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

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

3. Product Reminder

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

5. 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

6. 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

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

8. 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.

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

10. 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!

11. Possible choice: NiCd / NiMH

12. Battery Specification Discharge Curve

13. 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
= 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!

14. 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

15. 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

16. 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

17. Solar Panel V-I Characteristic

18. 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?

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

20. The Process Continues…