Through-Beam Photoelectric Beam Sensor

[saveearth123]

i need  low voltage sensors that will detect when something about the size of a tiny finger passes through a beam.  They will also need to be compatible with a low voltage load - about 5 volt dc.  likely small relays will be needed.  this is all under water but enclosed without water.  this is for an aquatic project.  the beam distance does not need to be more than one or two inches.  no life will be harmed, of course.  i see lots of sensors rated for something in your back yard or at your garage door available.  

 

[HarryA1]

I used a infrared  transistor as a detector and diode emitter to make measurements on my quadcopter by having the blade pass through a gap with one on each side. You can get similar optical detector devices already mounted in pairs and separate them I would think.

See this one for example: https://www.ebay.com/itm/402866611861?hash=item5dccb8a695:g:uUEAAOSwby1gql8L

or unmounted: https://www.ebay.com/itm/274298671466?hash=item3fdd79916a:g:j~YAAOSwWNNdp9Dl

The data sheet with circuit can be found here: https://www.digikey.com/en/products/detail/omron-electronics-inc-emc-div/EE-SG3/31139?s=N4IgTCBcDaIAQHkBKBWADAFgLQDkAiIAugL5A

Perhaps an enclosure like this?