Photoelectric sensors (photocells) are those who respond to the change in the intensity of light, allowing the activation or deactivation of a signal depending on the values received from that light. Photoelectric barrier, wherein the transmitter and receiver are separated in different bodies, standing in line and both being the components facing one another. This is the mode most fiable, because all the power emitted by the transmitter is sent directly to the receiver, causing dirt and humidity less adverse effect on its operation.