DIY: Laser Power Gauge

[drlava]

Good to see someone coming up with something new like this
You could probably improve the thermal time constants quite a bit using tiny SMT diodes and cal heater. Also very thin lead wires to the sensor. Finally consider enclosing the sensor from drafts which would greatly affect accuracy.

The sensor and ref diode bias current decreases as the battery is depleted, but this probably isn't too much of an issue.

 

[Benm]

You could probably improve the thermal time constants quite a bit using tiny SMT diodes and cal heater. Also very thin lead wires to the sensor. Finally consider enclosing the sensor from drafts which would greatly affect accuracy.

I actually tried to some extent: I isolated the airflow by placing the sensor disk in styrofoam with just a hole for the laser light to enter, and used very thin wires. I figured this would improve things, but it doesnt. The problem is in that setup, the only way for heat to exit the sensor is by IR radiation - which doesnt work fast enough. The result is that the disc will heat to a much higher temperature (well more than double) for a given power level, making things slower rather than faster.

External drafts should be avoided though: i've enclosed 3 of the side walls to some extent to prevent it.

The setup as pictured is the best i was able to build so far - it combines sensor cooling from convection, conduction in the wires and IR radiation, but the temperature-vs-power-rise of the whole thing is surprisingly linear (as good as ~1% comparing 250 and 63 mW heating).

Using tiny SMT components and perhaps an even thinner disc will likely improve speed - if you have them and want to build this, use them!

The sensor and ref diode bias current decreases as the battery is depleted, but this probably isn't too much of an issue.

True. The overal effect is marginal however, the V/T plot of a diode doesnt change much for say a 20% variation in bias, and since it affects both diodes equally its not much of an effect. Aslo, you can run this meter from any 9-15 volt labsupply or even stablized wall adapter if you want.

 

[CoZZm0_AU]

OK so i knocked one of these up and now that my lavadrives arrived today, i have some result (of sorts).

Firstly, big thanks to Benm for sharing this info and for helping me nut out some problems i had initially getting it cracking.

Here is my 2 versions, firstly got it layed out on a breadboard to test the circuit and see how it went, then put it onto some project board and wired it up. In the wired version i have since also added a thermal bridge between the heater and the sensor diode using a piece of black anodized heatsink shaved down to 0.5mm thick and bonded with a thermal epoxy (sparingly).

Results so far are:

PHR-803T @ 121ma = 85mw in aixiz housing with plastic lens.

now while that might seem a little low, there are 2 possible reasons.

1. i had some problems with the lavadrive in that i applied a 4 doide dummy load @4.6v set it to 100ma. I found that as the voltage dropped, the output increased, A LOT to over 120ma at under 4V, i wasn't expecting that and i think i'm going to get some 10440's and build a current regulated design which will be rock solid (or lower) as the battery voltage falls. 3x10440's in the housing i've got will easily give me enough voltage for a 317T type supply (but using smaller parts).

2. Once i hooked the diode up to the lavadrive and measured the current again, the minimum i could get reading on my DMM was 121ma before it flicked to 200+ ... NOW .. that's where the problem comes in, i might of damaged my diode in doing so (unfocused it has a dark spot on one side now). User error, live and learn, i have some fresh sleds to harvest so i'm going to play around with it and probably/ hopefully have one of my own drivers assembled to fit into the housing.


If anyone else is considering making this project, PLEASE DO. Its really not that hard, all the parts i got from my local electronics shop cost me <$15 and i had enough to make 2 complete circuits (i always buy extras just in case!).

I am eagerly awaiting Benm's results on the tested green unit that he's got coming to see how accurate it is.

 

[CoZZm0_AU]

Just thought i'd post of pic of the laser target. As you can see the back side is silver (because its filed down) and the front side that the laser hits is black anodized (not painted).

The epoxy is a special thermal epoxy made by Arctic Silver, common in the gaming/overclocking community for attaching heatsinks to VGA card memory etc.

 

[Benm]

Nice job on making the sensor!

For a final version i think it would be best to get the reference diode closer to the sensor, though i'm not sure how much of a difference it will make.

Another idea might be to increase (to 4k7/10k or so) the 1k pull-up resistors for each diode... i hadn't accounted for that in the original idea, but the diodes are dissipating a few mW each now which might skew results a little if they are not mounted equally.

 

[laserfreak3d]

ill keep on using my coherent powercheck for the moment but good job on the build

 

[Benm]

I've just received a measured DX200 red, kindly tested by forum member LikeitBright, with the following specs (3 measurement points)

5.49V/253mA -> 132.1/132.5 mW
5.98V/284mA -> 154.7/152.5 mW
6.48V/312mA -> 174.2/171.8 mW

The mW figures are the average power for the first minute and the instant power after one minuted. In requested those measurements because this power gauge is rather slow, so instant power at startup has little meaning.

