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DIY Laser Power Meter Interface Module






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dar303

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I think it is good to keep both the hardware and software simple.
The software could have just a few functions like power measurement, power vs. current and power vs time (that measures the power at a specific interval like once every minute to see long term stability) and the possibility to make and save data and graphs.
 
knimrod

knimrod

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I finally received all the parts and the PCBs.  I'll be doing more testing the next couple of evenings and over the weekend but initial testing looks good.  If all goes as planned, I'll have kits ready to go out on Monday.
 

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knimrod

knimrod

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I've done some preliminary testing of the interface module and it appears to work perfectly. :D

There is one problem however and that is that the output from the LPM amp is a bit noisy for an A/D process with no other filtering.  The noise is only a few LSBs but I'm not happy with it.  I've made a preliminary fix by soldering a .1uf cap across the amplifier feedback resistor (between pins 2 & 6 of the LTC1050) and that definetely solves the problem.  I suppose I could write a software filter for the PIC and I'll have to consider that.

Also, I'm wondering if there shouldn't be some offset built into the interface to allow for a reliable zero.  The PIC can't convert a negative voltage and a negative voltage is the same as 0 to the A/D converter (actually it's clamped).  Because of this, it would be possible to inadvertently put some dead-band in the measurement if the offset was set below 0.  
 

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amkdeath

amkdeath

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knimrod said:
I've done some preliminary testing of the interface module and it appears to work perfectly. :D

There is one problem however and that is that the output from the LPM amp is a bit noisy for an A/D process with no other filtering. The noise is only a few LSBs but I'm not happy with it. I've made a preliminary fix by soldering a .1uf cap across the amplifier feedback resistor (between pins 2 & 6 of the LTC1050) and that definetely solves the problem. I suppose I could write a software filter for the PIC and I'll have to consider that.

Also, I'm wondering if there shouldn't be some offset built into the interface to allow for a reliable zero. The PIC can't convert a negative voltage and a negative voltage is the same as 0 to the A/D converter (actually it's clamped). Because of this, it would be possible to inadvertently put some dead-band in the measurement if the offset was set below 0.
Click to expand...


solder a cap on the main board?? or on the interface module?

regards,

amk

P.S. LOOKS GOOD! and if the cap is on the interface module, can you revise the PCB's to accommodate this? if yes ill be in for one soon, for rev 2!
 
knimrod

knimrod

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The cap modification would go on the original amplifier PCB as pictured above.  But I'm not sure it's necessary yet..
I'm looking into some simple software filtering.
 

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amkdeath

amkdeath

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do the 2 wires going to the interface module from the DPM out on the main PCB solder in parallel with the original DPM setup?
 
knimrod

knimrod

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amkdeath said:
do the 2 wires going to the interface module from the DPM out on the main PCB solder in parallel with the original DPM setup?
Click to expand...

Yes.
 
TheMonk

TheMonk

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payment send to your paypal. :)

I also added some to support the project. :D
 
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dkelley

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I would opt for the best hardware solution possible and keep the software simple. If a cap will solve a jitter problem
I would go for that. Deadband may be another problem.
 
knimrod

knimrod

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dkelley said:
I would opt for the best hardware solution possible and keep the software simple. If a cap will solve a jitter problem
I would go for that. Deadband may be another problem.
Click to expand...

I think I'll put a filter in the firmware anyway but I'm convinced that adding that capacitor would be the best solution.  I'll include the extra cap as part of the kit.  I'll leave it up to the builder to decide whether to install it or not after seeing what kind of noise issues come up on a case-by-case basis.

As for the deadband, what I would propose is perhaps a one or two LSB -offset in the application software for determining end of range.  This would mean that the measurement from the DPM and the PC would disagree by a mW or two with respect to zero offset but it would make adjusting the zero offset via the PC more accurate.  The offset could always be disabled in software...
 
knimrod

knimrod

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Working on the PC application. The display seems noisy because the sensor is facing the window and I'm casting shadows. It's actually quite clean in subdued lighting.

[media width=480]http://www.youtube.com/watch?v=oC0vf8ooovQ[/media]
 
amkdeath

amkdeath

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OMG ILL TAKE ONE


see my PM


btw, I dont see what you mean by noise...
 
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