[mythtv-users] Where to buy CommandIR ?

[Brian Wood]

On Monday, August 02, 2010 03:12:15 pm Douglas Peale wrote:
> On 08/02/2010 02:01 PM, Brian Wood wrote:
> > On Monday, August 02, 2010 02:55:41 pm Gert van der Knokke wrote:
> >> Chris Hoff schreef:
> >>>> Otherwise, you could try attenuating the signal reaching the STB's IR
> >>>> receivers with something like multiple layers of white copy paper. If
> >>>> you come up with something that works extremely well, do pass it on!
> >> 
> >> If these emitters are simple IR LED's why not attenuate the current
> >> through them by means of a simple resistor. Just keep increasing the
> >> value up to the point the receiver stops responding then use a resistor
> >> value slightly less than this. You could even use a adjustable resistor
> >> in the range from 1 to 10 k or so.
> >> 
> >> Good luck,
> >> Gert
> > 
> > LEDs are not like incandescent lamps, you can only reduce the current so
> > much before you get below the firing point of the diode junction. You
> > can't dim them the way you can a lamp, the usual way to dim LEDs is to
> > vary the duty cycle, but that won't work when pulsing the diode for IR
> > transmission.
> > 
> > You might try a dab of not-quite-opaque nail polish at either end of the
> > IR link, it might take some experimenting to find a combination that
> > works, transparency to visible light may differ from the transmission of
> > IR energy.
> 
> You're thinking of neon bulbs.
> 
> You can dim LEDs by changing the resistance in series with them. LEDs
> operate at a fixed voltage, and the resistor is used to adjust the current
> through them. The lower the resister value, the higher the current and the
> brighter they get (until you exceed their current rating and they burn
> out). So yes you can adjust the brightness by changing the value of the
> series resister. _______________________________________________

Only to a point. It's not a smooth progression down to nothing, there is a "cliff" below which you get nothing.

Apple had to design a circuit board for the iBooks when they wanted the front LED to "breathe" when the unit was sleeping, 
the board pulsed the LED and varied the duty cycle, in order to have it appear to "fade out" and not simply dim a bit and 
then go off.

But, as long as the required amount of IR output falls withing the operating range of the junction, adding series 
resistance will work, though it throws away energy as heat.

It is similar to a neon or other gas discharge lamp, but not quite the same.


See:

http://www.mikesflightdeck.com/led_dimmer.htm

The best way to dim an LED is to control the duty cycle:

http://www.lunaraccents.com/technology-LED-truck-lights-dimming.html

and

http://www.reuk.co.uk/LED-Dimmer-Circuit.htm