Pulser circuit to interrupt the light beam

As shown in the figure below, the circuit outputs a pulse when an object on the conveyor belt blocks the light. The light source keeps the phototransistor on. This will produce a high logic level voltage on the Schmitt inverter and a low logic level compatible with TIL at pin 5 of the monostable circuit. When the object blocks the light, TIL81 turns off the Schmitt inverter to trigger the monostable circuit.