Just for reference, this combination worked for me initially.

 

Use a pair of emitter and sensor for each beam, with same circuit as for 1.

Shield both inside narrow opaque tubes! align the tubes in a clear straight line between them.


make sure:

1) no ambient light can get in, front side or rear.
2) make sure each sensor sees only the light from it's opposite emitter, no spill from the neighbor.
This may require moving the sensor and emitter deeper into the shielding tubes.

Connect the 4 outputs to an OR gate, so any of the 4 triggering causes an output.

 

I ran a short test on Fig. A using a 1W IR LED , 120 deg., operating at 20 mA, wavelength unknown with a
SMT IR transistor wave length unknown placed at outer edge. Dropping a AAA battery successfully detected.
Alignment of transistor is trick Rx . Fig. B might be easier with just 3 pairs. Would use T1, 3mm Tx & Rx with
wavelengths of 800 or 900 nm. Rx might be Rohm RPT-38PB3F & Tx OP266B.

 

If I were doing this, I would bore a hole in 2 wood blocks to fit pipe then drill 3- 3mm holes. TX - laser diode modules, 650 nm, 6 mm dia. 2 on top 1 bottom center. Rx- 3 mm photo transistor near 650nm. or 5mm
like Ledtech LT9593-91-0125.

 

View attachment 161149 If I were doing this, I would bore a hole in 2 wood blocks to fit pipe then drill 3- 3mm holes. TX - laser diode modules, 650 nm, 6 mm dia. 2 on top 1 bottom center. Rx- 3 mm photo transistor near 650nm. or 5mm
like Ledtech LT9593-91-0125.

Sorry, for the delay in responding.

Thanks for the simple idea of aligning the devices. You have made my day!