If the PIC doesn't have analog features it should wear a low value
protection diode current without needing the extra resistor.

Maxim have a data sheet showing almost the same thing using a display driver
chip and a large LED display. It really helps to have constant current drive
though.


----- Original Message -----
From: "Tal" <tal@ZAPTA.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 22 April 2003 19:35
Subject: [PIC]: Driving high voltage LED from a PIC


> Hello,
>
> I noticed the following circuit in a friend design of a very cost
> sensitive product and am curious what the people think about it.
>
> The PIC (e.g. 16F73) is to drive an high voltage LED such as digikey
> 441-1009-ND
> (http://rocky.digikey.com/WebLib/Sunbrite/Web%20Data/SSP-01TWB9WB12.pdf)
> . The general idea is to drive the LED directly from the PIC, saving an
> extra high voltage driver (actually, there are few LED in this circuit,
> each is driven seperatly).
>
> The circuit is as follows:
>
>
>   [14V]----[A LED C]----[R1]---(A)----[R2]----[GND]
>
> Where R1 is the current limiting resistor for the LED, point A is
> connected to a PIC digital output and R2 is a relativly large resistor
> (about 50K or so). The idea is that to turn the LED off, the PIC output
> goes to HIGH or TRI_STATE and because of the 'knee' of the LED curve and
> the resistor R2 that draws a minimal forward current, the voltage at the
> PIC will not exceed its VDD.
>
> Does this make sense ? How about if the PIC is operating on 3V only ?
>
> Thanks,
>
> Tal
>
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