Strategy for up to 40 ADC inputs using alert feature

Question

I'm working on a project where I want to monitor 40 phototransistors (IR breaker beams), but without having to constantly poll the ADCs. Ideally I'd want a digital pin pulled when one of the beams is broken and then I'll poll the ADC(s) to find out exactly which one it was. I'll also need to be able to detect a change on each input regardless of the state of the others.

I see there are ADCs with programmable GPIOs and dedicated alert pin(s). I'm just a beginner, and I find - particularly with the programmable GPIO varieties - the datasheets confusing and I doubt I have the experience to successfully make use of those.

Then the variety with dedicated alert pins seem to be easier to program but don't seem to come any larger than 8-channel. Fewest parts please, particularly since the incredibly small IC packages will likely be hand-soldered. AD7997 is currently the chip I'm considering, but would require 5 TSSOP footprints. I would probably AND the alert pins together to the host.

Are there any other ways to do this? Thanks for your input.

More project details: Beaglebone black, Node.js/bonescript, Phototransistor is the LTR-301.

UPDATE 1

Breaker beams will be broken or unbroken for large amounts of time.

Answer 1

If you have 40 inputs that are normally at zero and you want to know if one of those signals has gone high you can use logic gates to detect this or if you want, you can use diode orring to give you one signal that tells your one of the inputs has risen high.

The same can be done if the inputs are normally high and one goes to 0V, It's fairly trivial with real voltage signals but a little more care has to be taken when the signals are from an open-collector as per a photo transistor - you've got to make sure the loading of the diodes/logic isn't too great to cause the signal problems.

By the way - using diodes means that the inputs remain independant and still individually readable should you choose to do so.