CHAPTER 2 Input/Output Devices This chapter is a brief consideration of typical input and output devices used with PLCs. The input devices considered include digital and analog devices such as mechanical switches for position detection, proximity switches, photoelectric switches, encoders, temperature and pressure switches, potentiometers, linear variable differential transformers, strain gauges, thermistors, thermotransistors, and thermocouples. Output devices considered include relays, contactors, solenoid valves, and motors. 2.1 Input Devices The term sensor is used for an input device that provides a usable output in response to a specified physical input. For example, a thermocouple is a sensor that converts a temperature difference into an electrical output. The term transducer is generally used to refer to a device that converts a signal from one form to a different physical form. Thus sensors are often