Photo Cell responce time

[Chris W]

I was wondering if someone could point me to a source for a photo cell
that has a fast response time. I'm not sure where to find data sheets
on them.

Here is what I want to do. I want to hook the photo cell up to a
friends fluke scope meter and use it to measure duration and number of
pre flashes from my digital camera flash. I know a flash duration can
be less than 1/40,000 or a second so obviously the photo cell needs to
be faster than that.

--
Chris W

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deserve neither liberty nor safety."
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[Tim Wescott]

Chris W wrote:

I was wondering if someone could point me to a source for a photo cell
that has a fast response time. I'm not sure where to find data sheets
on them.
Here is what I want to do. I want to hook the photo cell up to a
friends fluke scope meter and use it to measure duration and number of
pre flashes from my digital camera flash. I know a flash duration can
be less than 1/40,000 or a second so obviously the photo cell needs to
be faster than that.

You want a PIN photodiode, not a photocell, and you want it in a fairly
fast circuit. Since the flashes are intense and well separated you
don't actually have to catch the flash -- just the fact that it happened.

You should be able to do some creative web searching and find a circuit.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Chris W]

Tim Wescott wrote:

Chris W wrote:

I was wondering if someone could point me to a source for a photo
cell that has a fast response time. I'm not sure where to find data
sheets on them.
Here is what I want to do. I want to hook the photo cell up to a
friends fluke scope meter and use it to measure duration and number
of pre flashes from my digital camera flash. I know a flash duration
can be less than 1/40,000 or a second so obviously the photo cell
needs to be faster than that.


You want a PIN photodiode, not a photocell, and you want it in a
fairly fast circuit. Since the flashes are intense and well separated
you don't actually have to catch the flash -- just the fact that it
happened.

Why not a photocell? Why do I need a fast circuit, or any circuit?

Won't the scope meter be able to measure changes in an applied voltage
drop with the light flashes? I'm also not sure what you mean when you
say "you don't actually have to catch the flash". I want to know how
many times it flashes and how long each flash is as well as the time
between each flash.


--
Chris W

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

 

[Anthony Fremont]

"Chris W" <1qazse4@cox.net> wrote in message
news:jCcJd.1096$al1.805@lakeread07...

Tim Wescott wrote:

Chris W wrote:

I was wondering if someone could point me to a source for a photo
cell that has a fast response time. I'm not sure where to find
data
sheets on them.
Here is what I want to do. I want to hook the photo cell up to a
friends fluke scope meter and use it to measure duration and number
of pre flashes from my digital camera flash. I know a flash
duration
can be less than 1/40,000 or a second so obviously the photo cell
needs to be faster than that.


You want a PIN photodiode, not a photocell, and you want it in a
fairly fast circuit. Since the flashes are intense and well
separated
you don't actually have to catch the flash -- just the fact that it
happened.

Why not a photocell? Why do I need a fast circuit, or any circuit?
Won't the scope meter be able to measure changes in an applied voltage
drop with the light flashes? I'm also not sure what you mean when
you
say "you don't actually have to catch the flash". I want to know how
many times it flashes and how long each flash is as well as the time
between each flash.

I think the problem is that the photocell is too slow in it's reaction
time. You are wanting to see pulses 25uS wide spaced maybe 100mS apart.
Rise and fall times of CdS photocells is measured in 10's of mS. IOW,
they are just not capable of giving you any meaningful data.

You said that you wanted to measure the pulse width, so you need
something fast (in terms of rise and fall time response). If Tim told
me that I needed a pin photodiode, then that's what I'd be researching.
;-)

 

Here is what I want to do. I want to hook the photo cell up to a
friends fluke scope meter and use it to measure duration and number
of
pre flashes from my digital camera flash. I know a flash duration
can
be less than 1/40,000 or a second so obviously the photo cell needs
to

I think you'll have less trouble opening the circuit and
reverse-engineering it. You'll probably find an output from the micro
turns on to dump the flash cap into the lamp. Looking at the waveform
there - where it's easy to measure - will tell you definitively how
long it is between flashes. It will also set an upper boundary on how
long the flash is.

 

[Tim Wescott]

Chris W wrote:

Tim Wescott wrote:

Chris W wrote:

I was wondering if someone could point me to a source for a photo
cell that has a fast response time. I'm not sure where to find data
sheets on them.
Here is what I want to do. I want to hook the photo cell up to a
friends fluke scope meter and use it to measure duration and number
of pre flashes from my digital camera flash. I know a flash duration
can be less than 1/40,000 or a second so obviously the photo cell
needs to be faster than that.



