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Seems like they should be a transceiver + retroreflector, not transmitter + receiver pair. Is there a cost or safety reason for this?

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    \$\begingroup\$ upvote for teaching me a new word ... retroreflector ... lol \$\endgroup\$ – jsotola Sep 10 '20 at 23:50
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    \$\begingroup\$ youtu.be/Bi_Tp1H9CDs for a good explanation on retroflectors and how they work. IIRC, it answers your question at some point of the video (gives the same answer as @ChrisStratton) \$\endgroup\$ – bracco23 Sep 11 '20 at 8:27
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    \$\begingroup\$ @bracco23 I thought when I watched that it poses this same question! But perhaps he poses it and then answers it, I might not have been paying attention! \$\endgroup\$ – Tim Sep 11 '20 at 9:20
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    \$\begingroup\$ "Why aren't garage door sensors retroflective on one end?" - Where I used to live, they were. \$\endgroup\$ – Mołot Sep 11 '20 at 14:44
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    \$\begingroup\$ An apartment building from around 2000 in Warsaw. Underground garage. \$\endgroup\$ – Mołot Sep 12 '20 at 9:33
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A reflector might accidentally be substituted by some reflective aspect of an obstacle, say a chrome plated fender on a bicycle, motorcycle, classic car... or even the reflector on one.

In contrast a transmit receiver pair is far more likely to indicate the true and complete path.

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A white card placed in the beam close to the transceiver end will fool the beam. A transmitter-receiver pair across the space to be protected is much more resistant to defeat in this fashion.

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  • \$\begingroup\$ Right. But isn't it trivial to detect a reflection that's too close, and not let the door shut on too-close reflections? \$\endgroup\$ – Tom Mercer Sep 12 '20 at 0:20
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    \$\begingroup\$ @TomMercer Try to think past the idea and to how would you actually discern the distance. Brightness? Not reliable. Time-of-flight? Really expensive. Triangulation? Limited distance. \$\endgroup\$ – DKNguyen Sep 12 '20 at 3:51
  • \$\begingroup\$ You can't make a good-enough certainty measurement based on diffusion and the weakening/decoherence of the returned beam? I imagine there are relatively low-tech solutions that'd be good enough for this mundane application. \$\endgroup\$ – Tom Mercer Sep 12 '20 at 4:56
  • \$\begingroup\$ @TomMercer I'd wager it's more of a problem of reliability. With a TX/RX pair with a specially encoded beam between them that only they understand - it's extremely unlikely to be "fooled" and do the wrong thing at the wrong time. With a reflector, you're making some big assumptions, even with distance measurements considered. There's no way for the TX/RX beam to get to the other side accidentally, the same cannot be said about a reflector. \$\endgroup\$ – SnakeDoc Sep 13 '20 at 1:14
  • \$\begingroup\$ @TomMercer And how do I tell the difference between a dark object close up from a light object far away? \$\endgroup\$ – DKNguyen Sep 13 '20 at 2:44
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The typical LiftMaster/Chamberlain safety sensor system is designed to prevent it being accidentally or even easily deliberately disabled.

Interposing a reflector, an electrical short or an electrical open anywhere external to the controller will not provide a false 'clear' signal. In fact no combination of passive components connected to the sensor terminals can defeat the safety device.

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    \$\begingroup\$ I think you could defeat it with fiber optics or some mirrors to route the beam around the door... \$\endgroup\$ – SaSSafraS1232 Sep 11 '20 at 22:46
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    \$\begingroup\$ @SaSSafraS1232 Oh, yes. Some people mount the sensor and transmitter facing each other on a board beside the drive unit. Not recommended, for obvious safety reasons. \$\endgroup\$ – Spehro Pefhany Sep 11 '20 at 22:48
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    \$\begingroup\$ Conversely, I have had the position of the sun at certain times of the year fool my system into thinking there is an obstacle when there is one. I had to place a piece of cardboard as a guard on the side to prevent this. \$\endgroup\$ – Michael Sep 12 '20 at 4:45
  • \$\begingroup\$ @Michael My old opener had that problem. It's replacement has never been fooled, though. There was no practical place to put a shield, there were days I got out of the car and stood in the right spot. Fortunately, it was seasonal (the sun had to be in exactly the wrong spot) and usually not an issue. \$\endgroup\$ – Loren Pechtel Sep 14 '20 at 3:51
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The main reason, for using a through-beam sensor for a garage door, would not be cost but safety, reliability and trouble-free operation.

A retro-reflective sensor could be considered unsafe for this application, should one take into account the rare event of it responding to the reflection from an obstacle.

