![\"what](\"https:\/\/potensilift.com\/wp-content\/uploads\/2026\/09\/What-Is-an-Elevator-Door-Sensor.jpg\")
<\/figure>What Is an Elevator Door Sensor?<\/h2>
An elevator door sensor \u2014 also called an elevator light curtain, an infrared door sensor, or a door detector \u2014 is a safety device that protects passengers in the door opening and prevents entrapment.<\/p>
A complete light curtain has four parts:<\/p>
- Transmitter<\/strong> \u2014 infrared emitting diodes on one side of the car door<\/li>\n\n
- Receiver<\/strong> \u2014 photo detectors on the other side<\/li>\n\n
- Power box<\/strong> \u2014 usually mounted on the car top<\/li>\n\n
- Flexible cable<\/strong> \u2014 connects the bars to the power box or door operator<\/li><\/ul>
The transmitter and receiver are installed on opposite sides of the car door. They are not part of the door panel itself, and they never touch the passenger. Detection is entirely non-contact.<\/p>
How Do Elevator Door Sensors Work?<\/h2>
![\"diagram](\"https:\/\/potensilift.com\/wp-content\/uploads\/2026\/09\/Diagram-of-an-elevator-light-curtain-with-two-vertical-bars-and-a-grid-of-infrared-beams-one-beam-blocked-by-an-obstruction.jpg\")
<\/figure>Step 1: The transmitter emits infrared light<\/h3>
The transmitter bar contains a row of infrared emitting diodes. They do not all fire at once. A microcontroller inside the unit switches them on and off in a fixed sequence.<\/p>
Step 2: The receiver scans the beams in sequence<\/h3>
As each transmitter diode fires, several receiving diodes read the light. This creates multiple scanning paths from a single emission. Repeating this from top to bottom, many times per second, builds a dense grid of infrared beams across the door opening.<\/p>
Step 3: A blocked beam breaks the light path<\/h3>
When a passenger, a hand, a trolley, or a pet interrupts any beam in the grid, that beam no longer reaches its receiver. The receiver circuit cannot complete the photoelectric conversion, and the sensor registers an obstruction.<\/p>
Because the beam pattern is dense, the sensor does not need to be touched to react. A thin object such as a leash or a walking stick can be enough.<\/p>
Step 4: The sensor outputs an interrupt signal<\/h3>
Once an obstruction is detected, the sensor sends an interrupt signal to the door operator. Depending on the model, this is either a switching output or a high\/low level output.<\/p>
Step 5: The door operator reverses the door<\/h3>
On receiving the signal, the door operator immediately commands the door to stop closing and reverse to open. The door stays open until the obstruction clears the protected zone, and only then closes normally again.<\/p>
That last point matters. The sensor does not simply pause the door. It triggers a reversal.<\/p>
What Signal Does the Sensor Send?<\/h2>
Understanding the output is essential when replacing a sensor, because a mismatched output will not work with the existing door operator.<\/p>
Two output types are common:<\/p>
Relay output<\/strong> \u2014 a mechanical contact, rated as normally open (NO), normally closed (NC), or 1NO\/1NC. Response is slower because the relay must physically operate. Typical response times run from about 47 ms to 81 ms.<\/p>
Transistor output (NPN or PNP)<\/strong> \u2014 a solid-state output. It responds faster than a relay, typically 42 ms to 76 ms, and has no moving parts to wear out. Its load capacity is limited, so it must match the door operator input.<\/p>
Two connection arrangements also exist:<\/p>
Direct connection, without a power box<\/strong> \u2014 the light curtain takes power directly from the door operator board, usually at DC 24 V. This is common in modernization work.<\/p>
With a power box<\/strong> \u2014 a separate control box on the car top supplies power and handles the output. This is common in new installations and where the door operator cannot supply the required voltage.<\/p>
A synchronization wire (often marked SYNC) runs between the transmitter and receiver to keep their timing aligned. It must be terminated at an isolated terminal on the car top.<\/p>
![\"signal](\"https:\/\/potensilift.com\/wp-content\/uploads\/2026\/09\/Signal-path-diagram-from-an-elevator-door-sensor-bar-through-the-control-unit-to-the-output-connectors.jpg\")
<\/figure>Types of Elevator Door Sensors<\/h2>
