Two photo-eye sensors. One on each side of the garage door opening. About 15 cm off the floor. Lined up across the threshold.

They are the cheapest safety component in the whole door system. They are also the one most likely to be broken, misaligned, taped over, or disabled when I show up for a service call.

This post is about why they matter, what they do, how to tell if yours work, and the bigger safety system they're part of.

The history

On January 1, 1993, Underwriters Laboratories standard UL 325 became mandatory for residential garage door openers manufactured for sale in the United States and (by trade harmonization) Canada. The standard requires that every residential garage door opener have a non-contact reversal system — meaning a sensor that detects an obstruction before the door touches it, and reverses the door's downward travel.

The trigger for UL 325 was a series of child deaths in the 1980s. Children getting trapped under closing garage doors. The contact-reversal systems of the era (the door physically had to touch the obstruction before reversing) were too slow and too weak.

Photo-eye sensors became the answer. An infrared beam between two sensors, parallel to the floor, low enough to detect a child or pet under the door. Break the beam, the door doesn't close.

If your opener was made after January 1, 1993, it has photo eyes. They were not optional. They were code.

How they work

One sensor is a transmitter (sometimes labelled "sending"). The other is a receiver (sometimes labelled "receiving"). The transmitter emits a low-power infrared beam. The receiver, when properly aligned, sees the beam continuously.

The receiver typically has a green or amber LED that lights up steadily when the beam is detected. If something breaks the beam, the LED flashes or extinguishes, and the opener:

It's a simple, robust, mature technology. The most common failures aren't electronic — they're mechanical and behavioural.

Why yours might be lying

In 1,240 service calls, I've seen these failure modes regularly:

1. Misalignment. The sensors get knocked out of alignment by everything — a basketball, a kid leaning on them, a contractor working on the floor. When alignment drifts, the receiver loses the beam and the door won't close. Most homeowners' solution? Tape one sensor in place with duct tape, pushing it into "approximate" alignment. This works until it doesn't.

2. Tape-over. When the sensors fail or get annoying (false reversals from a leaf on the sensor lens, kids hitting them with balls, the LED making clicking sound the dog hates), the homeowner's "fix" is to tape over both sensors. Both sensors now think they see each other. The opener thinks the safety system is working. The safety system is gone.

I have walked into Richmond garages where the photo eyes have been taped over for years. Sometimes by the homeowner. Sometimes by the previous owner. Sometimes by a contractor who got tired of explaining alignment problems.

3. Lens contamination. Spider webs, dust, leaf debris. Especially in detached older garages where photo eyes accumulate grit over a season. Clean the lenses with a soft cloth every six months. Takes 30 seconds.

4. Wire damage. Photo eye wires run along the inside of the door track to the opener. Rats chew them (see post 14). Lawnmowers and cars hit them. The wire breaks, the photo eye loses power, the system fails.

5. Dead sensor. Rare but real. A photo-eye sensor can fail electronically — usually a damaged LED in the transmitter. Replacement: $85–$165 for the pair, parts.

The test

This is one of the easiest tests in the whole system. Do it every six months.

Step 1: Start the door closing. Press the wall button or remote. Watch the door start to come down.

Step 2: Wave a broom (or your foot, but a broom is safer) through the photo-eye beam path. About 15 cm off the floor, across the opening.

Step 3: The door must reverse direction immediately.

If the door reverses and goes back up: the photo eyes work.

If the door continues closing: the photo eyes are not working. Stop using the opener for door closures until you fix this. You can still open the door with the opener; just close it manually for now.

That's the test. Five seconds.

The reversal force test (the second safety system)

Photo eyes are one safety system. There's a second one: the auto-reverse force limit. This requires the door, while closing, to be sensitive enough that a moderate obstruction in its path (a person's hand, a child, a pet) causes it to reverse without the photo eyes being involved.

UL 325 specifies that a closing door must reverse when it encounters a 7.6 kg (about 17 lb) force or less. The test: place a 2x4 flat on the floor across the door opening. Close the door with the opener. The door should land on the 2x4, sense the obstruction within a fraction of a second, and immediately reverse.

If the door crushes the 2x4 instead of reversing, or pushes it across the floor, the force limit setting is too high. This is dangerous. The door will crush a person at that setting.

Fix: adjust the down-force limit setting on the opener. Most openers have a screw or dial labelled "down force" or "close force." Adjust it down (less force) until the 2x4 test passes. Then verify the door still closes reliably without the test obstruction. About 10 minutes of work.

Or call somebody. $95–$165 service call.

Why both matter

The photo-eye and the force-reversal are redundant for a reason.

Both systems should work, independently, on every door. If either is bypassed or disabled or broken, the safety margin is gone.

Specific Richmond risk: the photo-eye height problem

UL 325 specifies photo eyes at 4–6 inches (10–15 cm) above the floor. This is intended to catch a small child crawling.

Some homeowners and some contractors install them higher — 25, 30, 45 cm off the floor — because the lower position keeps getting hit by snowblowers, basketballs, or boxes set down in the garage.

The higher mount fails the safety standard for children. A small child crawling through a closing door at floor level can pass under a 45-cm-mounted photo-eye beam without breaking it.

If your photo eyes are mounted higher than 15 cm, get them moved. Cost: $145–$225 service call to reposition. This is the cheapest fix in the whole post for the highest-stakes risk.

Cost reality check (2026, Lower Mainland CAD) - Photo eye realignment (existing sensors): $95–$165 - Photo eye pair replacement: $165–$245 installed - Photo eye repositioning (move to correct height): $145–$225 - Wire replacement (if rats or damage): $185–$345 depending on routing - Reversal force adjustment: $95–$165 - Full opener safety system inspection and service: $185–$285

A McLennan story

A young family near Cook Road and No. 4 Road in McLennan called me last June. Husband had been doing the photo-eye test before the family went on vacation — the one I just described. The door wasn't reversing. He'd assumed the sensors needed cleaning.

When I got there, both photo-eye sensors were perfectly clean. They were also covered with translucent packing tape — the kind that's nearly invisible at first glance. The previous owner had taped them eight years earlier. The new family had been living in the house for two years without realizing the safety system was disabled.

I peeled the tape, realigned the sensors, tested the system. Door reversed perfectly on the broom test. Also did the 2x4 force-reversal test — that worked too.

Total bill: $95 service call. He thanked me three times.

The family has two kids under five. They had been using the door for two years assuming the safety system was working. The cost of finding out it wasn't would have been measured in different units.

That's the post.

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