Industrial Sensors — A First Look

Every sensor on a factory floor belongs to one of three categories. Understanding the difference before you wire anything will save you hours of troubleshooting later.

Discrete sensors give the PLC a single bit — ON or OFF, 1 or 0. A photoelectric sensor that fires when a box crosses a beam is discrete. A float switch that closes when a tank is full is discrete. The PLC reads the signal on a digital input address like %I0.0. There is no middle ground: the input is either energised or it is not. Discrete sensors are the most common type in manufacturing, and they are almost always the first ones beginners encounter.

Analog sensors give the PLC a continuous range of values instead of a simple on/off. A pressure transmitter might output 4-20 milliamps proportional to 0-150 PSI. A thermocouple produces a millivolt signal that maps to a temperature range. The PLC reads this on an analog input word — a 16-bit integer that the program scales into engineering units. Analog sensing is what lets a PLC control a process precisely: it knows the actual temperature is 87 °C, not just "hot."

Safety sensors are a distinct category, not a signal type. They can be discrete or analog internally, but they are designed and certified to fail safely. A safety light curtain cuts machine power the instant any beam is broken — it does not just send a signal that software might ignore. Safety circuits use dual-channel architectures and self-monitoring relay modules to meet IEC 62061 and ISO 13849 performance levels. The hardware does the safety logic, not the standard PLC program.

The rest of Sensor School is organised around these three families. Start with discrete sensors — they are simpler to wire and debug — then move to analog once you are comfortable reading input addresses.