\u0422\u0438\u043f<\/th> How it detects<\/th> \u041f\u0440\u0438\u043c\u0435\u0447\u0430\u043d\u0438\u044f<\/th><\/tr><\/thead> Infrared light curtain<\/td> Grid of infrared beams<\/td> Most common today, non-contact<\/td><\/tr> Photo eye (photocell)<\/td> One or a few beams<\/td> Older, large blind spots<\/td><\/tr> Mechanical safety edge<\/td> Physical contact with the door edge<\/td> Older, reacts only after contact<\/td><\/tr> Ultrasonic sensor<\/td> Sound waves<\/td> Rare<\/td><\/tr> 3D \/ TOF sensor<\/td> Scans the area in front of the door<\/td> Growing in high-end buildings<\/td><\/tr><\/tbody><\/table><\/figure> For most projects today, a light curtain is the correct choice. It protects the full door height without contact.<\/p>
![\"types](\"https:\/\/potensilift.com\/wp-content\/uploads\/2026\/09\/Types-of-Elevator-Door-Sensors-1.jpg\")
types of elevator door sensors<\/figcaption><\/figure> Key Specifications That Affect Performance<\/h2>
\u0420\u0435\u0437\u043e\u043b\u044e\u0446\u0438\u044f<\/h3>
Resolution is the vertical distance between the centers of two adjacent infrared beams, measured in millimeters. A smaller resolution means denser beams, and a smaller object can be detected. In practice, resolution follows the diode spacing. Common spacings are 40 mm, 48 mm, 59 mm, 84 mm, and 118 mm.<\/p>
Protection Height<\/h3>
Protection height is the vertical range the sensor covers. It must fully cover the actual door opening height. If the curtain is shorter than the opening, a blind zone remains at the top or bottom, and a small object can pass undetected.<\/p>
Common curtains cover at least 1600 mm, with some models reaching 1800 mm or 1900 mm. The lowest beam usually sits 20 mm or less above the sill, so the sensor can detect a foot or a small object near the floor.<\/p>
Response Time<\/h3>
Response time is the delay between a beam being blocked and the output signal changing. It is measured in milliseconds. Shorter is safer, because the door starts to reverse sooner. This is the main reason relay outputs are slower than transistor outputs.<\/p>
Beam Count and Beam Pattern<\/h3>
Light curtains are often described by beam count \u2014 94, 154, 194, 234 beams, and so on. A higher count gives denser coverage.<\/p>
The pattern matters as much as the number. In a cross-beam design, the diodes do not only fire straight across. They also fire at an angle, so one emission reaches several receivers along different paths. A high-density curtain combines horizontal beams, upward-slanted beams, and downward-slanted beams into a single interlaced grid.<\/p>
This is why a modern curtain can detect a thin leash or a thin walking stick that a simple horizontal beam grid would miss.<\/p>
Other Specifications Worth Checking<\/h3>
- Operating voltage<\/strong> \u2014 AC 220 V or DC 24 V<\/li>\n\n
- Operating distance<\/strong> \u2014 up to 4000 mm<\/li>\n\n
- Ambient temperature<\/strong> \u2014 commonly -20 \u00b0C to +65 \u00b0C<\/li>\n\n
- Light immunity<\/strong> \u2014 models rated at 100,000 lux tolerate direct sunlight<\/li>\n\n
- Standards<\/strong> \u2014 EN 81-20 for lift safety, EN 81-70 for accessibility, EN 12015 \/ EN 12016 for electromagnetic compatibility<\/li>\n\n
- Alignment tolerance<\/strong> \u2014 typically \u00b115 mm vertically and \u00b13 mm front to back<\/li>\n\n
- Redundancy<\/strong> \u2014 some models allow one failed emitting diode to be masked for 120 seconds without losing protection<\/li><\/ul>
Common Problems and How to Troubleshoot Them<\/h2>
When an elevator door sensor stops working, the symptom usually appears as one of three behaviours.<\/p>
The door reopens for no apparent reason<\/h3>
Also known as phantom reopening.<\/p>
- Clean dust and dirt from the curtain surfaces<\/li>\n\n
- Check the alignment between the door operator and the curtain<\/li>\n\n
- Look for direct sunlight or a source of electromagnetic interference<\/li>\n\n
- If the problem continues, replace the curtain and retest<\/li><\/ol>
The door will not close, or closes part way and reopens fully<\/h3>
- Check whether the indicator lights on the receiver behave normally<\/li>\n\n
- Short the output to test whether the fault lies in the curtain or in the door operator<\/li>\n\n
- Check the connection cable between the curtain and the controller board<\/li>\n\n
- Swap in a known-good curtain of the same model to confirm<\/li><\/ol>
Nothing happens when you block the sensor<\/h3>
- Check that the NO\/NC wiring matches the door operator input<\/li>\n\n
- Test whether the output relay is operating<\/li>\n\n
- Measure the supply voltage against the rated value<\/li>\n\n
- Replace the curtain to rule out hardware failure<\/li><\/ol>
In all three cases, the first step separates the sensor from the door operator. Many reported sensor faults turn out to be wiring or door operator problems.<\/p>
![\"common](\"https:\/\/potensilift.com\/wp-content\/uploads\/2026\/09\/Common-Problems-and-How-to-Troubleshoot-Them.jpg\")
