Laser light barrier receiver

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Hi everyone,

 

I intend to design laser light barriers which are going to start a timer when a car passes trough the first one and stop it when the second one is passed.

 

My problem know is that this has to work outdoors, which means the ambient light will vary greatly. So my idea was to modulate the laser beam at a fixed frequency and make the receiver only react to said frequency.

But I'm kind of clueless how to do that. I know that there has to be a more or less simple solution to this since commercial products all do it that way. Even the IR receiver in DVD players an stereo system all work like this.

 

So basically, I need a circuit that takes in a square wave signal(the modulated laser beam) with varying DC offset plus noise (ambient light) and gives a logic signal based on whether said square wave is present or not. The modulation frequency is not yet fixed since I'm going to use an MCU to generate it.

 

Can somebody please point me in the right direction? Googling it did not reveal much useful information, probably because I did not use the right terms.

 

Thanks in advance,

Patrick

"Some people die at 25 and aren't buried until 75." -Benjamin Franklin

 

What is life's greatest illusion?"  "Innocence, my brother." -Skyrim

 

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Sounds like an interesting project.

 

Is the laser setup at a fixed location, or does it have to be moved now and then?

 

Having a long barrel on the receiver, carefully aimed at the transmitter, will block out the majority of the ambient light, and greatly increase your signal to noise ratio.

 

But, such a setup requires a little more attention to physical setup and aiming, and the barrel has to be wide enough so that you can aim it, but not too wide.

Having an LED mounted on the receiver that changes its flash rate or intensity based upon the strength of the received signal will  make it much easier to set up the aiming.

 

How fast of a response time does the system need to have?

 

The issue that comes to mind is that a simple laser on / off is very, very fast.

The speed is determined by the switching time of the photodiode or whatever the laser detector element is.

 

If you modulate the laser beam, then you might typically feed the photodiode output through an AC coupled band pass filter to detect the modulation signal.

 

The "tone detector" for the modulation signal will add a delay to the response, so you will have to run some tests on your setup to see how fast your tone detector is, and is it fast enough.

 

I'd think that the tone detector would likely have a faster response time with a 10K or 20KHz modulation signal, compared to a 1 KHz signal.

 

JC

 

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While modulating the laser is a good idea, the frequency would have to be rather high(upper KHZ) and the processing of the frequency will also have to be fast to minimise the trigger latency.

 

AS Doc says, place a tube in front of the RX to block ambient light somewhat.

 

We did something similar at the Fiber Optics place with R/C cars.  We used a fiber laser emitter and beamed it at a photodiode about a foot away.  The DC output of the transimpedance amplifier was fed into one of the two inputs of a voltage comparator, the other input had a reference DC voltage on it.  When the beam was broken by the car the comparator output would change state triggering a chronometer  we had a second one at the end of the drag strip to stop the chronometer.

 

By adjusting the DC reference you can 'filter' out the ambient light along with the tube as the laser will flood the receiver when the laser is missing, the output will be much lower so set the reference just below the laser level

 

Now mind you when we did this at the Fiber company the lasers and the photodiodes were designed for gigahertz frequencies so rise and fall times were negligible  couple of picoseconds....the parts were also blistering expensive and if the big shots had found out I used $1000 laser emitters, and $1500 photodiodes(avalanche diodes I think actually) I think my paycheck would have been very light for quite sometime.

 

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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You could use ir remote control receiver devices that have all the magic built in. You just need a IR laser and modulate it with 38kHz

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Forget the modulation!  Just use a visible laser (to make the aiming easier) and a retroreflector.  The ambient light will be reflected back to the source and the laser light back towards the laser.

 

In consideration of others, please RTFM!

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Thanks to everyone for your input.

 

DocJC wrote:
The "tone detector" for the modulation signal will add a delay to the response, so you will have to run some tests on your setup to see how fast your tone detector is, and is it fast enough.

The missing keyword was tone detector, thanks JC. I now found the LM567 which seems to be perfect for this. I think about 30kHz will do fine. As long as the response time is lower than about 10ms, I'm happy.

 

Kartman wrote:
You could use ir remote control receiver devices that have all the magic built in. You just need a IR laser and modulate it with 38kHz

This was actually my first idea but I couldn't find a receiver that would work with CW since they are made for remotes which only send short bursts. After trying about 5 different models I had in my inventory, I gave up. They all rejected the signal after about half a second. The TSSP4038 however is specifically made for light barriers. I found out about this thing only recently and haven't it bought yet but i will try this.

"Some people die at 25 and aren't buried until 75." -Benjamin Franklin

 

What is life's greatest illusion?"  "Innocence, my brother." -Skyrim

 

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Here's the background for all of this, for those who care:

 

My parents participate in classic car rallyes since about two years. These rallyes are not about going as fast as possible along a dirt road of course, no one wont's do damage their precious classics. It's more about timing and precision. There is a course to drive but you don't have to be the fastest but instead you have to be at the next checkpoint at exactly the given time, not too early and not too late.  And there are various skill tests at these checkpoints.

 

One of them is to pass between to light barriers as closely as possible within a given time. So there are two LBs at an unknown and arbitrary distance, say 100m apart. And then the goal is to take exactly 20s from the first to the second. The one who comes closest to these 20s wins this test.

 

And now my parents want to be able to do a little training, so they asked me to build such light barriers.

"Some people die at 25 and aren't buried until 75." -Benjamin Franklin

 

What is life's greatest illusion?"  "Innocence, my brother." -Skyrim

 

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Google "Hall effect vehicle sensor". There are plenty that would be a far easier, but not as cool as a laser, to implement. More durable as well.

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user