Tuesday, July 1, 2014

A cheap and simple Laser Power Meter LPM for small power source

A Laser Power Meter (LPM) is an instrument that measure the energy output of laser beams.
This project is my way to implement the MarioMaster LPM meter. You can find further information about this design in laserpointerforums.com the post here: http://laserpointerforums.com/f42/diy-thermal-lpm-under-50-a-51129.html

Warning! Laser diodes drive are emitting visible and invisible laser radiation and they are extremely dangerous! Their light can permanently damage the eyes. You must never look into the working diode even without the lens or point it on a reflective surface. Laser beam can cause burns or fire. This is usually a Class IIIb laser. Everything you do at your own risk.



This type of meter uses a ThermoElectric Cooling module (TEC) to measure the power of a laser. The TEC will absorb the laser light, and transform the heat generated by the laser beam to an electrical signal.
An operational amplifier is used then to amplify the signal and ouput it to a volt meter.
Voltage meter will display the power in W unit of the laser beam you are testing.
The TEC takes a little amount of time to heat, so wait until your reading became stable.


This type of meter is simple and cheap to build.
It can measure laser power up to 2W, with an accurancy of +-10mW.


You have to use a "small" (< 20mm x 20mm) TEC, for my circuit i've used a 6.6mm x 8.3mm TEC, Laird OT18,18,F2A 0608 HWS20mm.
TEC should be glued to an heat sink, and then paint with black to absorb laser beam of any frequency.
In my project the selected opamp is a common LM358. Only one channel of the opamp is used, the other is connected just to remove noise.
As a voltage meter use a any DC 0..5V Volt Meter with 3 digit precision at least.
As a power source, just use a 9v battery or any DC source.
Trimmer are used to calibrate the meter, multiturn trimmer are preferred.


Once you have build the circuit you have to calibrate it.
Power on your unit, and use the ZERO trimmer to adjust the volt meter to zero. Note that this step can be skipped if you use voltage meter with auto zero calibrating mode.
Heat the TEC with a laser with a know power, let's say 100mW. Wait until your reading are stable, then adjust the voltage meter output using the GAIN trimmer to fit the laser power source.

I've used a 100mW red calibrated laser to calibrate this meter.
Then i've tested it with a 200mW laser, and a 20mW (the one of the video) without problems.



Notes
  • read risk disclaimer
  • excuse my bad english

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