# Detecting a lazer beam that hits near but not on a sensor

Joined Feb 21, 2020
23
I'm making myself a lazer tag system and on the boards I have assembled photo-transistors that are spaced between 1 and 2.5 inches apart. I'm trying to make it so the areas hit between the sensors will be recognized.

So far, I have tried the following and got results as follows:

Result: Sensitivity was excellent but data rate is terrible. I'm aiming for 2400bps but In that test I might have been lucky to get 300bps

Tried: lower resistor (82K)
Result: Sensitivity a bit small but the data rate recognized by the sensor is acceptable.

Tried: Direct line-of-sight with test circuits from PC serial port to PC serial port, but lazer module and photo-transistor setup is the same and still got 5V. (serial port connects through MAX232)
Result: almost no problem with 2400bps, could get 9600bps if beam directly hits the top of photo-transistor itself.

I don't know if I can do anything electrically to solve my problem but when I looked at digikey, I saw they sell convex LED lens caps that fit on the light and can accept a beam from any angle which I think will help a bit, but what if the beam hits directly between the two sensors? Is there a specific product (that digikey could sell) I can use to take that beam and make it travel across the item so that (at least one of) the two sensors can sense the light?

Only thing that comes across the top of my head is coarse-sanded plexiglass but I'm not sure if that would be good enough?

Any ideas?

#### KeithWalker

Joined Jul 10, 2017
796
De-focus the laser a little so that illuminates a larger area.

#### djsfantasi

Joined Apr 11, 2010
6,335
I'm making myself a lazer tag system and on the boards I have assembled photo-transistors that are spaced between 1 and 2.5 inches apart. I'm trying to make it so the areas hit between the sensors will be recognized.

So far, I have tried the following and got results as follows:

Result: Sensitivity was excellent but data rate is terrible. I'm aiming for 2400bps but In that test I might have been lucky to get 300bps

Tried: lower resistor (82K)
Result: Sensitivity a bit small but the data rate recognized by the sensor is acceptable.

Tried: Direct line-of-sight with test circuits from PC serial port to PC serial port, but lazer module and photo-transistor setup is the same and still got 5V. (serial port connects through MAX232)
Result: almost no problem with 2400bps, could get 9600bps if beam directly hits the top of photo-transistor itself.

I don't know if I can do anything electrically to solve my problem but when I looked at digikey, I saw they sell convex LED lens caps that fit on the light and can accept a beam from any angle which I think will help a bit, but what if the beam hits directly between the two sensors? Is there a specific product (that digikey could sell) I can use to take that beam and make it travel across the item so that (at least one of) the two sensors can sense the light?

Only thing that comes across the top of my head is coarse-sanded plexiglass but I'm not sure if that would be good enough?

Any ideas?
Coarse sanding a plexiglass screen is inexpensive. Try it!

#### djsfantasi

Joined Apr 11, 2010
6,335
And if you’re making the sensor boards, can you try one that’s a little more dense? Photo transistors no more than 0.3” apart!

#### oz93666

Joined Sep 7, 2010
737
yes ...Keith is right ...De-focus ...Some cheap lasers have the lens secured to a threaded screw which you can adjust ....

This one has a knurled end so you can change the focus ($2 eBay) Thread Starter #### mike_canada Joined Feb 21, 2020 23 And if you’re making the sensor boards, can you try one that’s a little more dense? Photo transistors no more than 0.3” apart! I already made them. and they're placed close to the lights so if someone aims at the lights then the unit works but if someone aims in-between it might not register. I'm not in the process of redoing 40 complex boards just to order triple the number of parts only to make parts closer. yes ...Keith is right ...De-focus ...Some cheap lasers have the lens secured to a threaded screw which you can adjust .... This one has a knurled end so you can change the focus ($2 eBay)
The question is, is that particular lazer legal for use in Canada? From what I recall, the law is not to use lazers with a grade stronger than 3B I think.

One disadvantage with the focus idea is that it makes skilled players sad because they want to practice aiming not have a small beam become a large circle at the target.

Joined Feb 21, 2020
23
Let's just say my coarse idea works. question is how coarse must I go? and don't they already have such plastic-like pieces for sale at digikey? I don't know if diffuser is the right name. I mean I want to type in their search box to find the coarse pieces I need so I can spread the light because that seems to be my only decent option right now since I don't want cheaters in my game. As for focusing, I think I can get away with ordering convex lens caps and put them on the lazer beam light output.

#### djsfantasi

Joined Apr 11, 2010
6,335
I already made them. and they're placed close to the lights so if someone aims at the lights then the unit works but if someone aims in-between it might not register. I'm not in the process of redoing 40 complex boards just to order triple the num
Ok, I understand your position. Just be sure that you understand that you may not be able to get anything to work with that restriction.

