I assembled MK 120. It functions in the following manner: while the beam from the transmiter to the receiver is broken, the alarm sounds. But as soon as the beam is restored, the alarm stops. Is this how it is supposed to function? Is there a way of keeping the alarm on for a period of time, e.g, 20 seconds, after the beam is restored? This is useful to detect entry because the beam is only very briefly interrupted and the alarm is barely noticeable because it lasts for too brief a period.
Your kit is working as it should.
This is just a simple, educational circuit.
If you want pulse-stretching, you will need to modify the circuit (e.g. replace buzzer by a relay and trigger a timer…).