Electrical How to build a DIY laser power meter

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How do I build diy laser power meter. I tried shining 5 milliwatt laser on photodiode. Photodiode is connected to led. Led never turned on.
 

berkeman

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How do I build diy laser power meter. I tried shining 5 milliwatt laser on photodiode. Photodiode is connected to led. Led never turned on.
Do you have a DVM? Connect the DVM across the photodiode and see what you get for an output voltage (it will max out at a couple of volts).

Then better is to learn how to make a current-to-voltage converter circuit with an opamp and a reverse-biased photodiode. Have you worked with opamps or other circuits yet?
 

berkeman

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Do you have a DVM? Connect the DVM across the photodiode and see what you get for an output voltage (it will max out at a couple of volts).
See this reply from a different thread, for example:
Not true. Hook an LED to a DVM and shine some light on the LED -- you get a voltage from the photocurrent flowing through the DVM's measurement resistance. :smile:

There's an important reason to use at least a few volts of reverse bias across the photodiode -- it has to do with bandwidth. Do a little Google searching to see if you can find a good explanation of that, and post it here. :smile:

LED under a lamp with DVM measuring the voltage (Red positive lead on the LED anode):

View attachment 240669
 
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Actually I tried connecting the photodiode to a digital multimeter and shining a 5 milliwatts laser pointer onto the photodiode. The measured resistance Didn't change.
 
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I have worked with operational amplifiers before.
 

berkeman

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Tom.G

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The usual light meter / Laser power meter, is a photovoltaic cell, such a Selenium photo cell or a Silicon photo cell, connected to a load resistor and a moving-coil meter.

Since the photovoltaic cell current output is proportional to received power, the load resistor is connected across the cell and the meter reads the voltage.

Here is an old photographic exposure meter based on that principle.
ExposureMeter.png
ExposureMeter.png
Some (all?) of the exposure meters just use the resistance of the meter as the load, avoiding the additional load resistor.

On simple Laser power meters you change ranges by switching in different load resistors.

The main advantages of this approach are no battery or electronics are needed, very low cost, and the cells have enough active area that you don't need any optics to focus the Laser beam to a tiny photo diode.

A DIY meter will of course require calibration, that's why most folks buy a digital power meter. Google showed one for US$35 when I searched for 'camera exposure meter.'

Cheers,
Tom

Edit: added photo, reference to Silicon cell

p.s. the circular dial on the exposeure meter is just a circular slide rule to compute camera aperture and shutter speed, based on light level and film speed. It has no electrical connection to the metering function.
 
Last edited:

Henryk

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Measure junction voltage with the DVM, not resistance
Voltage is not linear with incident intensity. It is much better to measure the photocurrent.
You can add reverse bias with a single battery.
 

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