You want a PIN photodiode, not a photocell, and you want it in a
fairly fast circuit. Since the flashes are intense and well separated
you don't actually have to catch the flash -- just the fact that it
happened.

Why not a photocell?

Anthony's response -- because a photocell is s l o w.

Why do I need a fast circuit

because the flashes happen sufficiently close together that you want to
catch them. If you can't catch them you can't count them.

or any circuit? Won't the scope meter be able to measure changes in
an applied voltage drop with the light flashes?

Because any sensor hooked up to a scopemeter is a "circuit". In the
case of a PIN photodiode you need to provide power in the form of
back-biasing. You may be able to get away with a battery and a 1k-ohm
resistor and measure the drop -- but that's a circuit.

I'm also not sure what you mean when you
say "you don't actually have to catch the flash".

Sorry. If you only care about the time between flashes you don't have
to resolve the width of the flash -- you only need to detect that a
flash happened sufficiently fast to discriminate it from the next one.
Given that you know (or suspect) that it's 100ms between flashes this
helps to define "sufficiently fast".

I want to know how
many times it flashes and how long each flash is as well as the time
between each flash.


If you want to know how long each flash is then you need a fast circuit.

25 microseconds with a largely capacitive load such as you see with a
photodiode moves you right out of the cheesy 1k-ohm resistor and battery
circuit, and into the world of fast op-amps and/or RF amplifiers -- and
then you have to verify that the duration you see is the light pulse and
not an artifact!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Chris W]

Tim Wescott wrote:

Why not a photocell?


Anthony's response -- because a photocell is s l o w.

Why do I need a fast circuit


because the flashes happen sufficiently close together that you want
to catch them. If you can't catch them you can't count them.

or any circuit? Won't the scope meter be able to measure changes in
an applied voltage drop with the light flashes?


Because any sensor hooked up to a scopemeter is a "circuit".

good point, I guess I didn't think of it that way.

In the case of a PIN photodiode you need to provide power in the form
of back-biasing. You may be able to get away with a battery and a
1k-ohm resistor and measure the drop -- but that's a circuit.

I'm also not sure what you mean when you say "you don't actually have
to catch the flash".


Sorry. If you only care about the time between flashes you don't have
to resolve the width of the flash -- you only need to detect that a
flash happened sufficiently fast to discriminate it from the next one.
Given that you know (or suspect) that it's 100ms between flashes this
helps to define "sufficiently fast".

I want to know how many times it flashes and how long each flash is
as well as the time between each flash.


If you want to know how long each flash is then you need a fast
circuit. 25 microseconds with a largely capacitive load such as you
see with a photodiode moves you right out of the cheesy 1k-ohm
resistor and battery circuit, and into the world of fast op-amps
and/or RF amplifiers -- and then you have to verify that the duration
you see is the light pulse and not an artifact!

Let me see if I under stand this, the capacitance of the photo diode is
going to prevent accurate readings at high frequencies? And a fast
op-amp or RF amplifier circuit can compensate for that some how?

I found this thing on digikey.com
http://rocky.digikey.com/WebLib/Optek/Web%20Data/OPF480.pdf

it sounds like it designed for use with fiber optic networking. I would
think that would make it pretty fast, so I guess I will order one and
see what just it with a few volts and a resister will do to start out.
Also I see that it has a third lead labeled "case" is that just for a
ground to suppress noise or something?

I didn't mean my last post to sound like I was doubting anyone, I just
was trying to get more information.

--
Chris W

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

 

Chris W wrote:

I found this thing on digikey.com
http://rocky.digikey.com/WebLib/Optek/Web%20Data/OPF480.pdf

it sounds like it designed for use with fiber optic networking. I
would think that would make it pretty fast,

Chris,

Looks like that device is about 1000 times faster than your
needs. Not that there's anything wrong with that, but it
may have an extremely small sensor area (not specified,
oddly) and therefore a low signal strength from your flash.
You'll find out when it arrives!

Digikey has many cheap photodiodes, between $1 and $5.
If the one you ordered turns out not to work, try getting
several different ones with different detector areas (typically
between 1 mm^2 and 30 mm^2, for example the ones from
Photonic Detectors, Inc. on p. 1436 of catalog no. T051 that
recently came out.)

Mark