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FrozenGate by Avery

Improvised power meter for the rest of us...

Eudaimonium

Eudaimonium

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Feb 5, 2008
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Hiya all, i would like to post some idea that i came up with during very booooring school class.
A laser power meter, based on a thermistor and a resistance meter (multimeter set to ohms)
By now you should already guessed what I 'm aiming at,
Wrap the thermistor with black tape, and connecting it as a probe to the multimeter set to ohms,

Problem is as follows: I need tomeone with some degree in themics to do some math.
Lets say that a laser dot shoots on a piece of tape covering the thermistor (black tape, goes without saying). Laser dot is 4 mm in diameter, and is heating up the tape.
Thermistor changes resistance from R1 to R2. (during 30 seconds period or something)
Now, we make the difference between those two R2-R1= R
Now let's say that the beggining resistance was (I'm guessing, never dealed with thermos before) 500 Ohms, now it is like, 20 k Ohms.
That is R = 19.5 K ohms.
Now i need some one to possibly come up with a relation to:
1) Restistance to temperature
2) Temperature to wattage of a laser power.

I know it ain't simple, however thermistor are fairly cheap, and i cannot afford a power meter. Simply cannot . Period.
I also want to know the wattage of my laser that i make so if someone would make this possible, i would be very gratefull.
I am also aware that it is not accurate, give or take 20 % or so.

A'ight, anybody?
 





30 $ is waaay too much. Please i have pointed out that i cannot afford to spend another dime.
I live in Croatia you know! And I ain't exactly rich. Thing are streched enough as it is.
Thanx for a fast reply an' all but i simply cannot afford to buy something.

EDIT: and yeah, i make 405 nm lasers and high power (200+ mW) red.
So however you turn it around, it is not good.
 
Last edited:
Eudaimonium said:
Hiya all, i would like to post some idea that i came up with during very booooring school class.
A laser power meter, based on a thermistor and a resistance meter (multimeter set to ohms)
By now you should already guessed what I 'm aiming at,
Wrap the thermistor with black tape, and connecting it as a probe to the multimeter set to ohms,

Problem is as follows: I need tomeone with some degree in themics to do some math.
Lets say that a laser dot shoots on a piece of tape covering the thermistor (black tape, goes without saying). Laser dot is 4 mm in diameter, and is heating up the tape.
Thermistor changes resistance from R1 to R2. (during 30 seconds period or something)
Now, we make the difference between those two R2-R1= R
Now let's say that the beggining resistance was (I'm guessing, never dealed with thermos before) 500 Ohms, now it is like, 20 k Ohms.
That is R = 19.5 K ohms.
Now i need some one to possibly come up with a relation to:
1) Restistance to temperature
2) Temperature to wattage of a laser power.

I know it ain't simple, however thermistor are fairly cheap, and i cannot afford a power meter. Simply cannot . Period.
I also want to know the wattage of my laser that i make so if someone would make this possible, i would be very gratefull.
I am also aware that it is not accurate, give or take 20 % or so.

A'ight, anybody?
Click to expand...

This is not a new idea... it has been tried and there a few things
that need to be considered..

1) black tape is reflective and is reflective a different amount at
different wavelength.

2) using a thermistor with black tape wrapped around it is a very
large thermal mass... The response of measurements will be very
long.. if even repeatable..

3) You will need to do these experiments yourself and compare
them to a Calibrated LPM if you want to be able to use a mW scale.

4) You will only be able to tell which of your lasers is stronger than
another but not by how much unless you use a scale of Ohms or
Degrees..

5) by your own estimates.. a +/- Accuracy of 20% is a total Error of 40%..


If $30.00 is waaay too much... then you could always get a
thermometer and see which laser gets the red line higher..

There are also a few DIY LPM projects on the Forum... But you will
need to do a little research on your own...
The [Search] button will help you.. try different search words..:cool:

BTW... you have been a member on LPF for Over 1-1/2 years...
Haven't you read any of the DIY LPM threads to know how difficult
your project could be...:thinking:


Jerry
 
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Right... So my idea drops to the water...
Ok let's drop thermistor idea, let's use photodiode, photoresistor or photocell.
Could somezhing of that be made?
Ofcourse that would require it's own power source ...
Could somebody make use of that?

Somebody say yes...
 
It can be made, but it'll probably end up costing you a few bucks either way.

You can try the laser power gauge project i posted here, you should be able to make that for under $10 in components, provided you have a soldering iron and multimeter already. If you have general electronic components at hand you might already have everything you need really.
 
That's what this is....

Calibrated Laser Power Meter Sensor Module by J.BAUER Electronics (EBAYHLPM)

The problem with inexpensive Optical Sensors is that the start
to heat up (and could saturate) above 120mW...
You would need to use an ND filter to measure Higher...
The other Drawback is... inexpensive Optical Sensors don't
like 405nm ....

If you want to build one... Sam's FAQ has some circuits... but it
comes back to... you need a Calibrated LPM to calibrate your
DIY Optical LPM...


Jerry
 
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Not to discourage anyone from attempting to DIY, but even with all the DIY knowledge and ability in the world the laser hobby still requires the occasional cash infusion. In other words, playing with lasers will always end up costing you some money. Even to build a sensor will cost you money in parts (unless you happen to have every part handy already)..

I understand what it's like to be cash strapped. I live in such a state.. Getting the best value for my money is what I spend a great deal of time and effort trying to do, and I'm no stranger to DIY, but even with all of my efforts my wife still complains about how much my laser hobby costs..
 
I know it will cost me bucks :D i have nothing against it. I just said i don't want to buy it if i can build it for a lot less money.
I'll check out the link, thanx guys!

Edit: and yes i did look, just that i'm no good at it :D
The one with two diodes looks fairly simple however calibration is still needed.
The one with termopile is out definetly cuz termopile is 30 $ at ebay ?! (acoording to his parts list)

I'ma go woth the first one, have no idea how will i do it, I'm just putting it in plan somewhere ...

Anyway thanx one again.
 
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The one with the Thermopile is really out... They were on special
for a one time deal about a year ago...
You could pick one up on eBay nowadays for around $300.00-$400.00..
Brand new the Thermopile is about $800.00... :eek:

Jerry
 
Eudaimonium said:
The one with two diodes looks fairly simple however calibration is still needed.
Click to expand...

Needed is relative here. If you build the one with the heater resistor on the sensor plate, you can get 10% accuracy without any reference laser or calibration using external equipment.

If you need better accuracy than 10% or so, i think they only way it to buy a calibrated meter, like the laserbee or coherent lasercheck (though that is rated 5% average, it does better as long as you know the wavelength).
 
Isn't the laser check like $900?? (IIRC :D) I dont think thats in his price range!
 
You are right, waaay out of my price range.
Well as I said I'm going to go with the one with the diodes, however due to the status right now it ain't gonna be anytime soon.
When i start makin' it I will start a new thread, till then... Savin' money.
 
I have to say that not everyone is able to afford professional meters, such as the lpf members sell. It would be nice if some1 came up with an idea which would lead to a "cheap" working LPM. Current LPM's are way too expensive. I just can't afford such high prices like Eudaimonium.
 
I'd say, settle for DIY and limited accuracy then. 10% might sound pretty bad, but its still valuable to see if a laser you bought is at least near spec, and it will also clearly show any improvement in homebuilt lasers you obtain by increasing current, using better lenses and such.
 
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