Photo diode response time varies drastically with LED.

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#1
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I used a photo diode with peak sensitivity to wavelength of 900nm to detect flash light.

First i tried an LED with wavelength 450nm and it required a minimum of 50us LED on time to trigger the circuit.

Now with an apple ipod, it took ~7ms.

I have got not idea like what kind of led is used in an ipod or its internal led control circuitry.

I am aware of the fact that the intensity & wavelength of LED used plays a key role in the detection.

 

If anybody could shed more light on this, it could help understand and provide a sensible explanation to all these stuffs.

 

Thanks & Regards,

Hari

Silly beans are always silly
:)

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450 nm is blue, right? The photodiode is sensitive to the IR region, and probably also has good sensitivity for "hot" "warm colors like red, orange, etc. which have frequencies close to infrared.

For blue, the sensitivity will be much degraded.

I suppose the ipod LED is white, therefore it emits a range of frequencies, including red, so the photodiode is able do detect it fine.

Last Edited: Thu. Mar 29, 2018 - 08:39 AM
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an actual camera flash LED is designed to provide somewhat even illumination over a very large area (the whole camera field of view, say 45 degrees or so.   If you used a typical high-brightness LED for your testing, it was probably considerably more directional, and thus "dimmer"...

 

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Hi El Tangas,

Actually you got it wrong. Flash light from ipod took longer time to detect like 7 milliseconds.

On the other hand, the other LED used took only around 50 microseconds.

 

Regards,

Hari

Silly beans are always silly
:)

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Post a datasheet for the photodiode for starters.

When you say flash light from an iPod are you referring to the LED flashlight function? If yes that's white light and would expect the photodiode to be much slower responding.

Jim

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Hi Bill,

 

This really makes sense. But i wonder if its enough to increase sensing time from 50uSec to 7mSec.

I'll run some tests and will get back with some waveform. Lets see if we could figure out something it.

 

Thanks & regards,

hari

Silly beans are always silly
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Yup, you are right I was expecting the opposite of what you observed. So I guess the wavelength is not the important factor here.

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hi jim,

 

Here is the photo diode used.

https://www.vishay.com/docs/8329...

 

I wonder if its really normal to see such long delay to detect an LED turn on?

Could there be something to do with the ipod led control circuit?(just my speculation)

 

Regards,

Hari

Silly beans are always silly
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I think we need to see your circuit. I find it hard to imagine the diode itself would have such a delayed response.
 

Four legs good, two legs bad, three legs stable.

Last Edited: Thu. Mar 29, 2018 - 12:53 PM
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How are you activating the IPOD LED?

Could there be a delay between the activation and the actual turn-on of the LED?

 

What I think I am asking is, what defines the start and stop points of the 50us and 7ms intervals?

David (aka frog_jr)

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I'm with Frog_Jr on this.  I would not be surprised at all to find the iPod has a gentle LED ramp-up time, while your other test diode was (nearly) instantly switched to full power.

 

You'll never see the difference with your own eyes - This sort of timing issue is what oscilloscopes are made for.*  S.

 

* Try a photoresistor in a divider network, that'll show you ramp-up (and down!) time on a 'scope.

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I also think there is something missing here...distance between emitter and detector for example.

Also, when the led flashlight on a smartphone or iPod is turned on there is indeed a ramp up as the led has a substantial surge if it turns on full power instantly.
As already noted you can use your scope to watch this happen.

My guess is that the circuit has a comparator of sorts and the iPod LEDs slow turn on causes the delay you are seeing as it takes 7ms for the level to hit the comparator threshold

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Why do you blame your IR diode?

Are you sure you are not measuring the response time of your  LED?

It might be far fetched, but a lot of different technologies are used in LED's.

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White leds use a white phosphor excited by a uv led. the phosphor is much slower than the led, but not that slow.

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White leds use a white phosphor excited by a uv led.

Some.  Some use a blue LED to excite a yellowish phosphor.  There are >>many<< LED/phosphor combinations to achieve specific colour temperatures or CRI.

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Well, according to this: http://www.bostonati.com/product... , cerium doped YAG, a common phosphor that is used in some white LEDs, has a decay time of 70ns.

I think it's unlikely it will be responsible for the delay. Besides, some of the blue from the LED goes right through the phosphor coating without any delay.

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The rate of change of the diode output depends on the current generated by the diode. It is after all, the simple I = C dV/dt. The current, I, depends on two factors: (1) the wavelength/color distribution of the light falling on the diode surface and (2) the intensity AT the diode surface. Intensity includes source brightness, distance from source to detector, and orientation of the detector sensitive surface with respect to the light source.

 

Once you have ALL of these factors known and under control, the reason(s) for differences in response speed should be obvious.

 

Jim

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Whatever happened to LASCR's, anyway?  (Light Activated Silicon Controller Rectifier.)  These used to be available from most of the hobbyist electronics dealers, and there was a two-component slave-flash trigger that you could build (and was widely published) that was sort-of neat.   I haven't seen one in ages.  (come to think of it, I have no idea what these were ever used for, other than slave flash triggers...)

