Industrial Sensors — A First Look
Meet the three sensor families — discrete, analog, and safety — and learn which one your application needs.
01 / Recognize it
What this sensor looks like
Learn the housing, active face, mounting, and connector before you meet it on a machine.
Hardware recognition
Know what to look for
Use the silhouette, active face, and connection style to identify the device before checking its part number and datasheet.
- Body and mounting
- Discrete sensors are usually compact; analog transmitters often have a display; safety devices use distinctive red or yellow hardware.
- Active face
- Look for a lens, sensing face, process fitting, probe, or guarded actuator.
- Cable and terminals
- The terminal count hints at a simple switch, analog loop, smart link, or monitored safety circuit.
02 / Understand the principle
Watch cause become a PLC signal
Follow the physical event through the sensing element and into the exact controller value.
Signal story / live loop
Compare the three sensor families
Now showingDiscrete family
A carton breaks the beam → Photoeye switches → PLC reads one bit
03 / Test and commission it
Commission it on the bench
Move the process, adjust the setpoint, invert the logic and inject faults. Watch the PLC value respond immediately.
Commissioning bench
Industrial Sensors — A First Look
PLC channel
%I / %IW
RAW 9677
Engineering value
1.0
Signal path
Channel healthy
Signal is inside the expected operating range
Commissioning note: Follow the signal from the physical quantity to the PLC address.
Field guide
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.
Use this when…
- When you've never wired a sensor before
- When you don't know what 'analog' means in PLC speak
- When you're trying to pick which sensor a project needs.
Where you will see it
Bottling line (discrete)
A retro-reflective photoeye detects each bottle passing under a filling nozzle and sends a single ON/OFF bit to the PLC — classic discrete sensing.
Tank level (analog)
A pressure transmitter at the base of a water tank produces a 4-20 mA signal proportional to the head of liquid above it — the PLC reads the exact level, not just "full or empty".
Concept and commissioning lab
This topic is taught through the live bench and reference rather than a separate ladder challenge. Continue to a physical sensor when you are ready to wire a PLC input.