piclist 2000\05\28\200640a >
Thread: LED Challenged
www.piclist.com/techref/displays.htm?key=led
picon face BY : Ed Edmondson email (remove spam text)



I guess I am really "LED Challenged" as I can't find a circuit that will dim
an LED used as a backlight.  The LED has a Vf of 3.5 to 4.0 volts @ If of 20
mA.  I want to run it off 5 volts and want to vary the intensity from "Off"
to "On" (less the drop across the Vbe) using a 2N3904?  I figured the LED
limit resistor to be about 45 ohms (5 - .6 - 3.5)/.020) ?  Is this right?

How do I vary the base drive to vary the LED intensity?  A resistive divider
with a pot?  How do I calculate the values for the divider?  The 2N3904 has a
hfe of 100 to 300.  Does anyone care to help me out?

Regards,
Ed

<35.5c71e27.26630e69@aol.com> 7bit

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