DIY Thermal LPM for under $50

In my research for designing a new power meter I offer to you a simple circuit that can allow you to use a small thermoelectric cooling module as a laser power sensor. (you can call it a wafer thermopile if you want *cough*lasersbee*cough*, but it's not nearly the same as a professional radial thermopile)

The circuit uses an LM358 operational amplifier IC in a basic non-inverting amplifier mode. Coat the surface of the TEC with a matte black high temperature paint to absorb laser light. When you shine your laser on the tec, the heat generated is converted to a small electrical signal by the junctions in the TEC. This small voltage is then amplified by the LM358 to a more usable level.

EDIT: Version 2 released - see post #541

To build this meter you will need the following:
1x high powered diode laser with a known power output (anything >100mW should be fine)
1x LM358 IC
1x 10K ohm multi-turn precision potentiometer
3x 1K ohm resistor
1x 10K ohm standard potentiometer
1x 0.1uF capacitor
1x voltmeter
1x small TEC module
1x heatsink

Optional battery monitor circuit:
2x LEDs (recommend two different colors)
1x 1K ohm resistor
1x 10K ohm resistor
1x 10K ohm multi-turn potentiometer
1x zener diode (5v

The Circuit:



Making the Sensor:

-Glue the TEC to a heatsink with a thermal adhesive
-Extend wires to a reasonable length
-Paint exposed TEC surface with matte black paint (ensure coat is even but not thick to prevent high powered lasers from burning the coating)
-Let dry



Calibration:

-Connect the circuit as shown in the diagram
-Set zero potentiometer so voltmeter reads 0
-Shine known laser on painted TEC surface
-Adjust gain potentiometer so reading in mV equals laser output power in mW
-Adjust R8 to set the low battery setpoint. You can use a dead battery or set a power supply to ~6v or so then adjust R8 until the low battery led illuminates
And then you're ready to test the output of your other lasers This meter should easily be able to handle a watt or more. Share your designs, discuss improvements and have fun

You can buy small TEC modules from Mouser and DigiKey for about $15

Good job MarioMaster.... this will be another DIY project for the LPF
community members...

As to the "cough" Wafer type Thermopile "cough"... it has been
shown that shining a Laser over different points of a "professional Radial
type sensor" results in different readings... (I can supply a drawing of the
result we obtained using the OPHIR 20C-A heads using a ~1.8 watt Laser)...
while the Wafer type Thermopile Sensor exibits the same readings anywhere
on it's surface...

Either way... you did a nice job on this DIY project...:gj:

[EDIT] spelling error corrected... Thanks for
pointing out that Huge spelling mistake...:gj:


Jerry

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

Well waffer isn't a word, wafer only has one F in it.

I cannot duplicate your results with the heads I have, I'm willing to bet the slight differences in power you experienced were simply the result of some of the light from the high powered diode not hitting the surface of the thermopile completely. I know during my testing of high powered diodes from Chris that it is difficult to fit the entire beam of a multimode diode that is properly heatsinked onto the thermopile surface.

hey does anyone have the PCB layout of this i really want to make one
and on the case i shall write MarioMaster

i also cant get the 5 kohm res and the 2kohm i can only get 1kohm and 10kohm
and what type of TEC do i need is this ok http://www.shop.customthermoelectri...5F618F3D2301D2943AEB39D4753C90EF.qscstrfrnt03 what sort of voltage max???
cheers

That's a pretty standard op-amp circuit that you can probably just breadboard. Besides the TEC, the most expensive part is probably the pot. You can use it for other stuff too: audio, pressure, whatever.

Bionic-Badger said:

That's a pretty standard op-amp circuit that you can probably just breadboard. Besides the TEC, the most expensive part is probably the pot. You can use it for other stuff too: audio, pressure, whatever.

Click to expand...

I agree BB... quite a common circuit found in most Data Sheet
applications...
One could use trimpots.... they only cost a few cents...

I believe the biggest cost will perhaps be the Calibrated LPM
to calibrate this circuit to usable output...

Jerry

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

Yup, a very simple circuit that is also very effective.

As per request here's a circuit board layout




Red is the top layer, blue is the bottom. The circuit board has a large ground plane to eliminate noise.

Have you actually built this DIY circuit....:thinking:
If so... I think the members would like to see
pics of your build....:yh:


Just a heads up... your PCB designations of R2 and R4
are reversed...



Jerry

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

Yes, although it was on a breadboard and it has since been dismantled.

My 5k pot doesnt have 4 pins??
cheers
and my lm358 has 6 pins ???? http://jaycar.com.au/productView.asp?ID=ZL3358&keywords=lm358&form=KEYWORD

LOL, no, your LM358 cannot have 6 pins.

95% of the 6-pins ICs are simply photocouplers, NOT op-amps.

have a look at the link. and how comr in the diagram the pot has 4 leads?
and instead of 1kohm can i use 1.5kohm and instead of 5kohm use 3.7kohm?
cheers

You should perhaps be asking those questions to MarioMaster
since he offered the circuit... You could PM him or even read the
LM358 Data Sheet...

Jerry

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

If you mean on the PCB, the trimmers don't have 4 leads, only 3 ..... this pads disposition is made automatically from some softwares, cause in the market there are trimmers that have the 3 leads inline, and others that have the 3 leads placed in a "triangular" disposition ..... the central pads are connected together.

About the link you posted, the LM358 datasheet shows clearly that the IC is an 8-pin DIP ..... where you see it with only 6 pins ?

ok so that is the one i want awesome
thanks