laser level offset measurement

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xenon56
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laser level offset measurement

Post by xenon56 » Sat Sep 28, 2019 3:37 pm

I would like to measure contour lines of a sports field which is 125' square using a self-leveling laser red color and a linear array of photo-diodes (PDR). To build the measurement device I need to log which PDR of array is stimulated by each passage of the beam. One suggestion is to use a TIA current to voltage amplifier and latch circuit to acquire the data but no details are given. I gather, from a very limited discussion, that the passage of the beam gives rise to a very brief input. What I would like to know is whether one needs a separate input circuit for each photo-diode in the array. This would seem to preclude a large nummber of PDR's and therefore a useful measurement range. I would like plus/minus 100 mm and therefore about 40 PDR's if they are mounted at 5 mm intervals. If you have any insight in how to multiplex these signals while preserving the input origin I would appreciate greatly the help.

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haklesup
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Re: laser level offset measurement

Post by haklesup » Thu Oct 03, 2019 5:21 pm

Sounds a bit like a total station.

so when the Photodiode is illuminated by the red laser (you may need a red filter to reduce ambient glare) the diode generates some current. If you connect that to a digital IC (probably with parallel resistor to ground) with the right VIL and VIH levels, you may be able to cheaply convert the current to a buffered digital output. From there you have the buffering to do a variety of things but probably the lowest parts count would be to connect all those to IO pins o a microcontroller and scan to look for the changed input. Here is a good graphic but you will need to look at actual datasheets to confirm the final design
http://www.interfacebus.com/voltage_threshold.html

If Digital conversion through an IC won't work because the voltage pulse from the diode is too small, then using the Gate of an FET or Base of a BJT might be the level shifter you need. Many optoisolators use this configuration, maybe you can split one and just use the detector side
https://www.jameco.com/z/TCST1103-Visha ... gIWifD_BwE

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haklesup
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Re: laser level offset measurement

Post by haklesup » Fri Oct 04, 2019 3:13 pm

Have you picked a photo diode (PIR), can you post the part number. I am interested in output characteristics and the package it is in. you probably want to be picky about the lens and the direction the light needs to come in. Many have omnidirectional bubble like plastic lens and others are flat. Good selection in SMT but limited choices for thru hole.

This is the lowest cost Red Optimized PD on Digikey (using the search tool after selecting photodiodes)
https://www.digikey.com/product-detail/ ... -ND/480631
I'm not real sure how to read the output chart, What is A/W? I think it means 500mA at 1W of intensity max when the light is 900nm to 950nm, so if you are at mW laser power, expect uA of current.

An image search on "IV characteristic of photodiode" gives a good idea how they work. The diode is reverse biased (-10V for the example above) and when light hits the junction, the reverse leakage increases proportional to the illumination intensity in mW. -100uA to -400uA is typical in the charts I reviewed.

So if you reverse bias the diode on one terminal and put a series resistor on the other terminal to ground, when illuminated, current flow will increase causing the voltage drop to increase. Match that change to the input logic levels and you can convert the current into a digital bit at whatever sensitivity point you need by adjusting the Bias voltage and series resistor according to the design. What V and R values depend on the sensor you pick. (I didn't define anode and cathode because the Bias could be + or - voltage).

If you use 10V bias, you should probably use old 4000 series logic since it has a wide operating voltage but if you limit bias voltage to 5V and you can get 250uA through it when the laser hits (at your required range) then a (at least) 200 ohm resistor should give approximately 0V to 5V swing acceptable for most 5V logic families. At 3.3V you may not get much current in the PIR but higher bias and adjustment of the resistor should result in a workable operating point

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CeaSaR
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Re: laser level offset measurement

Post by CeaSaR » Fri Oct 04, 2019 7:47 pm

Land surveying is my profession. What you sound like you are saying is that you want to use a contractor's rotating laser level, something like this: https://www.amazon.com/Johnson-Level-To ... B00ME77KH8 and you want to build a logger that can be triggered by the laser in a +-100 mm range (I sure do hope the field is very close to perfectly level or you'll run out of sensor range in the array at less than +-4") as you walk around "taking shots" on the grid.

What you must determine is how you are going to collect that data. If you were to do it the old fashioned way and write it down by hand, there'd be no need for the array as the link I show comes with a rod, as do most other kits. BUT, if you want to be able to automatically store the information to a SD card or similar, you need to decide on what format you need and how best to divide up the array to process and collect that information.

So, before I get into any further speculation, please answer these questions:
1. How are you going to use the gathered information? Drop it into AutoCAD or similar drafting software?
2. What will be the format of the information? (The Data Collectors we use output ASCII text point files, usually in the format of P,N,E,Z,D {Point #, Northing, Easting, elevation or Zenith, Descriptor} which are imported into AutoCAD)
3. What will you be using to interpret and store the pulse of the laser hitting the PD array? Some sort of Micro Controller (PIC, Arduino, etc.)?

I have ideas and schemes that might work, but they would depend on what YOU have in mind. A more comprehensive picture of your fleshed out ideas would go a long way toward successful collaboration on everyone's part.

CeaSaR

**Have you checked out the wavelength of the laser level? It'll be important to know when selecting the photodiode. (I already have, but you should do that legwork.)
Hey, what do I know?

dyarker
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Re: laser level offset measurement

Post by dyarker » Sun Mar 08, 2020 11:07 pm

Shorted pair or wet junction will look like continuous "off-hook" to switch; gives busy to callers. After time will change to out of service, and gives same result as unpaid.

Cheers,
Dale Y

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CeaSaR
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Re: laser level offset measurement

Post by CeaSaR » Mon Mar 09, 2020 3:07 am

What'd I miss about the phone wires?
Hey, what do I know?

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dacflyer
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Re: laser level offset measurement

Post by dacflyer » Mon Mar 09, 2020 10:41 am

I've had times where i'd pick up my phone and i get a busy signal instead of a dial tone, But if i called my number it said it was out of service as well, and when i'd call the phone co. they said it might be 3 weeks before they can send out a repairman.
I belong to Centurylink..they don't even have a walk in office any more,. No such thing as customer service any more.

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haklesup
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Re: laser level offset measurement

Post by haklesup » Mon Mar 09, 2020 11:04 am

topic jumped the tracks didn't it

gerty
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Re: laser level offset measurement

Post by gerty » Mon Mar 09, 2020 8:46 pm

At least I'm not the only one that's lost... :-)

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