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# DIY Thermal LPM for under \$50

#### honeyx

##### Active member
You could use the peltier actively to keep both sides equally warm, then measure the necessary current. I think this should give you a very good linearity.
But you are measuring the voltage not the current. The current in this circuit is negligible. An yes, you can reach the phisical maximum and its different for every type of TEC.

im not sure about which fluctuations you are speaking, but yes they should fly perfectly linear, if they dont then external effects are at fault. c is a constant.
C is a constant but the space-time isn´t. Gravity for example is bending it and the light isn´t traveling in a absulute linear line through it.

If it were like you are saying, we would be also able to build lasers with a absolute perfect divergence with no spreading beams and the dots having always the same size no matter its 1 meter or 10 000 miles far away.

#### day

##### New member
Gravity for example is bending it and the light isn´t traveling in a absulute linear line through it.
this is not correct, i think (someone correct me if im wrong). The light is moving perfectly straight, but it bends at the same time.
You could maybe visualize it like this. Turn this '(' by 90 degrees, and it becomes this '|'

If it were like you are saying, we would be also able to build lasers with a absolute perfect divergence with no spreading beams and the dots having always the same size no matter its 1 meter or 10 000 miles far away.
this is not a light problem, its a optical/mechanical problem.
If you manage to reduce the emitter size to zero, then yes we would be a good step closer to it.

On top of that you are dealing with atmospheric influences which also scatters the light.

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#### honeyx

##### Active member
You are ignoring one simple rule. The universe is not a cube and it´s expanding all the time. therefore you will never get a absolute linear beam and the photons to have always the same distance to each other, even after passing a few billion years of space.

The same for reducing this to be only a mechanical/optical problem and the size of the emitter. There are not only diode lasers with a emitter and some optics. There are also gas lasers for example and those are divergating too. The emitter there is the gas plasma and when you are reducing it´s size, you even get a worse divergence.

#### day

##### New member
ive never claimed that its possible to get a perfect beam. Its all about improvements.

>The emitter there is the gas plasma and when you are reducing it´s size, you even get a worse divergence.

my knowledge about those converge to zero. Im guessing the beam doesnt require additional lenses? Because that would change everything.

Anyways, i was never talking about reaching perfection. We are talking diy here. no expensive lab equipment, only bits and pieces of knowledge. I just want to remove all unnecessary errors. Ive ordered the necessary parts, and will start playing around soon.

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#### honeyx

##### Active member
Gas lasers don´t require lenses, just perfectly aligned mirrors. There even exist TAE and DYE lasers with no mirrors at all.

Laserdiodes are also having microscopic mirrors and operate in principal the same way like gas lasers. Just because the whole laser is microscopic it´s RAW output has a horrible divergence compared to gas lasers with it´s huge emitters. That´s the only reason why lenses are needed to compenstate this up to a point.

And because of the nature of our universe you can´t reach infinite linearity. This you always only can reach for a relative short distance.

Therefore also a LPM only can give linearity for a given range. The circuit here is working really good when using the parts suggested here. All you need is a good TEC, the circuit and a good LED panel. To measure higher wattage lasers you also better go for a good CPU heatsing.

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#### day

##### New member
>All you need is a good TEC

what separates the good Elements from the bad?

#### MarioMaster

##### Well-known member
High junction count, requires less signal gain
Short junction height, reduces thermal mass

#### day

##### New member
damn i didn't know they differ a lot :<

I was only able to find one 15mm x 15mm element, all others i found were much bigger and i wanted to keep the mass as small as possible

the conductor blocks are 3mm high. (34 overall)

on both sides it has a 0.65mm ceramic plate (15mm x 15mm)

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#### hamzajaved780

##### New member
Hey can any one post a printable circuit of this project which i can etch on copper board as the one posted is much difficult to read>>>

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#### ARG

##### Well-known member
You should PM Kalmito and see if he'll share his.

It's a pretty easy circuit to make and route though, doesn't take long

#### ARG

##### Well-known member
Yup, that looks like a suitable TEC.

#### MarioMaster

##### Well-known member
Hi guys, a recent member's PM got me inspired to redraw the \$50 LPM project.

WHAT'S NEW IN v2.0

-dual rail power supply for better zero level performance (now requires 2x 9v batteries but can still be run on one battery by connecting jumper JP1)
-slightly cleaner schematic
-clearer solder pad labeling
-added circuit for unused op-amp to add battery indicator (optional)

PCB COMPONENTS: (prices are Mouser Electronics prices for quantity 1)
1x LM358 dual operational amplifier \$0.46
1x 0.1uF ceramic capacitor 6v or greater \$0.08
3x 10K ohm multiturn potentiometers \$1.13 (\$3.39)
4x 1K ohm resistors \$0.40
1x 10K ohm resistor \$0.10
2x 9v battery snap with leads \$0.64
2x 3mm or 5mm LEDs (use high brightness type) \$0.20
1x 5.1v 0.5W zener diode \$0.11
TOTAL: \$5.35

The following components are optional for the battery indicator circuit:
D1 zener diode
R7 1K resistor
R6 10K resistor
LED1 power ok LED
LED2 low battery LED
R8 10K multiturn potentiometer
Saves \$1.53

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#### MarioMaster

##### Well-known member
Also just a heads up for everyone, the board layout has not been tested yet so I will hold off on supplying the design files until I know it has been tested.

Thanks

#### paul1598419

##### Well-known member
Thanks for keeping these DIY projects alive. I designed one a couple years ago and they are pretty cheap to make and relatively accurate, too. For the price, they can't be beat. Great job!

#### gregma

##### Member
Question on the components. You list 3 10k pots, but there are only 2 shown in the circuit. R6 in the circuit shows as a regular 10k resistor.