Control the stop/start of a conveyor belt using a sensor (proximity or photoelectric)
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Circuit for AC powered motor.
This circuit switches the mains power for your motor.

When the sensor is triggered RLY1 is energised.
The motor is powered from an NC (normally closed contact). When this opens the motor stops.

The sensor and relay coil need to be rated for the voltage of your mains supply. The relay contacts need to be motor rated and go for a rating several times that of your motor's run current to be sure that it can handle the starting current.
Fuses are not optional (although your motor may have one already).