How to build a DIY laser power meter

In summary: So you need a DVM, some LEDs, and a resistor.You can connect the DVM across the photodiode and shine some light on the LED -- you get a voltage from the photocurrent flowing through the DVM's measurement resistance.Then better is to learn how to make a current-to-voltage converter circuit with an opamp and a reverse-biased photodiode.
  • #1
257
3
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.
 
Physics news on Phys.org
  • #2
David lopez said:
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?
 
  • #3
berkeman said:
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:
berkeman said:
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
 
  • #4
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.
 
  • #5
I have worked with operational amplifiers before.
 
  • #6
David lopez said:
The measured resistance Didn't change.
Measure junction voltage with the DVM, not resistance.
 
  • #7
David lopez said:
I have worked with operational amplifiers before.
This is the classic reverse-biased current-to-voltage converter circuit that is used with photodiodes to measure the photocurrent:

https://www.electronics-tutorial.net/wp-content/uploads/2015/09/I_to_V3.png
244272


The reverse bias on the photodiode makes it more linear, and improves the bandwidth of the detection (like if you were using it in a communication link as a receiver).
 
  • Like
Likes dlgoff
  • #8
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:
  • Like
Likes dlgoff and berkeman
  • #9
berkeman said:
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.
 
  • Like
Likes berkeman

Suggested for: How to build a DIY laser power meter

Replies
23
Views
2K
Replies
28
Views
2K
Replies
13
Views
2K
Replies
2
Views
1K
Replies
10
Views
2K
Replies
5
Views
1K
Replies
4
Views
739
Replies
19
Views
338
Replies
31
Views
2K
Replies
7
Views
2K
Back
Top