstraction & Synthesis 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 Solar Cell Charge Controller Rechargeable Battery Mode Selection Efficient Drive Circuit Flickering Control White LED 3.2V @ 20mA (worst case) 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 Solar Cell Charge Controller Rechargeable Battery Mode Selection Efficient Drive Circuit Flickering Control White LED 3.2V @ 20mA (worst case) 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 Mode Selection Efficient Drive Circuit Flickering Control LED 3.2V 20mA Focusing on the Solar Cell Photo Sensor Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Mode Selection Efficient Drive Circuit Flickering Control LED 3.2V 20mA 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 T 50m Solar Cell Current (A) Solar Panel V-I Characteristic 40m 30m 20m 10m 0 0.00 1.00 2.00 3.00 Solar Cell Voltage (V) 4.00 5.00 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 = 10 hrs Q: Is this realistic? T 50m Solar Cell Current (A) What voltage are we operating at? 40m 30m 20m 10m 0 0.00 1.00 2.00 3.00 Solar Cell Voltage (V) 4.00 Is this efficient? Does it matter? 5.00 The Process Continues…