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"noisy" flashlamp

G

George B

Jan 1, 1970
0
I recently attached a phototransistor to an audio amplifier (simple 4
stage transistor) after reading about observing ("listening") to
fluctuations in the light levels in the sky with such a device. If the light
is collected through a telescope one can "listen" to the light reflected off
clouds and the horizon. My amplifier, although it didn't have a great enough
gain to hear anything in the dark, made what sounded like white noise when a
torch light was directed onto the phototransistor.
Can anyone explain this considering that the filament is fed directly from a
battery(supposedly purely DC)? Could the noise be arising from radio
interference or could it be due to the nature of the electrochemical action
within the battery?
Before anyone suggests that something may have been amiss with my amplifier
I can assure you it was correctly biased and was able to amplify the 50Hz
mains hum from the lighting within the house quite nicely (the noise
disappeared if the photodiode was covered or taken outside in the dark).
Incandescent lights produce a soft hum, whilst fluorescent lights produce a
very loud and harsh 50Hz beat.
If I have time I might observe the waveform produced from the flashlight
using an oscilloscope.
I plan on getting either some higher gain transistors or building an IC
based amp to get the required gain in order to listen to fluctuations in the
light levels in the night sky.

George
 
F

Frank Bemelman

Jan 1, 1970
0
George B said:
I recently attached a phototransistor to an audio amplifier (simple 4
stage transistor) after reading about observing ("listening") to
fluctuations in the light levels in the sky with such a device. If the light
is collected through a telescope one can "listen" to the light reflected off
clouds and the horizon. My amplifier, although it didn't have a great enough
gain to hear anything in the dark, made what sounded like white noise when a
torch light was directed onto the phototransistor.
Can anyone explain this considering that the filament is fed directly from a
battery(supposedly purely DC)? Could the noise be arising from radio
interference or could it be due to the nature of the electrochemical action
within the battery?
Before anyone suggests that something may have been amiss with my amplifier
I can assure you it was correctly biased and was able to amplify the 50Hz
mains hum from the lighting within the house quite nicely (the noise
disappeared if the photodiode was covered or taken outside in the dark).
Incandescent lights produce a soft hum, whilst fluorescent lights produce a
very loud and harsh 50Hz beat.
If I have time I might observe the waveform produced from the flashlight
using an oscilloscope.
I plan on getting either some higher gain transistors or building an IC
based amp to get the required gain in order to listen to fluctuations in the
light levels in the night sky.

I know nothing about that torch, but how does the sky sound like?
Do you have a website with some wav-files?
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that George B <[email protected]>
Can anyone explain this considering that the filament
is fed directly from a battery(supposedly purely DC)? Could the noise be
arising from radio interference or could it be due to the nature of the
electrochemical action within the battery?

No, you are effectively hearing the thermal noise of the hot filament.
 
F

Fred Bloggs

Jan 1, 1970
0
George said:
I recently attached a phototransistor to an audio amplifier (simple 4
stage transistor) after reading about observing ("listening") to
fluctuations in the light levels in the sky with such a device. If the light
is collected through a telescope one can "listen" to the light reflected off
clouds and the horizon. My amplifier, although it didn't have a great enough
gain to hear anything in the dark, made what sounded like white noise when a
torch light was directed onto the phototransistor.
Can anyone explain this considering that the filament is fed directly from a
battery(supposedly purely DC)? Could the noise be arising from radio
interference or could it be due to the nature of the electrochemical action
within the battery?
Before anyone suggests that something may have been amiss with my amplifier
I can assure you it was correctly biased and was able to amplify the 50Hz
mains hum from the lighting within the house quite nicely (the noise
disappeared if the photodiode was covered or taken outside in the dark).
Incandescent lights produce a soft hum, whilst fluorescent lights produce a
very loud and harsh 50Hz beat.
If I have time I might observe the waveform produced from the flashlight
using an oscilloscope.
I plan on getting either some higher gain transistors or building an IC
based amp to get the required gain in order to listen to fluctuations in the
light levels in the night sky.

George

The "torch" biased the photo-transistor on to some collector current Ic
and there is shot noise directly proportional to sqrt(Ic) associated
with that conduction. Your amplifier was amplifying that shot noise- it
is a white broadband noise. With Ic=0, at no light, there is no shot noise.
 
B

Bill Sloman

Jan 1, 1970
0
George B said:
I recently attached a phototransistor to an audio amplifier (simple 4
stage transistor) after reading about observing ("listening") to
fluctuations in the light levels in the sky with such a device. If the light
is collected through a telescope one can "listen" to the light reflected off
clouds and the horizon. My amplifier, although it didn't have a great enough
gain to hear anything in the dark, made what sounded like white noise when a
torch light was directed onto the phototransistor.
Can anyone explain this considering that the filament is fed directly from a
battery(supposedly purely DC)? Could the noise be arising from radio
interference or could it be due to the nature of the electrochemical action
within the battery?
Before anyone suggests that something may have been amiss with my amplifier
I can assure you it was correctly biased and was able to amplify the 50Hz
mains hum from the lighting within the house quite nicely (the noise
disappeared if the photodiode was covered or taken outside in the dark).
Incandescent lights produce a soft hum, whilst fluorescent lights produce a
very loud and harsh 50Hz beat.
If I have time I might observe the waveform produced from the flashlight
using an oscilloscope.
I plan on getting either some higher gain transistors or building an IC
based amp to get the required gain in order to listen to fluctuations in the
light levels in the night sky.

How much torch-light was actually hitting the photo-transistor?

In other words, what was the level of the photon-generated base
current in the transistor?

You may well have been hearing the shot noise on this current.

The Art of Electronics gives an equation for shot noise in section
7.11 (page 432 in my copy).

I(noise)rms = (2.q.Idc.B)^0.5

where q is the the charge on the electron 1.6E-19 coulomb
Idc is the (base) current in Amps
and B is the bandwidth in Hertz.

Hope this helps.
 
T

Tim Auton

Jan 1, 1970
0
George B said:
I recently attached a phototransistor to an audio amplifier (simple 4
stage transistor) after reading about observing ("listening") to
fluctuations in the light levels in the sky with such a device.

If you really want to listen to light have a look here:

http://www.seeingwithsound.com/


Tim
 
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