<\/figure>How to Choose a Replacement<\/h2>
Replacing a light curtain is a compatibility problem more than a performance problem. Work through this list:<\/p>
- Protection height<\/strong> \u2014 measure the actual door opening. The curtain must cover it fully.<\/li>\n\n
- Supply voltage<\/strong> \u2014 confirm whether the system provides AC 220 V or DC 24 V.<\/li>\n\n
- Output type<\/strong> \u2014 relay or transistor, NO or NC, and NPN or PNP.<\/li>\n\n
- \u041f\u043e\u0434\u043a\u043b\u044e\u0447\u0435\u043d\u0438\u0435<\/strong> \u2014 with power box or direct connection, plus the cable length required.<\/li>\n\n
- Door type<\/strong> \u2014 central opening, side opening, or four-panel central opening. Each uses a different cable configuration. A central opening door commonly uses two 3.5 m cables, a side opening door uses one 2.5 m and one 4.5 m cable, and a four-panel central opening door uses two 5 m cables.<\/li>\n\n
- Environment<\/strong> \u2014 if the door faces direct sunlight or a bright atrium, choose a higher light immunity rating.<\/li><\/ol>
Price differences between models usually come down to four things: brand positioning, technical parameters such as resolution and response time, added functions such as foot detection or self-diagnosis, and after-sales support.<\/p>
\u0427\u0430\u0441\u0442\u043e \u0437\u0430\u0434\u0430\u0432\u0430\u0435\u043c\u044b\u0435 \u0432\u043e\u043f\u0440\u043e\u0441\u044b<\/h2>
What are elevator door sensors called?<\/h3>
The same device goes by several names. In the elevator industry it is most often called a light curtain, or an elevator light curtain. It is also known as an elevator door detector, a door detector, an infrared light curtain, an infrared door sensor, an optical sensing edge, a multi-beam door sensor, or a door protection device. Older single-beam versions are called photo eyes or photocells. The name changes with brand and region, but all of them describe the same non-contact safety device.<\/p>
Do elevator door sensors use infrared light?<\/h3>
Yes. Most modern elevator door sensors use infrared beams. The transmitter fires infrared diodes and the receiver reads them. Infrared is invisible to passengers, which is why the protection feels automatic.<\/p>
What is the difference between a light curtain and a safety edge?<\/h3>
A safety edge is a mechanical device that reacts only after the door physically touches a person or object. A light curtain reacts before contact, because the beam is interrupted first. Light curtains are the modern standard.<\/p>
How many beams does an elevator light curtain have?<\/h3>
Common models range from about 94 beams to more than 230 beams. More beams mean denser coverage, but the beam pattern matters as much as the count. Cross-beam designs detect thin objects better than the same number of parallel beams.<\/p>
Why does the elevator door keep reopening when nobody is there?<\/h3>
The most common causes are dust on the curtain lenses, misalignment between the transmitter and receiver, direct sunlight, or electromagnetic interference. Cleaning and alignment checks solve many cases. If they do not, the curtain is likely failing.<\/p>
Can I replace an elevator door sensor myself?<\/h3>
It depends on the model and the wiring. Direct-connection types are simpler, because they take power from the door operator. Units with a power box involve more wiring. In either case, match the voltage, output type, and cable configuration exactly, and test the door before returning the elevator to service.<\/p>
What do NO and NC mean on an elevator light curtain?<\/h3>
NO means normally open and NC means normally closed. They describe the state of the relay contact when no obstruction is detected. The setting must match what the door operator expects, or the door will behave incorrectly.<\/p>
\u0417\u0430\u043a\u043b\u044e\u0447\u0435\u043d\u0438\u0435<\/h2>
An elevator door sensor works by building a grid of infrared beams across the door opening, watching for any beam that fails to arrive, and sending an interrupt signal that makes the door operator reverse the door.<\/p>
The mechanism is simple in principle, but it depends on several details working together: beam density, protection height, response time, output type, and correct wiring.<\/p>
- Supply voltage<\/strong> \u2014 confirm whether the system provides AC 220 V or DC 24 V.<\/li>\n\n
- Protection height<\/strong> \u2014 measure the actual door opening. The curtain must cover it fully.<\/li>\n\n
- Operating distance<\/strong> \u2014 up to 4000 mm<\/li>\n\n
- Receiver<\/strong> \u2014 photo detectors on the other side<\/li>\n\n