Pulsed infrared/red laser detector

bookworm1706

Hello I have been trying to build my own job site laser grade receiver. The transmitter laser spins at 600 rpm, and so to a receiver it appears as pulses. I do not understand how commercial off the shelf receivers filter out direct sun. My system uses the lm567 tone detector with a photo-diode, (which idea I got off this forum) which feeds into a propeller so when the beam is spinning across the photo-diode at 36khz it triggers and the propeller reads the pin from the lm567. It works well inside, but as soon as the sun shines on it if floods the photo diode and it quits.

Does anybody have any idea's how the professionals do it? Judging by my amateur receiver's performance their receivers are almost miraculous as they perform perfectly on a wide open job site in direct sunshine all day when mine fails if indirect sun bounces into it! I am going to try the circuit attached which apparently is very famous and is on the Mars Rover, but it seems to me the fundamental problem is that in direct sunlight my photo-diode is just completely on. Besides, my part count for 5 photo-diodes would be quite large since i would have to do one of these on every photo-diode which would be 4 extra transistors for each photo-diode and when i look inside a commercial laser receiver i do not see that they did it this way with so many parts. But I am not knowledgeable enough to reverse engineer their multi-layer pcb. . If anyone has seen a rotary laser receiver they would also see that the diodes are pretty exposed, with the filtering only being a red plastic film. But I have seen one guy grading with a receiver where the film was scratched away multiple places and his system didn't seem to mind!

My own thinking is that it has something to do with the rotary laser spinning so it is pulsing the receiver, so the receiver is designed to be triggered by a pulse and ignores the very strong DC offset caused by the sun, This seems to be confirmed as if I shine a laser pointer at the receiver it does nothing, but if I wave it quickly back and forth it responds. This is good but it destroys my system with the lm567 as I need an exact frequency of pulses whereas all the commercial stuff just needs a repetitive pulse of any "reasonable" frequency. This is a great mystery to me that I have been thinking about for a while and searching on Google on and off for pulse receivers, infrared pulse receivers and getting nowhere, any insight and knowledge from the experts on here about how this is done, would be greatly appreciated. I have included links to a commercial laser receiver so one can see what they look like.

Thanks

http://www.tigersupplies.com/Products/Topcon-LS-B100-Machine-Control-Laser-Receiver__TOP57135.aspx?invsrc=adwords_ts&gclid=CjwKCAjwyIHPBRAIEiwAHPS-GMqAvjwEzobhhGm0Fb4HG431gS_q89AijDp--MBkl2yoNEPebNFnZRoCSY0QAvD_BwE

http://www.tigersupplies.com/Products/Topcon-LS-80L-Laser-Receiver-Sensor-Detector__TOP57181A__TOP57181.aspx?invsrc=adwords_ts&gclid=CjwKCAjwyIHPBRAIEiwAHPS-GI-tTTv5TA_T6OOJsVZ6fU1Fjia-nIHdyqesRm_Oxa-wqfFrzks0BRoCSL8QAvD_BwE

Phil Pilgrim (PhiPi)

Putting an optical bandpass filter in front of the laser diode should help -- especially in the infrared, where the sun radiates considerable energy. Edmund Scientific sells a variety of interference filters that have very narrow bandpass characteristics. Just get one that matches the wavelength of your laser.

BTW, these types of filters have a pretty narrow angle of acceptance at the designated wavelength. So your detector should be pointed directly at the transmitter.

Here's a link:

https://www.edmundoptics.com/optics/optical-filters/bandpass-filters/Laser-Line-Bandpass-Interference-Filters/

For half the price of one of those, you could also try a BG38 colored glass filter. It will filter out the near infrared, passing the entire visible spectrum, so it won't be as effective as an interference filter:

https://www.edmundoptics.com/optics/optical-filters/bandpass-filters/bg-38-vis-25.4mm-dia.-colored-glass-bandpass-filter/

Most lenses for TV color cameras use a BG38 or something similar to block the IR that the RGB filters on the sensor would pass.

-Phil

idbruce

bookworm1706

Does anybody have any idea's how the professionals do it? Judging by my amateur receiver's performance their receivers are almost miraculous as they perform perfectly on a wide open job site in direct sunshine all day when mine fails if indirect sun bounces into it!

Patent research would be a very good place to start. I am fairly certain that most of these lasers would be covered by one or more patents, and they should have patent numbers attached. Just research those patents, and then there will be "references" cited by the patent examiner, within these patents, that will lead to a lot of cross referencing and a wealth of further documentation.

erco

Your particular choice of which photodiode (of hundreds ?) is critical. Just as there are hundreds of similar-looking 38 kHz IR receivers with wildly differing specs. Some are have built-in noise filters, some are designed for sunlight rejection, continuous signal, etc. I can't tell you where to start, but clearly there's a solution since commercial units work so well. There's probably a lot of legwork ahead of you.

I will mention that I was amazed at these Waveshare laser sensors: https://www.amazon.com/Waveshare-Laser-Receiver-Sensor-Transmitter/dp/B00NJNYQ9G I posted about them at http://forums.parallax.com/discussion/comment/1393404/#Comment_1393404 and http://forums.parallax.com/discussion/comment/1394494/#Comment_

This is a reflectance sensor. My sunny outdoor tests worked easily at over 50 feet. This was detecting the reflection from a piece of Scotchlite reflector, so the laser round trip was over 100 feet. Just used a small cardboard box for sun shielding on each end. If you didn't use reflection and shot a direct beam from sensor 1 to sensor 2, I'm sure you'd get well over 100 feet, possibly even 200 feet.

Beau Schwabe

bookworm1706,

Notice the receiver diode is reverse biased in your schematic... THAT is a subtle and key feature to the design. The circuit is a dual balanced self biasing AGC(Automatic Gain Control) that "sees" the Photo diode as a capacitor since it is reverse biased. In this operation mode, the signal falling on the Photo diode causes the capacitance to change proportionally with the signal.

bookworm1706

Thanks for all the replies, Parallax has a very good forum because of it. I went out of town after posting the OP, so I am only responding now, but I will read through them and continue thinking about this and maybe ask some more questions on here and follow up on some of the suggestions when time permits me to work on it. Thanks for the patent idea, I have found several patents, I will see if I can make sense of them and go from there. Phil Pilgrim, the filter idea does help, and in practice all the laser receivers I have seen have red plastic over the receiver diodes, but as I was saying I have seen a guy using one receiver before with big chunks of the red plastic missing and he was still able to work in the daylight, I don't know if it was performing as well, but it is really impressive and a testimony to the engineers who designed this stuff. I do not think they are using any narrow aperture type filter as the receiver is designed to catch a beam from almost any angle across a job site.

Thanks again everyone,
Jonathan

bookworm1706

Beau I will have to think about what you are saying I think you are hitting on something very important and maybe the key. I will see if the patent schematics have a similar design.