At 03:39 PM 8/13/2009, you wrote:


>I didn't see an image attached to this email.
>
>Could I trouble you to post it again?
>
>Many thanks!
>
>dwayne

Okay, here's a link to a somewhat higher res version:

http://www.speff.com/backlights.jpg

As you can see, when then input is at +5V it provides a constant current of
around 180mA to the backlight LEDs (first-order temperature compensated
because the TC of the Vbe drop of the PNP compensates for the Vbe of the NPN
..although not perfectly because they are at different currents.. but more
than good enough for a backlight). LED Vf compliance  of the current sink is
up to around 4.5V.

When the input is at 0V, the backlight is off (but the circuit still draws
about 1.5mA due to the divider).

That could be avoided by tying the 3K to the input rather than to +5, in
which case only leakage current would flow when the input was low (but
the 270R might have to be increased a bit to account for voltage drop on
the relatively heavily loaded (~6mA) uP output).

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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