Mk 120 ir light barrier - modify for delay?

This kit is great! Just one thing that would make it better for my application would be to add a delay - so the alarm only sounds after the beam is blocked for more than 3 seconds.

Does anyone know how I could do this ??

The application is for an alarm when a stack of prints from a printer gets too high - only trouble with the standard kit is that the beam is broken for every print that drops - they only break the beam for a second or two, but the alarm sounds each time.

Thanks in advance for any tips or how too’s