Thanks guys,
the IRPDs are like those I have made in the good old 555 days, but I want quite a large range, and in sunshine.
I used to make a TV remote control extender back in the last millennium, that had the IR LEDs - 4 of them in series - on top of the TV rooms TV. I pulsed them at about 1 amp.
The pulses would beam out across the room and bounce back from any furniture or walls onto the front of the VCR / sat receiver etc.
( BTW The unit was on top of the TV as I sent the pulses through the coax screening, so it couldnt be installed across the room like a hardwired version.)
The longest range I personally measured was a hall about 20 meters long, and it worked fine.
Of the 1000 sold I never heard of a complaint that it didn't work.
I am making a security beam for gardens that has over 20m range in sunshine ( with 6 IREDS driven with short pulses of 38 Khz ) but I think for the new project I will need a continuous 38 Khz - commutated around the circle of IREDs.
I don't have to use 38 Khz, as I will be making my own receiver.
24uS is pretty fast Riva, so I think that will not be a problem, I can scan all 32 IREDs perhaps 4 times a second for my idea, so there should be plenty of time.
If I ran it slow enough, I wonder if I could sample the on and off receiver levels of the pulses to do the synchronous detection easily in the software ( subtract the off Rx level from the on level each cycle ? )