LunaLight PCB Rev 2.1 Mike Deagen (MATE) IME 458 May 25, 2012 Fully Integrated Solar-Rechargeable LED Lantern and Cell Phone Charger

The LunaLight is the result of a multidisciplinary senior project in collaboration with the non- profit organization One Million Lights LunaLight debut in Kenya (April 2012)

A solar-rechargeable LED lantern and cell phone charger has three sub-circuits IC TypePurpose Battery ManagementTake the input power from the solar panel and safely charge the rechargeable batteries within the lantern LED DriverProvide a constant current to the LEDs at the appropriate forward voltage DC-DC BoostIncrease the potential of the 3.7V Li-ion battery to meet the 5V USB standard voltage Battery ManagementLED Boost Driver & DC-DC Boost for Cell Phone Charger LT3652 MSOP-12 package Cost: $4.36 MC34063 SOIC-8 package Cost: $0.45

Rev 1.0 of the PCB had wires leading to the batteries, the LEDs, and the switch

The goal for the second revolution was to reduce overall assembly time by integrating all the electronics onto a single board Other Goals Use mostly SMT components Multiple brightness settings with a slide switch Li-ion battery instead of Nickel Metal Hydride Ports and Switches LEDs Battery Holder

I modeled the thermal management to see if the LED junction could rise above T max = 150°C Heat Generated = 0.84 W Total Est. θ = 102°C/W ΔT = 86°C If T ∞ = 25°C, T junc = 121°C 81% of T max

I used reflow soldering technique for SMT components (front side) and manual soldering for thru-hole components (back side) 91% isopropyl rubbing alcohol removed the solder flux residue

The basic functions of the PCB were confirmed with simple verification tests Illuminance from 12” away “HI” = 473 lux “LO” = 94 lux Solar Li-ion battery charging V batt_chg = 3.96 V I chg = 212 mA

Testing the LED voltage drop over time indicated a junction temperature of 50°C Temperature coefficient of voltage = -4.0 mV/°C [CREE XP-E datasheet] For two LEDs in series, temperature coefficient of voltage = -8.0 mV/°C However, the temperature coefficient of voltage is probably not linear!

Overall, I think the design was successful, and future LunaLight teams should consider implementing a fully integrated PCB

Questions?