A PLC is blind without sensors, which are how the controller perceives the physical world. There are dozens of types, and picking the wrong one wastes a commissioning day. This lesson is a field guide to what each sensor actually detects and how it wires in. Then a "pick the right sensor" simulator walks you through how a working engineer reasons from the application to the part number.
Every sensor produces one of two signals, and that decides which PLC input it lands on:
🔲 Discrete (digital): yes/no
"Is the part there?" "Is the door closed?" One bit: ON or OFF. Proximity switches, photo-eyes and limit switches are discrete. They land on a digital input card.
📈 Analog: how much
"What temperature?" "How full?" A continuous value, usually a 4–20 mA current or 0–10 V. Pressure, level, temperature and flow transmitters are analog. They land on an analog input card and get scaled in code.
🧭 First question on every sensor: do I need a yes/no or a measurement? That alone halves your search and picks the input card.
Selecting a sensor means reasoning from the application. What's the target made of? How far away is it? Is contact allowed? Work the scenarios below; each one is a real situation. Pick the sensor an experienced engineer would spec, and learn why the others fail.
| Sensor | Detects | Best for | Watch out |
|---|---|---|---|
| Inductive prox | Metal only, no contact | Detecting metal parts, end-of-travel, gear teeth | Short range (~few mm); ignores non-metal |
| Capacitive prox | Almost anything (liquid, plastic, powder) | Level through a tank wall, non-metal parts | Sensitive to dust/moisture; needs tuning |
| Photoelectric | Anything that blocks/reflects light | Longer range, fast counting, small parts | Shiny/clear targets fool reflective types |
| Limit switch | Physical contact / lever push | Rugged end-stops, doors, dead-simple reliability | Mechanical wear; slower |
| Ultrasonic | Distance via sound echo | Clear liquids, distance regardless of color | Foam, steam, soft targets absorb sound |
💡 Photoelectric has three modes: through-beam (emitter + receiver facing; longest range, most reliable), retroreflective (one unit + a reflector), and diffuse (one unit, bounces off the target itself; shortest, simplest).
Analog sensors turn a physical quantity into a current the PLC can read. Two things are worth understanding properly: how temperature is sensed, and why the loop is 4–20 mA, not 0–20.
Thermocouple (TC): two dissimilar metals make a tiny voltage that tracks the temperature difference between the hot (measuring) junction and the cold (reference) junction. Cheap, huge range (to 1000s °C), but low accuracy and needs cold-junction compensation. Types J, K, T differ by metals/range.
RTD (e.g. Pt100): a resistance that rises predictably with temperature (100 Ω at 0 °C for Pt100). More accurate and stable than a TC, narrower range. Wire it 3- or 4-wire to cancel lead resistance.
Why 4–20 mA? The "live zero": 4 mA = 0% of range. If the wire breaks, current drops to 0 mA. That is below 4 mA, so the PLC instantly knows it's a fault and not a real zero. A 0–20 mA loop can't tell "broken" from "empty."
2 / 3 / 4-wire transmitters: a 2-wire (loop-powered) device draws its power from the same 4–20 mA loop it signals on (simple, fewer wires). 4-wire has separate power and signal. Mis-wiring these is a common first-day mistake.