Philip Martin wrote:

> The main question is, is it feasible to use the analogue input of say
> a 16C622 to detect the ON state of an IR LED, instead of using a
> comparator.

  If all you wish to check is a logic level, why do you need to consider
analog values?  The PIC inputs are extremely high impedance, and
(without reference to the data sheet) just as likely Schmitt triggers,
so you should only need to provide the correct value of pull-up resistor
and ensure you have short lead lengths.

  Cheers,
        Paul B.