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Sensing a person's prescence in a corridor by a broken light beam

zenerbjt

Member
Dear Engineers,
Years ago I worked on a sensor system for blind and partially sighted people (in a huge home for the blind) whereby we had light beams going across the corridor, and whenever someone walked past it would tell them where they were in the huge building. (they would be walking whilst holding on to the rails on the wall) I cant remember what sort of sensors and LEDs we used for this. Do you think it is likely we would have used infra-rad? I mainly worked on the power supplies so cant remember the details of the “light beam break” sensors.
Can you possibly help with a suitable sensor/LED system for this?
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
The sensors would almost certainly have been infra-red.
There are plenty of off-the-shelf systems advertised (google 'beam-break sensors'), or did you want to roll your own?
 

AnalogKid

Well-Known Member
Most Helpful Member
As above, beam-break is a very common method (and my first science fair project). An alternative is a reflective sensor, where the emitter and detector are mounted close together (with a small light shield between them). Receiver sensitivity is such that the opposite wall is too far away, but someone in the hallway reflects enough energy to generate an output signal. An advantage for this system is that all parts are on the same side of the hallway. A small project box with nothing but a DC input from a wall wart (5 V, 12 V, whatever) could house the emitter, detector, audio player, speaker, and battery backup.

There is nothing magical about infra red in this application. Any LED color will work equally well. IR is used because it is not visible, and often that is desirable in a security use.

Thinking about this some more ... The whole thing could probably run on one 9 V transistor radio battery for a year. Pulse the LED for 250 us every 250 ms, which reduces the LED power by 99.9% compared to a steady-state beam. That's four pulses per second, more than fast enough to detect someone running down the hall. Until something is detected, power to the audio system is switched off with a MOSFET.

1 - battery
1 - CD4093 (LED oscillator, detector, output latch)
1 - Audio record / playback module (ebay)
1 - speaker

ak
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
If you want to make one, you can use a Vishay TSOP series IR remote control receiver. You have to continuously modulate the transmitter.
You configure the receiver to be a missing pulse detector. Use it to trigger a voice playback module. You need to power the LED side and the receiver side.

How did you do the power distribution? If it were me, I'd probab;y use something like an Alarm-Saf UL listed panel with battery backup. I think it had 8 or 16 class 2 protected outputs at either 12 or 24V DC. I'd make the emitters/detectors to use 12V DC. They would probably have to use some 5V. The panels are generally used for access control systems that interface to the FAP. Fire Alarm Panel,

Although you would like the receiver and emitter to be powered by the same supply, its not a requirement.

I might consider bringing an isolated output from the detector to or near the battery panel and connect them to LED's. Npt sure if it's arequirment and I'd have to think about it more.

An extra thought: keep the last LED triggered on. It could help locate someone, maybe.
 
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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I used a version of this http://www.alarmsaf.com/apddual.html. I would basically use these for the power supply. I can tell you how to do it cleanly with a suspended ceiling too.

The transmitter is easy. The 555 timer with say a duty cycle of 20% and probably 38 kHz. Some surge suppression on the power. then, there is a question of mounting, firewalls, plenum space, power sharing (daisy chain) etc.

The receiver, more difficult. You could probbably use a 4000 series part or possibly the Analog Devices TimerBlox series: or both e.g. https://www.analog.com/media/en/tec...C6993-6993-1-6993-2-6993-3-6993-4.pdf#page=21
 

AnalogKid

Well-Known Member
Most Helpful Member
If you want to make one, you can use a Vishay TSOP series IR remote control receiver. You have to continuously modulate the transmitter.
You have to do several other things, also. I think that level of complexity is way beyond what is needed for a controlled, indoor space.

At the risk of sounding like a snarky little twit, here's a thought: keep it simple.

ak
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You have to do several other things, also. I think that level of complexity is way beyond what is needed for a controlled, indoor space.

