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More  Lamps, LED, Solar

Shop Safety Light
November 13, 2012                designed by David A. Johnson, P.E.

My 1200 square foot metal shop only has one small entrance door and no skylights.  If the lights are turn off while inside, there is not enough light to safely find the door to exit the building.  On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking.  I was left in total darkness and only my very loud screams would bring her back to her senses.  I thought it would be wise to install some kind of light, which would turn on for a few minutes every time the ceiling lights were turned off.  The circuit below performs this task.
Dave's Shop "Basement" Electronics Lab Metal Shop Ceiling Lights
1200 Square Foot Metal Shop   Metal Shop Ceiling Lights

The shop has a tall ceiling with thick steel beams.  There are 12 long fluorescent lamp pairs proving illumination inside.  My plan was to install a box, containing a bright 3 watt white LED, up at the peak of the ceiling.  The box would have the LED on a small heat sink, an LED driver, a 3 minute timer and a battery pack.  The box could be held to the metal beam with a strong magnet.  A thin wire cable would emerge from the box and connect to a solar panel some distance away.  The panel would be held against a pair of fluorescent lamps with a Velcro strip.  When the shop lights are on, current would flow from the solar panel to the safety light’s battery.  Although the fluorescent lights are not a bright as sunlight, some measurements indicate that I should be able to collect about 100mw of power from a 6v solar panel. Since I typically spend several hours in the shop each day, even 90 minutes of ceiling lighting would be enough for one 3 minute safety light cycle.  A four cell pack of AA size 2300ma-hour NiMH cells should store about 10 watt-hours of energy.  That should be enough for about 40 3 minute 3 watt lighting cycles.  I will place the solar panel out in the bright sunlight for a day to insure the battery is fully charged.  After that, the energy harvested from the fluorescent lamps should be able to keep up with energy drawn during multiple 3 minute lighting cycles.

Ceiling Fluorescent Lamp Fixture  Rechargeable NiMH Cells  4 AA Cell Holder
3 Watt White LED  Plastic Box  10 Cell 6v Solar Panel
A schottky diode is placed between the solar panel and the battery pack.  The diode prevents battery current from leaking back into the solar panel when the panel is in darkness.  Four 1.2v NiMH cells power the circuit.  The four cells provide 4.8v, which is more than enough to power the 3 watt LED.  Unlike many lithium ion cells, NiMH cells have a fairly low internal resistance and can deliver the needed current.  The Cree LED I chose has a forward voltage drop of about 3.2v at 900ma of current.  A simple resistor, NPN transistor and n-channel MOSFET transistor form a class constant current sink, set for about 900ma of current.  An ultra low power Schmitt trigger IC is configured as a 3 minute timer.  The timer is triggered whenever the voltage across the solar panel drops to near zero volts, which occurs in darkness.  After the timer has done its job, the standby current drops to a very low level, insuring that the battery is not depleted when the system is in darkness.  The n-channel FET is mounted to a small heat sink, since it will dissipate about a watt of power. The white LED is mounted to a star shaped heat sink wafer.  The wafer is mounted to a piece of aluminum to act as a heat sink. The white LED has a broad pattern so it should flood the shop area with enough illumination for me to safely find my way to the door, whenever the shop lights turn off.

Click on Drawing Below to view PDF version of Schematic

More  Lamps, LED, Solar

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