Good luck!

#### DickCappels

Joined Aug 21, 2008
6,303
Yes, diffuser is the name of the filter you need to place in front of the phototransistors. The diffusor will need to be placed some distant out in front of the phototransistors so there is room for the light to scatter from where it strikes the diffuser to the phototransistors.

Joined Feb 21, 2020
23
ok, so I'm trying to think of how I can order such a diffuser. I do have a local shop near me that makes custom products from various different plastics, but I'm not sure which plastic material to start with.

This is their website: https://paplastics.com/

#### DickCappels

Joined Aug 21, 2008
6,303
I once changed a bunch of clear LEDs to diffused LEDs by painting them with clear fingernail powder and then dusting them with talcum powder.

Also got nice results by roughing up the epoxy lenses with medium grit sand paper. You just need to let light through your diffuser and have it scatter as i goes through.

Edit:
You may also want to check out arts and crafts stores for materials that are similar to these.

#### Bernard

Joined Aug 7, 2008
5,463
I believe that all lasers shown in post # 9 can be re focused, just reviewed refocusing of one; @ 7 ft. tight, spot went from about 4 in- 1"- .2"- 1"- 6". A small spring holds lens against end piece.

Joined Feb 21, 2020
23
Ok so my searches for diffuser (and from another source, opal) material has resulted me into looking into translucent ceiling plates which the hardware salesman at Home Depot also identified as "diffusers" as well.

He showed me about 5 different varieties of diffusers they had in stock and each one had different ripples to it, but I can't figure out which one is best for my purpose. Sadly, each one costs $15 because they only sell them as 2x4 (2 feet by 4 feet) sheets, and I'm not willing to spend$100+ just to take a tiny corner off each sheet for testing.

The question is which diffuser is the best from the varieties home depot or any other hardware store would have in stock?

I tried craft and office stores and got no luck.

#### jpanhalt

Joined Jan 18, 2008
9,057
Dispersion of the incident light is one way. Have you considered a fluorescent compound, such as rhodamine. It's incredibly cheap for the amount you would need. Suspend/dissolve it in some clear finish and paint the off-target areas.

Joined Feb 21, 2020
23
I don't know what "incident light" is. Maybe you mean incadescent?

But the source of light in my case is a red lazer beam. And where would I be able to get such a compound already made?

#### jpanhalt

Joined Jan 18, 2008
9,057
"Incident" light means light that hits it. Definitely not"incandescent;" although, incandescent light can produce incident light.

If you shine a light with the proper wavelength on something with fluorescein (https://en.wikipedia.org/wiki/Fluorescein) in it, the whole thing lights up. There are many substance that fluoresce with red light. Rhodamine B (https://en.wikipedia.org/wiki/Rhodamine_B) is one such substance and is really cheap.

#### djsfantasi

Joined Apr 11, 2010
6,335
I’ve used clear polystyrene sheet from Home Depot. It comes in approximately 3’ x 4’ sheets. Then, I use coarse sandpaper in a hand rotary / orbital sander to roughen up one side. If needed, I hand sand to even out the scratches. While I’ve never used it in your application, I have used it to construct large (1’) 7-segment displays.

Coating the non-sanded side with Rhodamine suspended in a clear finish may optimize the effect

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#### MisterBill2

Joined Jan 23, 2018
5,749
I'm making myself a lazer tag system and on the boards I have assembled photo-transistors that are spaced between 1 and 2.5 inches apart. I'm trying to make it so the areas hit between the sensors will be recognized.

So far, I have tried the following and got results as follows:

Result: Sensitivity was excellent but data rate is terrible. I'm aiming for 2400bps but In that test I might have been lucky to get 300bps

Tried: lower resistor (82K)
Result: Sensitivity a bit small but the data rate recognized by the sensor is acceptable.

Tried: Direct line-of-sight with test circuits from PC serial port to PC serial port, but lazer module and photo-transistor setup is the same and still got 5V. (serial port connects through MAX232)
Result: almost no problem with 2400bps, could get 9600bps if beam directly hits the top of photo-transistor itself.

I don't know if I can do anything electrically to solve my problem but when I looked at digikey, I saw they sell convex LED lens caps that fit on the light and can accept a beam from any angle which I think will help a bit, but what if the beam hits directly between the two sensors? Is there a specific product (that digikey could sell) I can use to take that beam and make it travel across the item so that (at least one of) the two sensors can sense the light?

Only thing that comes across the top of my head is coarse-sanded plexiglass but I'm not sure if that would be good enough?

Any ideas?
Sanding a suitable plastic piece is a very excellent idea, I have used similar. You will need a few sensors, but not a lot of them, and you will need quite a bit of gain . But it can work very well.