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Old 17th January 2009, 09:16 PM   #1
bigwill is offline bigwill  United Kingdom
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Lightbulb Can your amp do this?

Something I'm sure everyone has tried at some point. Messing with LEDs is fun

http://www.youtube.com/watch?v=8xcNwa8AJAA
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Old 17th January 2009, 10:46 PM   #2
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Bit of a waste of what looks like a Radord STA15. I tried a similar experiment with a light dependent resistor and xenon tube in my Physics class about 35 years ago, also tried the same thing but using infrared devices and managed about 20m.
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Old 17th January 2009, 11:05 PM   #3
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by theoldtrout
Bit of a waste of what looks like a Radord STA15. I tried a similar experiment with a light dependent resistor and xenon tube in my Physics class about 35 years ago, also tried the same thing but using infrared devices and managed about 20m.
It is indeed an STA15. I fix them The one in the video doesn't belong to me (although I do own one)

I am to fit brand new boards in it for someone.
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Old 17th January 2009, 11:21 PM   #4
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I have been using LED's since they were introduced. You know, you put current through them and light comes out. A varying current creates a varying light intensity, thus allowing modulated light.

I never knew until just a few days ago that they work the other way too. I was poking a scope probe around in a digital circuit which was running off of a regulated bench power supply and wondering where the 120 Hz ripple was coming from. The circuit contained 32 LED's connected to the outputs of a PIC chip. I noticed that the ripple changed as I moved my hand around over the board, and dissappeared when I turned off the bench light. A piece of cardboard casting a shadow over the LED's stopped the ripple.

Further experiments determined that the LED's do indead generate several millivolts when exposed to light. The little surface mount ones on my PIC board work the best.
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Old 17th January 2009, 11:29 PM   #5
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by tubelab.com

Further experiments determined that the LED's do indead generate several millivolts when exposed to light. The little surface mount ones on my PIC board work the best.
Playing around, I managed to get 2.2 whole volts out of a blue LED! Given enough light of the right wavelength the voltage you can get is surprising.

The interesting thing is that in that video, the ONLY thing on the amplifier's input is an LED. The cathode goes to ground and the anode goes to the input!
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Old 18th January 2009, 05:19 AM   #6
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"Playing around, I managed to get 2.2 whole volts out of a blue LED! Given enough light of the right wavelength the voltage you can get is surprising."

Hmmmm... and all those tube amps with LED cathode bias and LED biased CCS circuits. Hummmmmmmm......

Don
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Old 18th January 2009, 06:05 AM   #7
rdf is offline rdf  Canada
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Dab-o-black paint.
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Old 18th January 2009, 07:50 AM   #8
AndrewL is offline AndrewL  United Kingdom
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Quote:
Originally posted by rdf
Dab-o-black paint.

Precisely! Humbrol enamel should do it!
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Old 18th January 2009, 09:32 AM   #9
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Somewhat off topic, but where do you get new boards for an STA15? I could do with some!
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Old 18th January 2009, 10:07 AM   #10
Jaap is offline Jaap  Netherlands
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Quote:
Originally posted by theoldtrout
Bit of a waste of what looks like a Radord STA15. I tried a similar experiment with a light dependent resistor and xenon tube in my Physics class about 35 years ago, also tried the same thing but using infrared devices and managed about 20m.

With the right optical devices longer distances are attainable. I heard that secret agents used this kind of devices fifty years ago to communicate over the wall in Berlin. A small beam is difficult to intercept.

Another nice story is that the CIA digged a tunnel under the wall and tapped an important communicationcable from the ruskies. They used a lot of tube gear to do this. During the winter the heat of the tubes under the ground let the snow melt on the surface. That is how the Russians discovered and they fed the lines with fake information.

JH2312 (ex CIA)
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