This is a one direction motion sensor circuit. This motion sensor circuit is used to detects an object passing in one direction, ignoring an object that going to opposite way. This circuit uses two sensors to identify the movement only  in one direction. The basic principle of this circuit is simple, where one sensor is used to generate a short pulse, and the other sensor is used to block of turn of the gate. The phototransitors give high output on their collectors when there is an object blocking the light.  By a 0.4uF differentiator capacitor, the interruption of light at Q2 sensor will produce short pulse at point C. But this short pulse will only appear at the output if  a high signal appears at A. This condition will be satisfied if the light to  Q1 is blocked by the object when the object is passing through Q2, means that the direction should be from Q1 to Q2. Here is the schematic diagram of the circuit:

 

Unidirectional Motion Sensor circuit schematic