On 11/12/05, Dmitriy Kiryashov <vze27bym@verizon.net> wrote:
> Hi Olin.
>
> CCP pin to base of NPN  ( 2N4401 or 2N2222 , Vcbo about 50v )
> collector to +5v, emitter to current limiting resistor
> 2nd pin of current limiting resistor to anode of LED
> cathode of LED to ground
>
> One resistor less and smaller pcb estate.
> Any objections ?

Many. Switching the LED will be like vodka with water...
:)
If you was thinking to a constant current switcher, than keep the
sensing resistor in the transistor's emitter and the IR led in the
transistor's collector. Else will be a mixed analogic/switching
design.

cheers,
vasile
>
>
> WBR Dmitry.
>
> PS. 2N4401 with hFE = 50 on 100 mA sounds scary :)
>
>
>
> Olin Lathrop wrote:
> > Not a good idea.  First, there is no need for a separate pin to
> > enable/disable the 40KHz pulse output.  You can do that just as easily in
> > software by enabling/disabling the CCP module.  Second, you need a resistor
> > in series with the LED if running it from 5V.  The forward voltage of an IR
> > LED is nowhere near 5V, so without a resistor something will have to give.
> > That will probably be one or both of the PIC pins.  Third, IR LEDs can take
> > a lot more current than most visible LEDs of the same size.  Check the data
> > sheet.  Most likely you can put at least 100mA thru it when it's on, for an
> > average current of 50mA.  Fourth, you can't drive that much current with a
> > PIC pin.  Connect the CCP output pin to a resistor, other end of resistor to
> > base of NPN transistor, emitter to ground, collector to LED cathode, LED
> > anode to another resistor, and the other end of that resistor to 5V.  Size
> > the LED resistor to get 100mA (depending on your LED) thru the LED when the
> > transistor is on.  Size the base resistor to make sure the transistor is
> > saturated when sinking the LED current.  With a jellybean transistor like
> > 2N4401 you can count on a gain of at least 50.  That means you need at least
> > 2mA base drive to support 100mA LED current.  A 2Kohm base resistor will get
> > you a little more than that, which is probably a good answer.
> >
> > > a 40khz pulse should be output on the IR LED for a short period of time
> > > (about a second).
> >
> > That's not a short period of time.  The IR detector will probably want to
> > see pulses from 10 to 50 cycles.  Much beyond that and it's automatic gain
> > circuitry will decide the pulse is ambient and set its threshold to filter
> > it out.
> >
> > > I have included a simple Excel spreadsheet I made to help calculate
> > > the settings for the CCP Module for the PIC 16F628, which I used to
> > > come up with my values for the program.
> >
> > That's silly.  What happens when you change the PIC oscillator frequency or
> > want to use a different IR modulation frequency?  These should be parameters
> > directly in your code, probably a top level include file, and everything
> > else should be computed from that at assembly time.
> >
> > ******************************************************************
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> > consultant in 2004 program year.  http://www.embedinc.com/products
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