2
\$\begingroup\$

I have a working set-up to detect beam breaks using an IR LED, modulated at 36kHz, and a TSOP receiver connected to a comparator. It outputs the count via Arduino. The range is about the width of a door, say two meters maximum.

The transmitter and receiver are on a separate breadboards. This has its disadvantages, such as two sets of batteries.

Is there a way to reflect the IR LED off a reflector and having the TSOP receiver on the same breadboard as the LED? Perhaps behind it? The German term 'lichtschranke' seems to be what I vaguely have in mind, but I don't speak German and can't find it in English.

Does anyone have suggestions on how to modify my current set-up or can anyone point me to some schematics?

I am guessing something will need to be done in order to maintain a straight or angled beam and not have it reflected back all over the place.

How can I mount an IR emitter and receiver on the same circuit board to detect a reflection instead of a straight beam?

\$\endgroup\$
  • \$\begingroup\$ You probably want to know about the corner reflector. As far as I can see from German Wikipedia, Lichtschranke is just the German name for what you're trying to build: An optical beam (break) detecting circuit. \$\endgroup\$ – The Photon Jul 22 at 16:05
  • \$\begingroup\$ Another old-fashioned word for this circuit (particularly for detecting people passing through doorways) was magic eye. \$\endgroup\$ – The Photon Jul 22 at 16:07
3
\$\begingroup\$

You need a retroreflector, which is a special type of reflector that sends light back towards its source (unlike a simple mirror).

This is what you'd put on the rear of your bicycle to reflect the headlights of incoming drivers back at them so they see you at night. You can get them for cheap in any bicycle store or supermarket. Even better, shown below is one that is designed to be screwed at the back of a trailer. If you want to mount it with a screw, that would be a nice choice, as it already has a hole.

enter image description here

Next, put your IR LED and TSOP receiver close to each other, both aiming at the reflector, and perhaps a black plastic or cardboard separation between the LED and TSOP... Adjust LED power down to make sure the signal is detected with the reflector, but not with IR light bouncing off the people you want to detect, and you're all set.

\$\endgroup\$
2
\$\begingroup\$

Yes, you can put the IR LED and IR detector next to each other with a reflector on the other side.

A corner reflector has the advantage that it doesn't need to be aligned perfectly, it will reflect the light back where it came from. The retroreflector that peufeu referenced is composed of many tiny corner reflectors. These should work with near infrared, but it would be safest to find one that specifically says it is good for near IR.

You can also use polished aluminum as the reflector. For a recent project I used 3" x 3" x 1/16" aluminum. I sanded it smooth with ultra-fine sandpaper. Then, I polished it with car polishing compound until I could see my reflection clearly.

To minimize stray light on the detector, you can put a small tube over the sensor.

\$\endgroup\$
  • \$\begingroup\$ This will not work well as a 'break detect'. You will get reflected IR from any object crossing in front, and no means to distinguish. You COULD check for sudden changes in the received IR level, but you have not said so. Even then it would be tricky and unreliable \$\endgroup\$ – Kripacharya Jul 24 at 3:58
  • \$\begingroup\$ The OP asked if the concept is viable. I have successfully done this and Amazon sells many different retro-reflective beam-break products. Of course, there will be details that need to be worked out. And, there is a risk that a highly reflective object near the sensor will not be detected. \$\endgroup\$ – Mattman944 Jul 24 at 8:14
  • \$\begingroup\$ Hey Mattman! Of course your concept will work. My point is that it is not a RELIABLE solution. There are many situations where it would fail. And I would not trumpet goods sold on Amazon as a standard to go by. Cheers! \$\endgroup\$ – Kripacharya Jul 24 at 8:51
2
\$\begingroup\$

You can mount your transmitter and receiver next to each other, but you'll need to block the direct light path. Here are two examples of pre-build products. I would model a design after the first example; I'm not sure the isolation of the second example is sufficient.

examples

And, yes, you can use a reflector, but make sure it is appropriate to your IR wavelength. For example, a standard bathroom mirror won't work well. These mirrors are often aluminum under glass. The aluminum reflects well, but the glass absorbs IR.

Some plastics absorb IR, whereas others reflect it. You may need to test different options.

The best IR reflectors are often metal (copper, aluminum, silver, etc). Here is a chart showing the reflectivity of different metals at different wavelengths. It looks like copper, silver, or gold will outperform aluminum at typical near-IR frequencies.

enter image description here (source)

\$\endgroup\$
  • \$\begingroup\$ This will not work well as a 'break detect'. You will get reflected IR from any object crossing in front, and no means to distinguish. You COULD check for sudden changes in the received IR level, but you have not said so. Even then it would be tricky and unreliable \$\endgroup\$ – Kripacharya Jul 24 at 3:59
  • \$\begingroup\$ Hi @Kripacharya! I see you're new here. On the Stack Exchange sites, we try to answer the given question and assume the person asking the question can work out the rest of it. If they can't, they can ask another question! Also, a reflective light sensor is used everywhere from occupancy sensing, industrial automation, automatic doors, etc etc. Just because a "better" solution exists, doesn't mean the given solution is a bad one. By the way, you make a good point about IR reflection off the moving object instead of the reflector. Thanks for that! \$\endgroup\$ – bitsmack Jul 24 at 17:24
  • \$\begingroup\$ Dear sir @bitsmack, I may be new here but I'm not a newbie. As I have pointed out that the IR reflector scheme WILL work, but it will be unreliable and error prone. Imho that makes it a bad concept unless some design method is indicated to overcome this limitation. In fact I have suggested one such method, but no one has picked up on it. That's all I'm saying. \$\endgroup\$ – Kripacharya Jul 24 at 17:33
1
\$\begingroup\$

Look for line or proximity IR sensors. Some even detect distance. Many are compatible with bread boards or can be wired to them with 0.1" jumper wire or headers. They have the detector and sensor built into the same module as the one shown below.

enter image description here
Source: https://www.digikey.com/product-detail/en/sparkfun-electronics/ROB-09453/1568-1272-ND/5762422?WT.srch=1&gclid=EAIaIQobChMImKyvqPLI4wIVhcpkCh2mpwiNEAQYBSABEgJe5fD_BwE

\$\endgroup\$
  • \$\begingroup\$ This would work only for very short distances. Nowhere near 2 mtrs requested. \$\endgroup\$ – Kripacharya Jul 24 at 8:46
0
\$\begingroup\$

Yes, it is very possible. You just need to arrange the LED and detector appropriately.

IR is just light that your eyes can't see. If you were trying to do this with a visible LED, what could you do?

For example: You could put the LED inside an opaque box with a hole at one end, you could make it more of a "pinpoint" type of light. You could put the LED and detector next to each other with a wall between them.

\$\endgroup\$
  • \$\begingroup\$ This will not work well as a 'break detect'. You will get reflected IR from any object crossing in front, and no means to distinguish. You COULD check for sudden changes in the received IR level, but you have not said so. Even then it would be tricky and unreliable. \$\endgroup\$ – Kripacharya Jul 24 at 3:56
0
\$\begingroup\$

Putting a reflector on the other side is the obvious and trivial answer. But it won't work completely reliably as a beam break detector because of reflections from the very objects which are breaking the beam. For exactly this reason a distance measuring is SPAD LiDar module is a better bet. Since your objective is a doorway of less than 2 mtrs, this is perfect. Simply do fast, continous distance measurements. If the result shows less than (say) 4ft, then count it as a beam break. There some excellent cheap modules available now... $5-10 range. I'll post the part number as soon as I remember it.

Ok... VL53L0X is one such.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.