Sensors

Sensors give the control circuit information about the process. As far as the control circuit is concerned, a sensor is a switch: its output is either closed or open, depending on whether the measured limit has been exceeded.

Output types

• Contact output. Float switches, pressure switches and thermostats, for example, switch with a mechanical contact. They are wired like any other contact.
• Transistor output. Electronic sensors, such as photoelectric sensors, are usually 24 V DC three-wire sensors: brown +, blue 0 V and black output. A PNP output supplies positive to the load, and an NPN output switches the load to negative.

Common sensors

• Level. A float switch, or electrodes with a level relay, tells whether the liquid is above or below the limit. Tank filling uses a high and a low limit so that the pump does not keep starting and stopping.
• Pressure switch. Switches at a set pressure. There is an adjustable difference between the cut-in and cut-out pressure, so the contact changes state only at the set limits.
• Photoelectric sensor. In a through-beam sensor the transmitter and receiver are separate, in a retro-reflective sensor the light comes back from a reflector, and in a diffuse sensor it comes back from the object itself. In Light-ON mode the output is on when the light reaches the receiver, and in Dark-ON mode when the beam is broken.
• Twilight switch. Switches when the light level falls below a set limit.
• Thermostat. Switches at a set temperature, for example in heating control.
• Door contact. A magnetic contact tells whether a door is closed or open.

Remember

It is good practice to design alarms and protection so that a broken wire shows up as a fault. For example, in dry-run protection the sensor contact is closed when there is water, and open when the water runs out or a wire breaks.