coding
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Joined: Sun Jul 20, 2014 7:23 am

Connecting a photo diode and measuring light's frequencies?

Wed Jun 15, 2016 4:54 pm

I think maybe it could be possible to use a Raspberry Pi and a photo diode to measure the frequencies of light sources. To hold the photo diode near to a incandescent bulb and then get the value 100 Hz or 120 Hz (depending on country).
It is probably very easy for an experienced GPIO user, but I don't know where to start. Do you think this is possible? I also would like to hold the diode near a computer monitor or smartphone and then see the light's flicker frequency which can be up to 10.000 Hz or even more.
Some PC monitor review sites test flicker rates. I wonder if it can be done with a Raspberry Pi.

drgeoff
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Joined: Wed Jan 25, 2012 6:39 pm

Re: Connecting a photo diode and measuring light's frequenci

Wed Jun 15, 2016 5:55 pm

I expect it is possible but you'd need an AC coupled amplifier and limiter before feeding into a RPi GPIO.

(Note that you are not measuring the light's frequency but rather the modulation frequency.)

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bensimmo
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Location: East Yorkshire

Re: Connecting a photo diode and measuring light's frequenci

Thu Jun 16, 2016 6:39 am

I'm guessing you mean the light 'flicker' frequency and not the actual frequency of the light wave.

You'll want to capture/analyse the input. Give it a go and see what you can 'see'.

coding
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Joined: Sun Jul 20, 2014 7:23 am

Re: Connecting a photo diode and measuring light's frequenci

Thu Jun 16, 2016 9:05 am

I now connected an IR sensor as shown here: http://learn.adafruit.com/assets/5909

Where do I start from here? Can you give me any links to tutorials on how to save and analyze the incoming electric current?

drgeoff
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Joined: Wed Jan 25, 2012 6:39 pm

Re: Connecting a photo diode and measuring light's frequenci

Thu Jun 16, 2016 9:45 am

coding wrote:I now connected an IR sensor as shown here: http://learn.adafruit.com/assets/5909

Where do I start from here? Can you give me any links to tutorials on how to save and analyze the incoming electric current?
An IR sensor like the one you link to is not suitable for what you want said you want to do. It is specially "tuned" to:

1. Infrared as emitted by remote controls, not visible light. Yes, the photojunction will have some response to visible light but see #2 next.

2. it has additional circuitry for demodulating the carrier frequency of about 38 kHz that IR remotes use.

You will not see any useful output that corresponds to the 100 or 120 Hz flicker from a light or flicker from an LCD backlight or the fresh rate of a CRT.

Think about it. IR remote control receivers are expected to work receiving signals from IR remotes in places illuminated by such sources of visible light. That is why they are designed not to respond to what you are trying to measure.

coding
Posts: 40
Joined: Sun Jul 20, 2014 7:23 am

Re: Connecting a photo diode and measuring light's frequenci

Thu Jun 16, 2016 10:15 am

I understand, thanks. I think I have a BPW34 photo diode lying around here somewhere. But I believe I can't directly connect it as simple as the IR receiver. I should probably give up on this project as I have no electrical knowledge. I read about a 10K resistor I'd probably have to use, maybe soldering skills are needed, circuit boards and other stuff. On top of that, maybe python programming skills. Probably too much to learn in a reasonable time.

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