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Investigating Photo transistors

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Posted: Apr 17th, 2011 01:12 AM

mikesndbs
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Hi

After reading here about using phototransistors for EVP and seeing the article here:

http://archive.atransc.org/aaevp_news/2007_aaevp_news/spring_2007.htm

I decided to give it a go.

I have the exact same recorder and so have copied exactly what the author did.

One discovery I have made is that the set up still can record audio! now, I am not sure if this is due to the internal microphone not being fully out of the circuit or if the phototransistor actually picks up audio.

Strange as it might seem I think it is the latter!

Here is a test I just made so you can judge.

Starts off with me speaking into the set up.

http://www.box.net/shared/hnrhhqj7oa

Then I hold the sensor in sunlight, compact fluorescent light, ambient room daylight, torch light.

Interesting that sun light seems to produce a more soft sound!

I shall be making some tests under differing light sources soon.

Posted: Apr 17th, 2011 07:20 AM

EVPDave
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Good to see someone finally experimenting in this area. I wrote that article as a follow-up article to one about the optical flame microphone.

I have two recorders I modified by removing the internal microphones, completely replacing the element with a photodiode. Under certain lighting conditions some photodiodes can become quite microphonic, an interesting internal effect.

Of the various devices I tested such as phototube, photomultiplier tube and photodiode, a simple solar cell arrangement proves most sensitive. Here is a liink to a demo video I made a few years back demonstrating the optical flame microphone.

http://www.youtube.com/watch?v=j5-WOuWPHOY

Looking forward to reading of your future experiments.

Dave

P.S. Link to the very first optical microphone article I wrote from 2006, using the very sensitive OPT101 . http://atransc.org/techniques/techniques_optical_microphone.htm

Posted: Apr 17th, 2011 09:00 AM

clockdryve
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Hmmm, seems as though you found another link to the "phototransistor" description. From the same site that I sent you, but a larger page with other information also...never noticed that page though

To decide if the photodiode is actually picking up your voice you could just connect a stereo/mono type adapter or simular (a broken headphone's connector or something) to the microphone input on your recorder...and see if it still "hears" you. I have been told recently that SOME recorders do not completely isolate the internal microphone when using an external.

Last edited on Apr 17th, 2011 09:03 AM by clockdryve

Posted: Apr 17th, 2011 09:11 AM

clockdryve
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As noisy as these "light" microphones are...that is why I used mine "with success" right away...in a darkened room. Then it is near silent (except for internal recorder noise) until an EVP is captured. The message I got with mine said "It works Jim" (and then a female voice said "I told you it would"). Good luck with your new "microphone" Mike. It sounds like it is working correctly

Posted: Apr 17th, 2011 09:18 AM

clockdryve
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I have seen that video before (over a year ago) Nice!! Thanks for the article at "aaevp" EVPDAVE.

http://atransc.org/techniques/techniques_optical_microphone2.htm

Last edited on Apr 17th, 2011 09:20 AM by clockdryve

Posted: Apr 17th, 2011 11:53 AM

mikesndbs
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How is the output from the solar cells connected to the recrder? do you use a capacitor to isolate the dc from the recoder?

Posted: Apr 17th, 2011 12:08 PM

mikesndbs
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Update! OK you know me I can't wait LOL

I love the concept of solar power and have many cells employed on battery charging etc.
I had a large spare panel that used to drive the motor of garden water feature.
I isolated the recorder with a 100nF capacitor and connected up to my computers mic socket.

WOW!

A strange new world of sounds opened up, the screen of my PC, flashing a torch and the light from my computer mouse.
I had no idea that sound could come from light so simply.

Be assured I shall be experimenting with this as well!

Posted: Apr 17th, 2011 12:25 PM

EVPDave
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mikesndbs wrote: Update! OK you know me I can't wait LOL

I love the concept of solar power and have many cells employed on battery charging etc.
I had a large spare panel that used to drive the motor of garden water feature.
I isolated the recorder with a 100nF capacitor and connected up to my computers mic socket.

WOW!

A strange new world of sounds opened up, the screen of my PC, flashing a torch and the light from my computer mouse.
I had no idea that sound could come from light so simply.

Be assured I shall be experimenting with this as well!

I looked at my setup and I had used a 0.1uf (100nF) cap for isolation. On the latest version I also use an IR filter of 950nm. A large panel has many possibilities :biggrin:

:biggrin:

.

Dave

Posted: Apr 17th, 2011 12:29 PM

mikesndbs
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LOL so we both chose 100n then :-)

Well, I have lots to test, have you any clear EVP by this method?

Posted: Apr 17th, 2011 01:19 PM

EVPDave
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mikesndbs wrote: LOL so we both chose 100n then :-)

Well, I have lots to test, have you any clear EVP by this method?

Yes, that value works well to reduce some of the low frequency components. I did post just a fraction of my results using the optical flame microphone on this site. Just use the search function to find the examples using the terms 'optical microphone'.

There are countless ways to utilize light in experiments. Think of light or even heat as a carrier wave. Think of the frequency of the carrier as a specific wavelength or color . A red laser pen for example has a narrow bandwidth and specific frequency (vibration). White light has a wide bandwidth. A simple TV remote control outputs a specific frequency of IR light. The IR light is pulsed (modulated) at a specific rate to perform a function depending on the key that is pressed.

Have fun,

Dave

Posted: Apr 18th, 2011 12:08 PM

mikesndbs
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Hi guys, I have been thinking about this all day and did some experiments this evening!
I was so surprised at the results that I have made a short video so I can share with you all.
Please see my new thread 'LED Spiricom'

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