A metal / non-metal sorting line controlled by a simulated PLC. Press START, watch the sensors fire, follow the ladder logic rung by rung, then break things with fault injection and see what happens.
Open the Playground → wire your own panel and write your own ladder logicInductive proximity sensor PX1 flags metal parts. A FIFO shift register tracks every part down the belt until photo-eye PE2 tells the PLC it has reached the pneumatic diverter. Capacitive level sensors stop the line when a bin is full. Click any component on the machine to learn what it is.
Each sensor on its own test bench. Move the target, change the material, and watch the output LED.
An oscillator coil creates a high-frequency magnetic field at the sensor face. A metal target lets eddy currents flow, which damps the oscillation. When the amplitude falls below a threshold, the output switches. Non-ferrous metals damp less, so the working range is multiplied by a correction factor. A small hysteresis band stops the output chattering at the edge.
The sensor face is one plate of a capacitor and the surroundings are the other. Anything with a dielectric constant higher than air raises the capacitance. That's why it detects plastic, wood, powder and liquid, even through a non-metal tank wall. You tune the sensitivity pot so the wall is ignored and the product is detected. The simulator uses two of these as bin-full level switches.
Through-beam uses a separate emitter and receiver. It has the longest range and is the most reliable. Retro-reflective uses one housing plus a reflector; a polarizing filter helps with shiny objects, but clear bottles can still fool it. Diffuse relies on the object reflecting light back, so dark or clear objects may be missed. Dark-ON means the output is ON when the beam is blocked.
The sensor sends an ultrasonic burst and times the echo: distance = speed of sound × time / 2. It outputs a 4–20 mA current loop. 4 mA is a "live zero", so 0 mA means a broken wire. The PLC analog input card converts the current to a raw count (for example 0–27648 on Siemens S7). The program then scales that count to engineering units.
Most DC proximity sensors have three wires: brown +24 V, blue 0 V and black output. The output transistor either sources +24 V to the load (PNP) or sinks the load to 0 V (NPN). The PLC input card must be the opposite type.
What each device does and how it connects to the PLC.
A PLC doesn't react to inputs the moment they change. It runs one loop forever, typically every 1–20 ms. The simulator above runs exactly this loop.
| Symbol | Name | Meaning |
|---|---|---|
--] [-- | XIC / NO contact | True when the bit is 1 |
--]/[-- | XIO / NC contact | True when the bit is 0 |
--( )-- | OTE / output coil | Bit follows the rung condition |
--(S)-- --(R)-- | Set / Reset (Latch / Unlatch) | Bit stays on until it is explicitly reset |
--|P|-- --|N|-- | Rising / falling edge (one-shot) | True for exactly one scan on the edge |
TON | On-delay timer | Q / DN turns on after the input has been true for the preset time |
CTU | Count up | Accumulator +1 on each false→true transition |
FFL / FFU | FIFO load / unload | Queue used to track parts between stations |
Note on Stop and E-Stop: they are wired normally closed, so the input reads 1 when healthy and the program uses a normally-open contact. If a wire breaks, the input drops to 0 and the machine stops. This is called fail-safe wiring. Real E-stops also cut power through a safety relay, not just through the PLC program.