Author Topic: How to filter reflections  (Read 1437 times)


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How to filter reflections
« on: February 04, 2017, 03:43:11 PM »
We've built a DIY-Lasertag system - Here are some images (too big for attachments):

It does work: shots and hits are registered, with corresponding beeps.

But we've run into a fundamental problem.
For the system to work outside, the LED signals need to be rather strong.
But if that configuration is used inside, reflections can be strong enough to trigger a hit.
Basically you can shoot the ceiling and hit everyone in the room.

To give you some technical details:

The system uses a Vishay TSAL 6200 with a resistor as IR emitter.
On the receiving side there are 5 TSOP 4838 (38khz) in parallel.

The problem seems unsolvable. As soon as we decrease the strength of the signal, the system becomes unusable outside.
But if we keep the signal strong, the reflections are problematic.
We tried optimizing the message protocol: Different number of bits transmitted, different lengths for marks and spaces, etc.
Nothing seemed to work.
Then we set up a test-station. One of the weapons connected to a notebook, over serial. To display the received message.
Behind the weapon was a sheet of cardboard.
A direct hit on the receivers and a reflected signal from the cardboard was virtually identical.
We believe that this can't be solved in software.

But experiments in hardware are expensive.
And we don't really have the know-how to make an educated guess about how to solve the problem.

I took a look at your sensor domes.
There is a number of differences:
1. 3 sensors to each dome
2. 56khz
3. 10uF capacitor.

We have 5 sensors, but they are wrapped around the head. I think that at most two sensors can be hit at once.
We use 38khz sensors.
We do have capacitors.

Is this a potential fix?
How many sensors are usually hit in your systems ?
Is the 56khz frequency important ?

And we tested our optical system.
The lens is a Google cardboard lens: 25mm diameter and 40mm focal length.
This is glued into a PVC pipe optics system.
We optimized this to have a spread of about 10cm per meter.
In a 10m distance, the cone will have a diameter of about 1m.
Is this a sensible value ?
Maybe outdoor lasertag systems need to be much more focused.


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Re: How to filter reflections
« Reply #1 on: February 04, 2017, 04:56:09 PM »
Hi mintwurm --

Don't try to find the perfect medium.  There isn't one.  For exactly the reasons you state, I know that both Milestag and Adventure Sports HQ equipment allows you to set either an Indoor or Outdoor mode (which essentially just adjusts the resistance to the IR emitter).  For example, make the Outdoor mode as strong as you like, and then set the Indoor mode with an extra resistor.  I know that Milestag and Adventure Sports also have options to further adjust the IR output.  Read the manual for Milestag to see the settings and maybe get a better description of them. 

As far as your lens and focal length, 1 meter at 10 meters sounds pretty good to me.  If you adjust it further could you get it any tighter of a radius than that?  If not, you found your sweet spot. 

Looks like a really cool project, though!  Good luck with it.

One question...Any particular reason for choosing 38khz?


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Re: How to filter reflections
« Reply #2 on: February 05, 2017, 02:07:09 PM »
Hi ezio,
Thanks for the quick feedback.
It's very reassuring to know that different modes for inside / outside games are used.

It's hard to optimise the spread of the light.
We want the matches to be fun, so you should be able to hit moving targets. And so far we only use the headband for sensors. The 1m at 10m seemed like a good idea.
At this point we're pretty much committed, since we have a huge stock of lenses. We used Google cardboard lenses, they are incredibly cheap.

The 38khz frequency was simply the easiest to get.
Only for this model,  we got reasonable bulk pricing.

Something else : you use plastic domes over your sensors? So far I thought this would only work as protection. Maybe it helps with the reflections, too. The reflections should be much weaker than the direct hits.

Can you tell me more about the plastics used in the protective domes?


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Re: How to filter reflections
« Reply #3 on: February 06, 2017, 01:05:18 PM »
The plastic domes are nothing special.  They are not calibrated for optics, nor do they provide any substantial focusing or blurring.  They're just for protection.  I doubt they would help in anyway with reducing ricochets.  As long as the domes are clear, the receivers should have no problem picking up the IR from the guns, and they shouldn't reduce the intensity of the incoming IR by any noticeable amount.

As far as the lenses go, you'll probably find you get what you pay for.  Google cardboard lenses are probably plastic, I'm guessing?  You might try purchasing a real glass lens just to see what kind of difference it makes.  Here's a place to get some reasonably priced ones:

Hope this helps!  Good luck!


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Re: How to filter reflections
« Reply #4 on: February 13, 2017, 03:53:08 AM »
That's a great link, thanks.
Unfortunately, I live in Germany which makes shipping more expensive.

Currently, the lenses seem to be good enough.
We have an existing playing field, some kind of scrapyard behind the house of a friend.
The usual distances for engagements will be about 20m, which seems possible with the cardboard lenses.

The biggest help was your feedback that different modes for indoor and outdoor are a viable solution.