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LPM Design: A Beginner's Guide

Atomicrox said:
Why would we need .25mV per 1mW? It should still work above 5W at 1mV/mW, as long as the supply voltage is high enough. But I agree the supply might be an issue..
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Most, if not all, sensors they offer at those powers are 0.1V/W or less. It's just not practical to have a 150V+ "signal" voltage from the sensor. :p

Trevor
 





Part Four - Creating a Sensor

Now that you've got a functional LPM with a screen you need a sensor. OEM heads such as the Ophir 20C-A will work, but if you're on a budget you need to make your own sensor.

First, the sensor is needed. For this tutorial a peltier plate (TEC) will be used to create a thermal sensor. When the laser is pointed on the peltier plate it will create a small voltage across the leads.

There are a few things that need to be considered when looking for a peltier plate to make a suitable sensor.
  • Amount of junctions. Ideally the more junctions the better, as there will be more voltage on the output. The amount of junctions often isn't specified in the datasheet, but the more junctions a peliter plate has the higher the voltage will be; therefore choose one with a high voltage rating. A peltier plate with a voltage rating under 4V is not recommended.
  • The smaller the surface area is, the faster the response time will be. Keep it under 20mmx20mm for a response time under 60 seconds. Around 10mmx10mm is ideal.
  • Unpotted (not sealed) peltier plates typically have better performance.

The power from the laser will heat up the peltier plate as it measures the laser. This heat has to be somewhere - so adhere the peltier plate to a heat sink with thermally conductive epoxy. Make sure the hot side is face up. Regulator heat sinks are commonly used as they are cheap and allow for the peltier plate to be recessed, preventing it from damage.

Now that you've got the peltier plate and heat sink assembly it needs to be coated flat black to better absorb the laser power. Common coatings include heat resistant paint for brake calipers, barbeques and radiators.

Next we need a amplifier to boost the voltage from the sensor; this is easily built with a generic OP amp such as the LM358. Set it up as seen below.
xtwk01c.png


The formula for the voltage boost is Vin*(1+R1/R2). To calibrate the sensor find a laser with a known power, shine the laser on the sensor and adjust the potentiometer R1 until you reach the desired volts per watt ratio.

The once you have the sensor calibrated as desired feed it into the ADC on the Arduino.
 
There are ICs and very good instrumentation amp typologies that are good for reading individual thermocouples. AD595 for example.

They cut out the noise greatly via a very CMRR and provide a cold junction compensation- and some can linearize the curve.

Do you think any of those are real advantages over the lm358?
 
I mention the LM358 because it's a common OP amp and lots of people have a few in their parts box, and it's available at most parts stores if someone doesn't.

It's a very old OP amp and can only amplify to about +Vs - 4V :p

There are plenty of low noise, rail to rail, ect. that would be better suited than the LM358.
 
Okay, so, this weekend is fall break. So far, we have:

- How to make an Arduino talk to the computer.
- How to read voltage using an Arduino.
- How to add a display to your basic LPM.
- How to make your own sensor.

What should I spend some of this weekend working on? :)

Trevor
 
This reminds me, I never sent you guys those DIY quick response sensors! Actually I never even put them together. Been pretty busy and I dont really have the right tools to do so here at my dorm. I'll order a few things and see if I can get that taken care of :p

If it passes your guys critique I can write up a tutorial here.
 
Oh that sucks! I limited myself to one bag, about the size of a plastic shopping bag from a grocery store for my stuff. Made absolutely sure my fluke was in there, also my portable little butane iron and solder spool (forgot my solder paste), a ziplock bag of heatsinks and copper modules, a ziplock of uCs and related items, li-ions, and a altoids tin stuffed with >$500 of diodes. Forgot my pcbs and parts, but I cant have a toaster oven anyways. The only working laser I brought is a green pen and a 405 pen, left all my high power stuff and labbies.

I never realized how much stuff I have, I feel like I cant build anything because I had to leave so much stuff!
 
I've got two toaster ovens of the same make/model.

They work just as well for reflow (with some additional circuitry) as it does for making toast :D

Just got to make sure you don't put the wrong thing into the wrong toaster :shhh:

benmwv said:
I never realized how much stuff I have, I feel like I cant build anything because I had to leave so much stuff!
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Yeah, that feeling sucks :p
I've mainly been doing PCB design.
 
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Lol, what do your guys roommates think of you having a toaster oven for reflowing? We aren't allowed to have toaster oven because they think we will burn the place down.

I got a crappy roommate. He thinks i'm weird just for the small amount of stuff I have. He also is very awkward and boring. He thinks hes better than me because he never goes to parties or comes home drunk or smokes. The funny thing is I have better test scores :p Hes also an in-the-closet gay. Im not homophobic but that doesn't mean I want to live with a gay guy, its awkward. Every person i've had over, girls and guys and even my mom, says he is too so its not just me.

We are getting a bit off topic here. :D Alex did you ever get those heatsinks and graphite disks we were talking about this summer machined?
 
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My roommate is a computer engineering major. We both have a lot of embedded development stuff. :D

Trevor
 
Heh. Well, I dun have a roommate XD But this year, I plan on reflowing outside because I am afraid the smoke might set off my alarm :\
 
Trevor said:
My roommate is a computer engineering major. We both have a lot of embedded development stuff. :D

Trevor
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I wish I have a roommate like yours. I am a physics major and although my roommate is a ECE major, he deal with Power Line most of the time....
 
Trevor said:
Okay, so, this weekend is fall break. So far, we have:

- How to make an Arduino talk to the computer.
- How to read voltage using an Arduino.
- How to add a display to your basic LPM.
- How to make your own sensor.

What should I spend some of this weekend working on? :)

Trevor
Click to expand...

How about
-How to make ARM microcontroller based LPM :D
 
What are you guys majors? Im doing a double, EE and CS. That can still change though, and i'm not sure if I should go CPE, CPE/CS, EE only, or keep EE/CS.

And how to make an ARM LPM would be awesome! Though there is a lot of material to cover...
 
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