At the risk of sounding like a snarky little twit, here's a thought: keep it simple.
The advantage of the TSOP chips is that it IS simple - you really need some sort of modulation to avoid ambient light, and the TSOP makes it very simple to do, and do well.

Otherwise you're talking IR filters (to reduce ambient light), and tubes with the photodiode at the bottom to further reduce ambient light problems, not to mention amplifiers on the photodiode.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
An embedded engineer friend made one, and it wasn't too hard. The TSOP chips really cut down the complexity. Doing it with a 100 kHz frequency is really going to be hard. The chip takes care of automatic gain control, the modulation does most of the ambiant light filtering and CFL and florescent lamp interference, The receiver has a built in filter. Choosing one that supports NEC5 coding would probably be a good choice. That protocol specifically sends a burst just for the AGC even though you would not be using the protocol.

Reading has suggested using both a narrow and wide emitter for remote controls.

In this case, there is the audio problem that has to be dealt with too. Hopefully, some mode which can play audio based on some trigger.

It's not even clear we are trying to build the "same" project in this thread, but I could see a single gang box for the emitter and a dualgang box for the detector side. The detector could contain a speaker. I'd consider braille markings on the IR plates.. You could put say a 1/4" thick, 1" wide strip of wood from floor to ceiling with a cutout for the box. That would allow a blind person to find the plate and "read" the braille markings. Primary use would be direction of travel. Make one side 1" wide. and the other 2" wide. You could code N, S, E and W by board sizes, above and below the detector.

e.g Count in binary N (00), S(01), E(10), W(11), S; So, N would be two "wide boards", south would be a "wide" at the bottom and a narrow board at the top.
the Mneumonics are you need to remember N, S, E and W order, the LSB (least significant bit) is toward the floor
Wide represents a "0" and narrow represents a "1", Note that a 0 is "wider" than a "1".

How does one determine the direction of travel?

You could get weird and put the speaker in the ceiling. It's not necessarily weird because ceiling speakers are available. and it saves the expense of cutting a grille in an outlet cover plate and having a small speaker. IC's that support 12V power are widely available and you may find many ready made amplifier PCB's. All likely stereo.

You might have to use PTFE jacketed fire alarm cable and you have to buy that stuff in large rolls. The conductors are usually solid..

Sparky ran the cables and mounted the big stuff. I only had about 10 cables and a few locations, so the cables were labeled like 1A2 and 2A1 which is cable "A: from location 1 to location 2 and from location 2 to location 1.

So, when you have to have a bunch of cables from say the "power supply", the best way would be just run the cables into an enclosure with DIN rail terminals. The use a piece of conduit to the power supply. If you have to replace the power supply, all you have to do is replace the patch cables.

It ALWAYS makes sense to terminate the wiring to DIN blocks and use patch wires within the enclosure. I can;;t stress that enough.

Anyway, the cables ran in conduit on the wall to above the ceiling and the conduit had a large junction box mounted to the top. The junction box had individual cord grips for each wire, so it was basically a pull port. Fire is unhappy because it can;t travel or emit nasty smells. Class II wiring doesn't need conduit.

Sparky also mounted flush strobes and mushroom panic buttons. I did not pay attention to how that was done.

During a move and expansion, I did have to penetrate a firewall with LOTS of cables. They wanted to insert a PVC pipe, then bundle the cables and fill with firestop - Yuk. 6 or so cables that went to hydrogen sensors had to be handled separately and in their own conduit.

Sparky was nice and came up with a large box for running cables that helped connect everything low voltage together.
A wiring trough or wireway e.g. https://www.zoro.com/wiegmann-wirin...x-36ind-rscg040436/i/G0749384/feature-product my panel, the power supply, the battery box, the hydrogen alarm (wall mounted). Six pieces of tubing to sample the gasses. One cable to the fire alarm panel. There were at least 3 other cables that had to exit and penetrate a firewall..

These are just ideas when "brainstorming".
 

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