For a through-beam sensor, the actual distance traversed by the beam would be its range, whereas, in the case of a retro-reflective type, the actual distance would be twice the range. Secondly, for the retro-reflective type, loss of signal in the reflector would also need to be factored in. Hence the typical range of a through-beam sensor would be more than twice that of a retro-reflective one.

Thus, compared to a retro-reflective sensor, the through-beam sensor with its higher excess gain (more transmitted light, than that required to activate the receiver, impinging on it), would give consistent and trouble-free operation even in dusty and dirty environments.

Thus, inherent safety and higher reliability of the through-beam sensor would dictate its usage, notwithstanding its higher cost.

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    \$\begingroup\$ If cost were the issue, the mirror setup would likely be cheaper... \$\endgroup\$ – BlueRaja - Danny Pflughoeft Sep 11 '20 at 21:29
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    \$\begingroup\$ How the heck could a through-beam transmitter-receiver pair be cheaper than a single transceiver and retroflector!? \$\endgroup\$ – Tom Mercer Sep 12 '20 at 0:22
  • \$\begingroup\$ Hi BlueRaja - Danny Pflughoeft and Tom Mercer, Thanks to your comments, my answer has been reviewed and edited. \$\endgroup\$ – vu2nan Sep 12 '20 at 5:52
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    \$\begingroup\$ @TomMercer it's only cheaper than a reliable retroreflector based system. \$\endgroup\$ – Jasen Sep 13 '20 at 3:44
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Instead of a retroreflective prism or tape, a set of mirrors could return the beam to the combined transmitter and receiver that receives a beam from above. That would overcome the "safety" objections posted by Luddites above. Three simple flat mirrors at 45 degrees is all it takes-- no wiring. Of course my trailer hitch would still interrupt the beam and prevent the door from closing and the additional mirrors would be additional spider web accumulators.

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  • \$\begingroup\$ Three simple flat mirrors at 45 degrees comprises a retroreflector, the subject of this thread. \$\endgroup\$ – Tom Mercer Sep 22 '20 at 3:46
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This may be a great idea for product improvement!

There are some very simple chips out there to ease the design of a light beam, that bring the cost down and ease of use and reliability up - it's a solution that's been in use for more than 4 decades at least. So nothing new or exciting about that.

However, using a retro-reflector requires sending out a very very short light pulse (assuming a 5m distance, the reflected pulse, now greatly attenuated by distance, will return in ~33 nanoseconds, requiring measurement of nanosecond pulses with phenomenal accuracy (ideally you'd need to detect anything more than a 10cm variance? How big is a leg in white trousers, or a hand with reflective glove? 10cm corresponds to something like 300 pico-seconds!). At install, the sensor must be calibrated or "trained" to open a detection window of a few 100 pico-seconds at the precise time to expect the response. Some kind of "learning" algorithm is required. Also, the returned light needs to be distinguished from ambient light clutter and noise.

This can all be done with modern electronics and optics, however, it would require a fast micro-controller, some high-spec photo-diodes and perhaps even a laser LED capable of generating extremely short pulses. 20 years ago I used a laser distance sensor that used a multi-watt laser (and a big lens), pulsed for an EXTREMELY short time, and then just a PIC micro-controller that used a huge amount of oversampling to improve the measurement accuracy. Technology has certainly moved on since then.

Why would you use a retro-reflector? Because, despite being more costly to make, it is potentially easier to install (one less set of wires to run). In my experience install costs usually dwarf the cost of the electronics for most household installs - in most 1st-world countries labor is expensive.

Plus, the installer often chooses the brand of product to install - most installers will favor the more expensive unit that is easier to install. So this may be an idea whose time has come - well done Tom for asking this question.

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    \$\begingroup\$ You are describing an optical range sensor but that's not what's required in this case. A simple "light is / is not reflected" sensor with adequate range is all that would be required unless you are intending to make a more fail-safe system that checks that the reflector distance is as expected (the width of the garage door) and not a lesser distance (that to a reflector mounted on the vehicle). Banner and Sick used adjustable lenses to sense fixed distance. Keyence projected the reflected light onto a linear CMOS sensor array to determine distance. No time of flight! Welcome to EE.SE. \$\endgroup\$ – Transistor Sep 13 '20 at 10:03
  • \$\begingroup\$ Thanks! Yes, I was assuming time of flight as being the safest, thinking that a significant risk to human life is involved. But I'd forgotten most garage doors can often sense the increased current draw from the motor caused by the motor attempting to crush a human or car - this makes the design of the sensor much easier, if you know that failure won't result in injury. \$\endgroup\$ – Julian Gerber Sep 13 '20 at 11:58
  • \$\begingroup\$ Retroreflectors are dirt cheap, not "more costly to make". It's both easier and cheaper to install, simpler and cheaper to manufacture. \$\endgroup\$ – Tom Mercer Sep 14 '20 at 3:29

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