The sending sensor emits a modulated infrared beam (940nm wavelength) at ~38kHz to distinguish from ambient light. The receiving sensor has a photodiode with a bandpass filter tuned to this frequency. If the received signal drops below threshold, the controller triggers a reversal within 150-250ms. The beam is invisible to humans but can be seen through a phone camera as a faint purple glow.
The sensors mount no higher than 6 inches from the garage floor per UL 325. This height is chosen to detect the smallest expected obstruction — a small child crawling under the door. Below 4 inches, debris and snow accumulation cause false triggers. The sensors must also be within 6 inches of the door plane to ensure the detection zone covers the entire door opening.
Misalignment is the #1 cause of sensor problems — bumping a sensor shifts the beam off-target. Sunlight interference can overwhelm the IR receiver, especially in west-facing garages at sunset. Dirty lenses reduce signal strength. Wiring damage from lawnmowers, pets, or rodents can disable the safety circuit entirely. LED indicator lights on each sensor show alignment status.
Place a 2×4 piece of lumber flat on the floor under the door. Close the door. It must reverse upon contact with the 2×4 within 2 seconds and with no more than 15 lbs of force. If it doesn't, the force limit needs adjustment — usually via the "down force" knob on the motor unit. Test monthly. This is your legal compliance check.
The motor monitors current draw. When the door hits an obstruction, the motor stalls slightly, causing current to spike above a calibrated threshold. The controller interprets this as an obstruction and reverses. Older systems used mechanical clutches; modern openers use electronic current sensing with adjustable sensitivity via "force" or "limit" knobs on the unit.
UL 325 specifies the door must reverse when meeting 15 lbf (66.7 N) of resistance. This is roughly the force a small child can resist before injury. The standard was developed through biomechanical testing — it's the maximum force that can be applied to a child's chest/neck area for 2 seconds without causing serious injury. Below this threshold, bruising may occur but not crushing injury.
If a spring is broken and you pull the emergency release while the door is UP, the full weight of the door (~130-200 lbs) is released instantly with nothing to counterbalance it. The door will slam down. Always be prepared to control the door's weight when pulling the release with the door up. With a broken spring, consider the door trapped — call a professional.
The release cord connects to a spring-loaded lever on the trolley carriage. Pulling the cord rotates the lever, disengaging the trolley from the rail drive (chain, belt, or screw). The trolley stays on the rail but slides freely, so the door can be lifted and lowered by hand. The springs still provide counterbalance — the door should feel light (~8-10 lbs) to lift manually if springs are functional.
To reconnect: pull the cord toward the motor (not down), or simply activate the opener — the trolley carriage will catch the drive mechanism on the next cycle. Some models have a lock position where the release lever stays disengaged for extended manual use. Always re-engage before relying on the opener's safety features, as manual mode has no auto-reverse protection.
The emergency release can be a break-in vulnerability. A wire hook pushed through the top of the door weatherseal can snag the cord and disengage the trolley from outside, allowing the door to be manually lifted. Solutions include a zip-tie on the release arm (breaks during emergency pull but prevents fishing), a shield plate, or simply locking the interior deadbolt.
Millions of garage door openers installed before 1993 are still operating without photoelectric sensors or compliant auto-reverse. These units are legal to keep using but are not legal to sell. If your opener predates 1993 (brands like old Genie, Sears Craftsman, or early Chamberlain models without sensor wires), it has no secondary entrapment protection. Retrofit sensor kits are available, or replace the opener entirely.
Common when homeowner disconnects sensors "because they keep blocking the door." The beam sensor is the PRIMARY entrapment protection — it detects obstructions BEFORE contact. Without it, the door relies entirely on the force-limit auto-reverse, which requires physical contact with the obstruction. A child's body may not provide enough resistance for the force sensor to trigger reversal before injury occurs.
Homeowners sometimes increase the "close force" setting because the door struggles to close (usually a spring or track issue they're compensating for). This defeats the auto-reverse by raising the threshold above 15 lbs — potentially to 30-50+ lbs. Now the door can apply crushing force before reversing. Combined with working sensors, this is survivable. Without sensors, it's potentially fatal.
This is the worst case — no beam detection AND no force reversal. The door closes with full motor force (potentially 50-100+ lbs) and doesn't stop. This is the scenario that killed children pre-1993. Modern dual-requirement (UL 325) exists specifically to prevent this. Even if one system fails, the other catches it. Both failing simultaneously requires either severe neglect or deliberate tampering.
Test monthly: (1) Close door, break beam with your foot — door must reverse. (2) Place 2×4 flat under door, close — door must reverse on contact. (3) Pull emergency release — door must disconnect and move freely. (4) Check sensor LED indicators — both should show steady green/amber. (5) Inspect sensor wires for damage. Takes 2 minutes. Do it.