 

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westfw wrote:
I have no idea what these were ever used for, other than slave flash triggers
Optical triggering of thyristors  has been used with the real high power semiconductor stuf. Think power station here, not electric car. 100's of kV where whole stacks of high voltage thyristors are put in series to reach high enough breakdown voltages. The advantage of optical triggering with long glass fibres is obvious here.

It could be that this is old technology, maybe IGBT's are taking over in that area and I don't know if those can be optically triggered.

 

Some saying goes: "When you hear hooves, think horses, not unicorns".

When response times change drastically by changing the led, I suspect the led (or led incompatibility with the photo diode).

Or something even simpler, it might be an alignment error, where most of the led's light misses the photo diode.

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Paul's last paragraph is the same as what I wrote in #17.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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No unicorns in #17.angel

 

Edit / Addition:

Different human's brains work in different ways. Different wordings can help a lot if the "other" explanation fits the target's brain better.

I had to re-read and think about #17 to see the similarities...

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Last Edited: Sat. Mar 31, 2018 - 04:24 PM
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True!

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Jim, you are right. The detector circuit is pretty much a comparator circuit & as you suggest, ipod flash light circuit might be the root to this problem.

Hari

Silly beans are always silly
:)

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No, that is not what I suggested.

 

I suggested that the light sensor is a current source and the "strength" of the current varies with intensity of the light reaching the sensor and color of the light source. Blue light will generate a LOT less current because the sensitivity of silicon to blue is MUCH lower than it is to red. Other factors include the brightness of the source, itself, the separation between the source and the detector, relative orientation of the detector with respect to the source. ALL of these factors effect the response speed because a higher current produces a faster detector output change. 

 

Specifically, with respect to your comment, the ipod CIRCUIT is not the issue. What could be the issue is the amount of light output and the color of the light output from the ipod. BUT, how the detector is oriented also matters. 

 

Jim

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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In post #10 Frog mentioned the true unknown, the trigger signal in the apple device from which the 5 mS interval is determined. Unless H. Opened the case and put a scope probe directly on the led then I think any time measurements are inherently inaccurate.

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Hi David

 

I use the torch application to turn on ipod flash LED.

To measure the on timing, i use a logic analyser and the time is measured from the start of dip in comparator in voltage till it reaches the threshold.

I'll attach the analyser output as well.

Hope this makes things it much easier to understand.

 

 

Silly beans are always silly
:)

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Hi Jim,

 

I can definitely say that ipod led is less brighter compared to the LED that I used for testing. So this could be the point us in the right direction.

 

Thanks,

hari

Silly beans are always silly
:)

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Harikrishnan911 wrote:
less brighter
The apparent brightness to your photo diode is also very dependant with the distance (and possibly alignment errors).

Why use the led from a phone?

Try some different led's, different colors, etc.

... And try some different photo diode's.

PIR sensors (for outside lights and such) very often also have an ambient light sensor. These are often linearized, and they might be fun to play with.

Old fashioned LDR's are probably too slow for you. And phased out because of RoHS.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

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Perception of brightness by eye is notoriously difficult, especially if it is not a side-by-side comparison. First off, the eye, like many sensory organs of the human body, is logarithmic.

 

Second, memory of brightness is actually quite poor. This comes into play if you look at an illuminated surface, then after a period of time, look at a different illuminated surface and try to judge which one is/was brighter. It is much easier if you can look at the two surfaces, simultaneously.

 

Yet a third problem arises with flashes. When the flash is short compared to the visual time constant of the eye, your perception of the brightness may not match the actual brightness.

 

It is almost impossible to compare brightness for two different colors, because that is strongly affected by the frequency response(s) of the sensor (the eye, with its rods and cones that are sensitive to different wavelengths). Here, you get into the very significant philosophical issue of what "brightness" is. Is it simply the number of photons per second arriving at the eye (photon flux)? Or, is it the perception of brightness which is photon flux modified by the color response of the eye? Even light sensors such as photo-diodes, are subject to the latter constraint. 

 

Personally, I think that the OP is on a wild goose chase, with no clearly defined end-point.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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If anybody could shed more light on this, it could help understand and provide a sensible explanation

Your explanation of what you hope to do is quite vague.  You want to detect a flashlight  or a flashing light?   Why; to do what?  How fast?  How bright...give some useful details.  

What do you mean by "detect"?  Is that yes/no or providing a numerical or analog response ?

 

 

from JGM

 as the led has a substantial surge if it turns on full power instantly

That might be true of the power driver/supply driving the led, but not the raw led itself.

Of course an old-time light bulb can draw 2 amps running, but 20 amps surge during turn on.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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ka7ehk wrote:
Even light sensors such as photo-diodes, are subject to the latter constraint.
Gosh, I never envisioned that IR diodes have a limited frequency response smiley

ka7ehk wrote:
Personally, I think that the OP is on a wild goose chase, with no clearly defined end-point.
Well said Jim, you have my vote.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

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avrcandies wrote:
Of course an old-time light bulb can draw 2 amps running, but 20 amps surge during turn on.
You haven't watched Photoninduction yet, have you?

He has a 20kW Halogen lightbulb (or was it 50kW? It's about half a meter high and the filament wire is... Well, it's thick).  He has to pre heat it with a 200Amps current limited low voltage power supply and wait a minute or so till the thing is red hot before he can even dream of turning the thing on without blowing every fuse in a 10km radius from his attic.

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