Re: A-Bus IR LED flicker

[nobody@xxxxxxxxxxxx (Dave Houston)]

Pulse meters that slip over your finger use IR in the 900-925 nanometer
range. They transmit the IR through the finger and receive it on the other
side. Surges in blood flow modulate the IR at your pulse rate. Consumer IR
uses 940 nanometer IR. A finger is not the best choice to block 940nm IR
although it will attenuate it. Even black plastic may not block CIR.

We emit IR in the 8-14 micron range but, as I understand it, it's the
optical filtering used that determines what wavelength an IR detector
responds to rather than the detector itself. I have no idea (and suspect you
have none either) about the specs of the filter/detector used in this
application. I assume (as you have) that it's designed for consumer IR but I
don't know how it may be affected by extremely near far-IR. Again, I don't
think a finger is the best choice for a filter.

Vishay has a good white paper on interference sources for their Consumer IR
receivers.

     http://www.vishay.com/docs/fmod_disturbance_sources.pdf

Plasma screens also give most IR receivers fits.

Consumer IR receivers rely on modulation (usually in the 36-40kHz range) to
separate a signal from ambient IR.

"John O" <johno@#no^spam&heathkit.com> wrote:

>> Your finger will not block all IR. Your body also emits IR.
>
>Waitaminute...we need a tech timeout.
>
>We emit far-IR radiation, but this sensor is looking for near-IR light. Very
>different things, very far apart on the spectrum. A remote control sensor
>cannot ever detect IR radiation. Otherwise PIR motion sensors and IR
>receivers would be the same, and they're not.
>
>-John O
>