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Solid State Penlight LED Circuit DesignThis solid State Penlight LED Circuit Design is one of the simplest electronic design that one can embarked on. The only challenge is how to put the circuit into the pen holder and incorporating a switch in it. This solid state LED as a penlight is far better than the incandescent penlights as the batteries for incandescent type run out very fast. With the advancement of LED technology, more and more applications are turning from incandescent to LED in many real life application. Using a 5000 milicandela LED brightness, this penlight will have a running life of greater than 4 weeks and the LED hardly burn out. Circuit Description The schematic diagram shows that the power to the LED is provided by two AA alkaline batteries, B1 and B2 in series. Switch S1 turns the unit ON or OFF. A 1N4001 or 1N4003 diode, D1 drops the battery voltage to 2.3V (3.0V - 0.7V) which supply the power to LED1. An ultrabright LED should have a property of sharply increasing its internal resistance as the applied voltage drops. As a result, as the batteries fade, LED1 will demand less and less current thereby maximizing the life of the two batteries.
Circuit Description The schematic diagram shows that the power to the LED is provided by two AA alkaline batteries, B1 and B2 in series. Switch S1 turns the unit ON or OFF. A 1N4001 or 1N4003 diode, D1 drops the battery voltage to 2.3V (3.0V - 0.7V) which supply the power to LED1. An ultrabright LED should have a property of sharply increasing its internal resistance as the applied voltage drops. As a result, as the batteries fade, LED1 will demand less and less current thereby maximizing the life of the two batteries.
LED Circuit Design Parts List The parts list of the project is as shown below. 
Source : Extracted from Popular Electronics April 1996, By Dean Poeth.
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