What a vehicle detector does
Many gates need to know when a vehicle is present. One use is convenience: a car approaching from inside the property can open the gate automatically, with no remote or button. The other is safety: a gate should not close on a car sitting in its path. Vehicle detectors do both, and the most common type is the inductive loop.
How an inductive loop works
A loop is a length of insulated wire, typically laid in a rectangle, buried in a groove cut into the pavement or placed under pavers before installation. The two ends connect to a small electronic unit, called a loop detector, usually in the opener’s control cabinet. The detector sends a small alternating current through the loop, and that current creates a magnetic field above it.
A large piece of metal such as a vehicle changes the loop’s electrical behavior. The detector notices that change and closes a relay, which tells the opener a vehicle is there. When the car leaves, the relay opens. No contact is needed, and the sensor does not need to see the vehicle, so it works in the dark, in rain and in snow.
Detection depends on metal. A bicycle, a small motorcycle or a lightweight trailer may not be detected, because of how little metal there is, how high it sits above the loop, or both. Sensitivity settings on the detector can help, but they are a trade-off with false detections.
Loop roles
The same sensor hardware can be used for different jobs, depending on where the loop sits and how the opener is wired.
- Free exit loop. Placed inside the property, it opens the gate as a vehicle approaches from the inside. It is meant to help vehicles leave without a transmitter.
- Safety (or shadow) loop. Placed directly under the gate’s path, it detects a vehicle in the way. While it sees a vehicle, the gate does not close, or it reopens if it is already closing. This protects vehicles from being struck. It is not designed to protect people, since a person on foot may not be detected.
- Reverse or “closing” loop. Used on some gates to stop or reverse the gate if a vehicle is in the area.
- Arming loop. Placed outside the gate and used in some commercial systems to control access.
The wiring and the opener’s input settings determine what each loop does. Mis-wiring a loop can have undesirable effects, such as a gate opening when it should not, so how loops are connected should follow the opener’s documentation.
Why a free exit loop affects security
A loop that opens the gate for anything that rolls across it is convenient, but it also opens the gate for anyone driving up to it from the inside, and in some cases from outside if the loop sits too close to the gate. Placement and sensitivity matter. A person could also reach a loop with a large metal object. The tradeoff is the same as with any automatic opening: more convenience, a slightly wider path for unintended entry. For a household that cares about this, a keypad or remote might be a better fit. Also, a free exit loop does not remove the need for a safe gate; photo eyes and sensing edges remain essential.
Other exit and presence sensors
Loops are not the only option.
- Radar or microwave sensors. Mounted on a post or wall, these detect motion in an area. They are sometimes used for exit triggers. They can respond to things other than vehicles, such as people, animals and swaying branches, depending on the model and aiming.
- Magnetic sensors. Some detect the disturbance a vehicle causes in the earth’s magnetic field. Specific products vary in how they are installed and how they behave.
- Infrared or beam sensors. Beam sensors can detect that something is in the way, and the article on safety sensors and photo eyes covers them in detail.
- Push-button or wired exit devices. A button inside the property, or a keypad on the inside, is a simpler alternative.
Which one fits depends on the pavement, climate, budget and use. Loops can be hard to retrofit into existing concrete or asphalt, since someone must cut into the surface. Before any cutting, utility lines should be located; calling a locating service before digging is standard good practice.
What commonly goes wrong
- Broken loop wire. Pavement movement, frost heave, heavy vehicles, settling and poorly sealed saw cuts can break the wire. The detector may show a fault.
- Water in the cut. Poor sealing allows moisture to degrade the insulation.
- Detector settings. Sensitivity or frequency set incorrectly can cause missed vehicles or phantom detections.
- Interference. A nearby loop on the same frequency, or electrical noise, can cause false triggers. Many detectors let you change frequency.
- Power or fuse problems. A tripped breaker or blown fuse can take the detector offline.
- Loose connections at the detector terminals.
A typical sign of trouble is a gate that suddenly stays open, or one that will not close because it thinks a vehicle is present. Another is the opposite: a gate that does not open when a car approaches.
Basic checks
A homeowner can look at the detector’s indicator lights, if there are any, and read the manual for what they mean. Some units show a steady light for a detected vehicle and a flashing pattern for a loop fault. Beyond that, loop testing uses a meter and some experience. Loop repairs generally involve cutting pavement, so they are best performed by someone who handles them regularly.
Once the gate is working, keep the area over the loop clear of large metal objects, and avoid parking on the loop. Seal cracks near the loop wire where practical, because water entry shortens its life.
Choosing with care
If you are considering a free exit option, ask an installer about the loop layout and safety features. The system should always include protections for people and vehicles in the gate’s path, and a loop is not a substitute for them. Treat convenience features as an addition to a well-protected gate, and test them with the rest of the safety equipment from time to time.