Photo-Fork Sensor (Slot Sensor)
A U-shaped optical sensor with emitter and receiver built into opposite tines — detects objects passing through the gap.
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
- A U-shaped housing leaves a narrow slot for a label, web, or small part.
- Active face
- The emitter and receiver oppose one another across the fork gap.
- Cable and terminals
- The cable normally exits the solid base of the fork.
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
Photo-Fork Sensor (Slot Sensor): cause to controller
Now showingPhysical event
Label edge enters the fork → Receiver changes state → %I0.5 = 1
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
Photo-Fork Sensor (Slot Sensor)
PLC channel
%I0.5
RAW 0
Engineering value
35 %
Gap interrupted
Channel healthy
Signal is inside the expected operating range
Commissioning note: Teach the backing and label separately when contrast is low.
Field guide
A photo-fork sensor — also called a slot sensor or fork sensor — is a self-contained through-beam sensor shaped like the letter U. The emitter sits in one tine and the receiver sits in the opposite tine, with a precisely defined gap between them. Any object passing through the slot interrupts the beam, switching the output.
Because the emitter and receiver are mechanically fixed relative to each other, there is no alignment step during installation. This makes photo-forks extremely repeatable and reliable in high-speed applications where positional accuracy matters.
The slot width is fixed at manufacture — typically 3 mm to 30 mm depending on the model. Choosing the right slot width is critical: too wide and thin objects may pass without reliable detection; too narrow and thicker parts will jam.
Photo-forks are the sensor of choice for label-gap detection on liner-less and liner-backed webs, for encoder disc reading (the slots or slots-and-teeth trigger the sensor to produce a pulse train), and for edge detection on thin films.
Electrically they follow the same NPN/PNP, NO/NC conventions as other discrete sensors. Most photo-forks output a clean square wave at high speed — models rated for 50 kHz switching are common for encoder applications.
Use this when…
- Detecting individual sheets, labels, or thin film in a gap
- Counting encoder disc slots for position feedback
- Detecting the presence of a circuit board or thin part
Where you will see it
Label applicator
Photo-forks detect label gaps on a backing liner, timing the applicator head to peel and place each label precisely.
PCB handling
Slot sensors detect the leading edge of circuit boards entering a solder wave machine, starting the conveyor-speed synchronisation.
Next skill
Connect it to PLC logic
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