I just measured it on batteries, running at 310 mA, and the resulted reading was 187 mW. This should have been close to the highest datapoint of 171.8 mW since the currents are almost equal. As an inital test, this figure is off by +9%, which is within the 10% i aimed for with this project.

I will try to measure this laser on a regulated power supply at the voltages specified tomorrow, and show some more results here. That should shed some more light on the actual accuracy.

My first impression is that this thing produces usable results, not bad considering i built it from components that were laying around the workbench here.

ill keep on using my coherent powercheck for the moment but good job on the build

Obviously its no replacement for any professional instrument, so if you have a proper LPM, rely on that

 

[CoZZm0_AU]

Nice work Benm, within your design goals is an excellent achievement!

Thought i'd try my hand at etching a PCB version of this. I did this for 2 reasons, 1 i've always wanted to be able to make my own PCB's but put it in the too hard and messy basket and 2, i am going to make a driver to suit the host i'm using that will be more stable than the flexdrive and i didn't want to goto the cost of ordering 100+ from china and try and recoup costs. I used the PCB etchant detailed Here: http://www.instructables.com/id/Stop-using-Ferric-Chloride-etchant!--A-better-etc/ . Surprisingly easy to obtain and worked VERY well. Just beware of the fumes, they are very nasty if you're not in a ventilated environment!

As you can see, i did 4 boards, only 2 of them actaully turned out properly (didn't iron it long enough it seems...)

Result, well it works, but i don't have a target on it yet so its not "functional" but i was happy with my first PCB effort. Its not 100% perfect, so i'll probably do another one, the Pots are backwards (grr!) and the resistor i feel really needs to be under the diode once mounted, but still, at least it works!

 

[Benm]

Isn't that nice - you made a pcb for it I have used the HCl/H2O2 quite often in the past, and it is a really good etching process, probably the best you can do at home with reasonable means.

If you do another: try leaving as much copper on the board as possible - make a big all round groundplane, powerplanes and such. For some reason many homebuilders make boards with just traces, but this is actually wasteful since you spend time and chemicals on removing copper, not on leaving it sitting there. If you use a groundplane that goes all around the board including the edge, you can also wind-proof the meter by mounting two slabs of fully coppered pcb on the sides like i did... just solder them in place, its an often used technique especially in high frequency electronics, but works for these things too.

Also, it would be better to get reference and sensor closer. Perhaps rotating the opamp 90 degrees will make wiring easier.

 

[Zom-B]

I also once considered making a laser power meter on a similar principle (before i bought a knimrod DIY laser power meter).

The principle, measure the absorption of the laser beam on one sensor by mimicking it on another. The sensors don't have to be linear, but exactly equal to each other. The one sensor heats up by the laser beam, the other sensor is heated by a resistor. Using a closed loop you can heat up the resistor until the sensors register the same. When they do, simply tell he power pushed into the heating resistor, which will be equal to the laser power.

It works great, in theory. I didn't build it, for the following reasons:
- The coating on the first sensor needs to be equichromatic, as to absorb every wavelength equally. I don't expect heatsink coatings to possess this property
- Some light is still reflected even from the best coating (Ideal black doesn't exist), so the measured power is pessimistic
- The resistor which heats the second sensor also heats surrounding air, resulting in less efficiency. With this, the measured power is optimistic.

All these things together, and you still won't know the exact power without calibration.

This concept is temperature independent though. Your design will drift when the meter setup is in a different ambient temperature, unless you twist the offset pot every time (as is needed with most 'real' thermal laser power meters, too)

 

[Benm]

I did a few more measurements. My lab supply is out, but i set up a construction where i measured current so to get a good comparison point. Values obtained:

250 mA -> 139 mW (should be just over 131.5)
300 mA -> 183 mW (should be just under 171.8)

As far as accuracy goes comparable to the quick initial test, and i still believe this is a project many people could built for little money while giving a usable indication of laser power.

I am a bit puzzled why the results are too high, i expected them to be too low due to light not being absorbed and all. Somehow the laser seems to do a better job at heating the sensor than the resistor does... perhaps a matter of mechanical design.

The principle, measure the absorption of the laser beam on one sensor by mimicking it on another. The sensors don't have to be linear, but exactly equal to each other. The one sensor heats up by the laser beam, the other sensor is heated by a resistor. Using a closed loop you can heat up the resistor until the sensors register the same. When they do, simply tell he power pushed into the heating resistor, which will be equal to the laser power.

