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.
Spehro Pefhany --"it's the network..." "The Journey is the reward"
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See also: www.piclist.com/techref/displays.htm?key=led
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