piclist 2000\05\29\001724a >
Thread: LED Challenged
picon face BY : Spehro Pefhany email (remove spam text)

At 08:06 PM 5/28/00 EDT, you wrote:

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?

Assuming you just want to vary the brighness with a pot, not under program

If you intend to use a pot to control the brightness, one easy way would
be to use a 555 to do PWM and also switch the LED.

Have a look at the data sheet for the 555. If you connect the backlight
from +5 , through a series resistor, to pin 3 the 555 will easily drive
20mA (even a CMOS 555 will easily sink the 20mA).

If you use a fixed resistor from +5 to pin 7, perhaps 10K, then use a pot
of, say, 100K with a 1K series resistor from pin 7 to pins 2 & 6,
you can get from 10% to 90% brightness. Use a timing capacitor such as
1n0 that will give a reasonable frequency.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspam_OUTspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com


In reply to: <35.5c71e27.26630e69@aol.com>
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