This is a very good method of going about it. One big advantage over the design above is that it will be much faster, as it gives accurate results when things are heating up, so you dont have to wait to reach a stable temperature. Disadvantages for me would be:

- sensor equality, might be more difficult to build
- diffulty getting a stable yet rapid control over the heater power
- added complexity of readout, since the power is squared the voltage over the heater

For the last point there is only the option of calucating manually, or using for example the ADC in a PIC controller and then reading it out to a LCD. I might give it a go at some point, but the whole thing would be more complex, expensive and difficult to construct for others.

Now that i have the reference laser though, i'll see if there are points i can easily improve to make it more accurate.

 

[woop]

wow good job. very interesting project.
could you use a temperature sensor chip which already has amplification built in? like the lm 335. or a i2c sensor, have a picaxe or something to calculate the power based on calibration and ambient temperature

and you said most of the heat is radiated from the sensor by IR. wouldn't a lot of heat be conducted by the coper leads of the resistor and diode? you should probably have large areas of copper trace to dissipate this conducted heat which might make the equilibrium temp come slightly faster and drift less

 

[Benm]

You could use temperature sensors like the lm35, but those are usually larger than the 1n4148 diodes, which would make things slower. Plain diodes are pretty good for measuring temperature differences though, so there is no need.

Also, in the current design, most heat is lost from the sensor by air convection and conduction through the leads. To get mostly-IR loss, the sensor would have to be isolated from airflow (in a styrofoam block or something) and connected by whisker wires. But even then i doubt it would work - the sensor would become extremely hot if even 100 mW would have to be radiated off as blackbody radiation.

 

[andrea87]

Nice idea, I've just built one of these power meters on a piece of breadboard, waiting for my supplier to get some pre-coated boards to build up something better. Powering it with a 9V cell, I've got good results for the 250mW measurement, some oscillating values around 65-70mW for the 60mW one (my heater resistor is powered at 4.9V so 240mW @ 100 ohms, 60mW @ 200 ohms). Adding about 5mW from the diode biasing power, results look good. I'm just waiting for my DX order to arrive to test this on the 200mW red pointer, and on a true 30mW greenie.

just copy the addresses here and edit the * with an H

Build:
*ttp://img179.imagevenue.com/img.php?image=55237_lasermeter_build_122_514lo.jpg

Measuring 250mW
*ttp://img196.imagevenue.com/img.php?image=55240_lasermeter_measure_122_245lo.jpg

Sensor
*ttp://img40.imagevenue.com/img.php?image=55252_lasermeter_sensor_122_953lo.jpg

 

[lasersbee]

Nice idea, I've just built one of these power meters on a piece of breadboard, waiting for my supplier to get some pre-coated boards to build up something better. Powering it with a 9V cell, I've got good results for the 250mW measurement, some oscillating values around 65-70mW for the 60mW one (my heater resistor is powered at 4.9V so 240mW @ 100 ohms, 60mW @ 200 ohms). Adding about 5mW from the diode biasing power, results look good. I'm just waiting for my DX order to arrive to test this on the 200mW red pointer, and on a true 30mW greenie.

just copy the addresses here and edit the * with an H

Build:
*ttp://img179.imagevenue.com/img.php?image=55237_lasermeter_build_122_514lo.jpg

Measuring 250mW
*ttp://img196.imagevenue.com/img.php?image=55240_lasermeter_measure_122_245lo.jpg

Sensor
*ttp://img40.imagevenue.com/img.php?image=55252_lasermeter_sensor_122_953lo.jpg

Nice Build... and great pictures... [smiley=thumbsup.gif]

Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[andrea87]

I'm getting better results now, today I had tuned up the board on a (maybe) too much ventilated area... I was getting very instable results. Also, I think its a good idea to partially close the entire build into some sort of box, like benm did... Some airflow works well, too much gives really unstable values.

I've noticed also some long reply time with my sensor on low powers, like the 60mW calibration mode, probably due to a too thick target (about 0.5mm steel black-coated). I was so thinking to use an smd as sensor with just the heating resistor as laser target for lower power levels, for a faster response. Tomorrow I'll try this too.
smd sensor: *ttp://img204.imagevenue.com/img.php?image=69331_lasermeter_sensor_smd_122_544lo.jpg (don't get fooled by the large image, its little , the resistor is a standard do-204 1N4148)

I've done a preliminary design of a pretty compact board for this circuit, if anyone doesn't want to build it on a breadboard I'll attach also a PDF version ready to print and photo-etch.

board layout: *ttp://i40.tinypic.com/21j7lm8.png (It may look large, but it's 5x7cm / 2x3 inches approx )

pdf to print: *ttp://filebeam.com/ef37da7b4bc7383e05af1b51276263e0

can't wait to get my Dx order for some measurements