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. An analog input module converts the electrical signal to a raw numeric value; its range and resolution vary by platform and module. The program then scales that raw value into engineering units. Analog sensing is what lets a PLC control a process precisely: it knows the measured temperature is 87 °C, not just "hot."

Safety sensors are a distinct category, not a signal type. They can use discrete or more complex internal signals, but certified devices provide monitored safety outputs for a safety relay or safety PLC. When a light-curtain beam is interrupted, its safety outputs switch off within the device's specified response time; the complete safety-related control system then performs the risk-assessed stopping function. Total stopping time also includes the controller, final switching devices, and machine stopping time. A standard PLC input and ordinary application logic are not a substitute for that validated safety path.

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.