#### NewbieDoug

Joined Feb 4, 2012
15
For many years I have not been able to find the concept I wish to build (have contacted some manufacturers / retailers)

No-one have a circuit diagram then please?

Where else can I try please?

#### mhastie1234

Joined Feb 10, 2012
29
I know where you are coming from. I used a pick up for years and it can be a pain. It seems that all you need is a bunch of photo transistors on a pole spaced apart every few mil. when the laser hits the transistor the circuit is completed and the led turns on at that specific measurement. I am not an engineer but i will see what i can dig up for you.

#### NewbieDoug

Joined Feb 4, 2012
15
Its such a relief to read someone who has an understanding of the system / issue. The wonderful manufacturers over engineer the concept and turn 300mm of detection into THREE LIGHTS?!?!?! Uuuurrrgggghhhh... I'm going to wet my pants if I can find / make the receiver circuit I'm wanting. Surely a programmable chip (PICAXE) could be a huge help here? I'm not educated in electronics, I have a slight grasp on the simple stuff, and I was sure hoping there was a relatively simple answer out there for me.
Thanks again hastie.

#### mhastie1234

Joined Feb 10, 2012
29
Im thinking that you will need a circuit like this. Its pretty simple you will just need a lot of them for every measurement. Are you doing a lot of fine grading? You could add 555 circuit to the mix so you could have a different tone for every led. Might be better so you don't always have to look at it. What do you think. Is this close to what your looking for?

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#### NewbieDoug

Joined Feb 4, 2012
15
Well done. Thanks.
That looks like what I need.
On another forum someone posted similar, but with a few more transistors to amplify the signal. Someone else also posed a pdf of a photo (diode) array with transistors in it, I'm yet to work out who it is that sells them.
Cheers.

#### Bernard

Joined Aug 7, 2008
5,524
mhastie1234's circuit may be fine to test photo tranistors in a dim room, or the laser at night. Using 100k for R1 & 5V battery, measured V from collector to emitter of Q1; dark 4.99V, outside, bright sun, 2:30PM, slightly shaded, .9 to 1.1V. A laser pointer could bring down V to 50mV to .5v. So I believe the comparator ckt will work-- except the photo transistor does not respond well beyond +_ 15 deg.

#### Bernard

Joined Aug 7, 2008
5,524
Thumbnail re drawn to use 558, quad timer & 339, quad comparator. When laser dot is traveling at 500 to 600 RPM, the LED will appear to be on all the time even tho each strike triggers it for 1/10 sec.
To broaden acceptance angle, I filed the dome lens from U1, will try it next Monday- rest of this week is spoken for.

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#### Bernard

Joined Aug 7, 2008
5,524
Tried a small solar cell, S875-16R @ $1.99, 2mm X 7mm, has beam acceptance angle over 90 deg. Cost is the problem, but in despiration might try " Easy Solder Solar Cell Pieces", 5 pieces for$ 2.00, possibly could cut them into usefull sized pieces, say 5mm X 10mm ? Untill next wk.

#### NewbieDoug

Joined Feb 4, 2012
15
Thanks Bernard. VERY appreciated!

#### Bernard

Joined Aug 7, 2008
5,524
I've ordered some " Easy Solder Solar Cell Pieces " to see how easy it really it is- curiosity is an expensive thing. It may be necessary to capacitvly couple cell output to reduce background.

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#### ifixit

Joined Nov 20, 2008
650
Hi

I have been thinking about what you are trying to do and here are some things for you to think about as you develop your novel idea

Based on previous posts, I have assumed the following requirements & conditions:
• laser rotates at 600 RPM
• detector bar to work at a range of 100m from the laser source
• 30cm long detector strip
• 4 detector strips required for 360 degree coverage
• Detector device diameter is 5mm
• Laser beam spreads to 15mm at 100m distance

Evaluating design requirements:
• 600 rpm is 10 RPS, or 100mS per revolution
• Velocity of the beam at 100m is; pi X 2 X radius / 100mS = 6,280m/S
• This translates to 6.28mm/uS
• So the 5mm detector is fully exposed to the 15mm wide beam for only 10mm of travel, which is 10/6.28 or 1.59uS.
• The switching speed for the detector should be at least 10X as fast, or 150nS
• The beam intensity will be reduced a lot by a dusty environment
• Youll need to have a top notch, fast, sensitive detector for this application
• I suspect this is why we pay $500 or$600 for a receiver unit.
• Each detector strip contains (30cm/5mm) 60 detector circuits
• Times 4 strips is 240 detectors plus related circuits
• If the parts for one detector and related circuits cost $5 then thats$1200 for all, plus the cost for everything else. \$??
Scaling back the design requirements will help with cost, but may I suggest that you build only one circuit and test in the field under actual (dusty, hot) conditions first. Once you are happy with the performance & cost then proceed further.

Bernard, if you build a test circuit using the solar cell detector, it would be good to do a response-time test also.

Good Luck,
Ifixit

#### Bernard

Joined Aug 7, 2008
5,524
ifixit, where did you get the 15mm beamwidth, I,ve asked OP several times for this info with no result. Maybe the laser cannot be slowed down enough to measure. Welcome to the problem, we need all the help we can get.

#### ifixit

Joined Nov 20, 2008
650
Hi Bernard,

The 15mm beam width is based on doug's comments in post #5 and some experience with communication lasers. With an accuracy 5mm over 100m, the beam obviously doesn't spread much in a professional quality instrument, and they have receivers that can detect this beam.

This worst case scenario seems to be what doug would like, but he may have to settle for something less.

If the requirements are relaxed toward your assessment in post #4 then the task is a little easier, but still quite challenging.

This is why I suggested the electro-mechanical solution in post #6.

Regards,
Ifixit