Cheap DIY Laser Tester
I have a laser engraver/cutter with a supposed 3W diode laser. I always want to test things and have been looking into determining how to measure the output power of a laser. Not only is it useful for knowing how inflated the ratings are, but I could also check colored acrylic and safety glasses for attenuation.
After a bunch of research I landed on using heat as the method and went as foolproof as possible with the design.
I will be pointing the laser at something while checking the temperature and using a known measurement we can determine how much power is being transfered.
The thing I will be heating is a piece of copper that has been painted black such that it has more than 98% of light absorbtion.
The known measurement will be a power resistor stuck to the back of it.
The method of measuring will be an NTC also stuck to the back.
Materials
- Seeeduino XIAO SAMD21 microcontroller
- 10kΩ NTC thermistor
- 1/2" copper pipe coupling
- SSD1306 128×64 OLED display
- 100Ω resistor
- 2x 10kΩ resistors
- 12 V DC power brick
- 2.1 mm barrel jack
- 5V buck converter
- Foot pedal with internal momentary switch
- PCB
- JST headers
- 5W+ 5Ω - 25Ω power resistor
- Thermal tape
- Flat black paint
Tools and Equipment
- Soldering iron
- Multimeter
- Typical stuff like hammer, pliers, paintbrush
- Variable power supply with constant voltage setting
Log
Cut pipe coupling in half lengthwise with dremel and pull apart into a rectangle, then bang it with a hammer until flat. Sand it down and clean it well with dishwashing detergent and acetone and deionized water.
Dry it and paint it with ‘Darth Black’ paint. Four light coats. Notice how the light gets sucked into it.
Adhere power resistor and NTC to the back of it with thermal tape. Thermal tape is a giant pain in the ass to remove the liner from. I hate it.
Here is a cheap foot pedal with a momentary switch inside that I will use for a deadman’s switch.
Built the PCB using project board and flashed the measuring sketch to the Xiao. I picked the Xioa SAMD21 because it is tiny, it is dirt cheap while being decent quality, and it has a 12bit ADC. I used a buck converted to knock the 12VDC input to 5VDC for the Xiao input. The laser can be controlled either by a relay triggered by a jellybean transistor or by PWM if it is capable. Refer to wiring diagram.
Calibrate by powering the resistor with a fixed voltage until it is steady and do some math with the current and voltage and resistance we can find the thermal constant. Once that is entered into the sketch we upload it and plug in the laser and point it at the slug.
Press the pedal and watch the numbers go up.
Settled at ~2W for my 3W laser. Actually not that bad.
Github repo contains arduino sketch and guides for building it yourself.