Re: Infrared RC oscillator

On Wed, 11 Jan 2006 23:09:16 GMT, Mike Harrison <mike@xxxxxxxxxxxxxxx>
wrote:

>On Wed, 11 Jan 2006 15:05:22 -0500, "Michael" <newszz10@xxxxxxxxx> wrote:
>
>>So many of the xmtr circuits I've seen for infrared remote control use a
>>crystal or ceramic resonator in a cmos oscillator.
>>
>>Is there any reason why an R/C network oscillator using low tolerance parts
>>wouldn't be accurate enough at such a low frequency?
>>(38 or 40 khz).
>
>Just because the frequency is low doesn't mean the accuracy required is any less. In practice it
>gets tricky getting RC repeatability better then about 5%, especially where supply voltage is likely
>to be variable, partly due to the high pulse currents driving the IR LED.

Actually a circuit using a 555 timer will have pretty good immunity
to voltage variations, OTOH the 555 may cost as much as a ceramic
resonator, and the frequency is still dependent on high-tolerance and
temperature-varying resistors and capacitors.

>A 5% frequency error will
>put the carrier far enough out of band for some IR sensors to reduce range noticeably.
>Resonators are cheap and stable and provide the best solution for this application - the proof of
>this is simply that it's what's almost universally used in an extremely price-sensitive market.

.