Direct answer: A loop detector is a sensor that detects the change in magnetic field created when a vehicle passes over a copper wire loop embedded in the ground. It relays this information to the barrier control board, preventing the arm from closing while a vehicle is under it — protecting both the vehicle and the barrier from damage.
How does it work?
A rectangular or square wire loop is laid at a specific depth in the ground and connected to a detector board. When a vehicle with a metal mass passes over it, the loop's inductance changes; the detector picks up this change and sends a "vehicle present" signal to the control board.
The loop detector's role in a barrier
Two loops are typically used: one under the arm (safety loop) and one near the entrance (call loop). The safety loop overrides the close command as long as a vehicle is under the arm; the call loop can be used to detect an approaching vehicle and trigger the barrier early.
Loop detector vs. photocell
Because a loop detector is embedded in the ground, it is unaffected by weather conditions (fog, snow, sun glare) and reliably detects metal vehicles; a photocell, on the other hand, is an optical beam-interruption sensor, usually at arm level, used together with the loop detector to also detect pedestrians or thin objects. The two systems complement each other rather than replacing one another.
Quick facts table
| Operating principle | Magnetic field change in a ground-embedded wire loop |
|---|---|
| Typical placement | Under the arm (safety) + near the entrance (call) |
| Main benefit | Prevents closing while a vehicle is under the arm |
| Loop vs. photocell | Loop detects ground-based/metal, photocell detects optical/arm-level |
Frequently asked questions
What is a barrier loop detector?
It is a sensor that detects the change in magnetic field created when a vehicle passes over a wire loop embedded in the ground.
Is a barrier safe without a loop detector?
Without a loop detector, a barrier closes purely on a timer, which carries a risk of closing on a vehicle under the arm.
Last updated: July 20, 2026 · Technical content: Gray Teknoloji project team
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