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Loop Detectors and Vehicle Presence Sensing
A coil of wire that can tell a car from a cat
Look closely at the pavement near some driveway gates and you will see a thin rectangular cut sealed with black material. Under that cut lies a loop of wire, the heart of a vehicle detector. Loops do not see people or animals; they sense large pieces of metal. That specialty makes them useful for two jobs: opening a gate for a vehicle approaching and keeping it from closing on a vehicle that is in its path.
The principle in plain terms
A loop detector module sends a small alternating current through a coil of wire buried in the ground. The current creates a magnetic field around the loop. When a vehicle sits above or passes over it, the metal of the body changes the field slightly, which alters the electrical behavior of the coil. The module measures that change and flips a relay.
The relay output connects to the gate control board, which can treat it as a free exit command or as a safety input. Because the module compares the loop to its own baseline, it adapts to slow changes in temperature and moisture, and responds only to sudden changes caused by vehicles.
Three common loop roles
The first is the exit loop. Placed inside the property, a distance back from the gate, it senses a car approaching and opens the gate with no remote or button. Residents leaving simply drive up.
The second is the safety loop, also called the shadow or presence loop. It sits directly under the path of the gate. While a vehicle is above the loop, the control board refuses to close the gate, or reverses it if it has started. This protects cars from being struck by the leaf.
The third is the reset or free-exit loop outside the gate on the entry side, used with a keypad or card reader. It detects a vehicle that has passed through and allows the gate to close promptly.
Many gates use two loops in a pair, one for presence and one for exit. Each has its own detector channel, or a two-channel module covers both.
Shape, size and placement
Loops are generally rectangles, six feet or so on a side for a car, with the long dimension across the lane. The number of turns of wire is chosen according to the loop perimeter; smaller loops need more turns. Corners are usually cut at an angle to avoid sharp bends that can damage insulation.
The loop must be placed where a car's body will pass over it. For a presence loop, that means the area directly under the gate's swing or slide path, with a margin on each side. For an exit loop, it sits far enough back that the car is not touching the gate when it triggers, giving time for the leaf to open before the car arrives.
Avoid placing a loop near rebar, which can interfere. If the pavement contains reinforcing steel, the loop must be positioned or sized to account for it, or sensitivity must be adjusted. Large metal objects such as the gate itself can also influence the field, so the loop is positioned a few feet away from moving metal when possible.
Installation details that matter
The groove is cut with a concrete saw, wide enough for the wire and a backer, and deep enough to keep the wire a bit below the surface. Corners are cut at forty-five degrees to avoid sharp wire bends. The groove is cleaned and dried. The wire, usually cross-linked insulated cable designed for loops, is laid in without kinks, and the leads are twisted tightly together along their run to the detector to reduce noise.
Sealing is the next step. The groove is filled with a flexible loop sealant that protects against water and traffic. A sealant that cracks lets water in, and water in the groove eventually means a failed loop. The twisted lead runs through conduit to the detector module.
Lastly, the module is tuned. Sensitivity is set so a small car triggers it but a bicycle does not, and so that it holds while a vehicle sits still. The module should also be set to reset its baseline at power-up. Some have a delay feature that extends presence for a moment after a car leaves, covering the gap between detection and clearing.
What goes wrong
- False triggers from nearby metal gate parts, moving shopping carts or heavy trucks in an adjacent lane.
- Failure to detect low or small vehicles such as motorcycles and trailers.
- A broken wire from pavement movement, seen as a constant fault.
- Water getting into the cut, shown as drifting sensitivity or erratic behavior.
- Poor connections in the lead run, which cause intermittent faults.
Most detectors display the loop status and have a fault indicator. A steady fault light suggests an open or shorted loop. A flickering light may point to a loose connection. Measuring loop inductance with a meter helps confirm. If the loop is damaged, the usual repair is to cut a new groove and lay a fresh loop, since splicing in the pavement is unreliable.
Loops alongside other sensors
A loop does not replace a photo eye. It will not see a person standing in the gate path, or a child on foot. It does an excellent job with vehicles, including those that may park a bit too long under the gate. Combining a loop with beams and edges produces layered protection that covers different threats.
Loops are also free of lenses and thus unaffected by dirt, fog, sun glare or spider webs. In wet, foggy settings, that is a real advantage.
A short planning list
- Decide the purpose: exit, safety, reset or all three.
- Mark positions so a car's body, not just its wheels, passes over each loop.
- Check for rebar and large metal objects nearby.
- Choose a detector module that supports the needed number of channels and features.
- Specify cut depth, sealant and conduit for the leads.
- Test with different vehicles and adjust sensitivity.
Done well, a loop disappears into the driveway and works for years, quietly letting cars through and keeping the gate from closing on one.