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'[PIC] LED as a light measurements'
Someone of you mentioned that an ordinary led can be used as a light sensor.
How sensitive is that? How fast is it -- I mean can be it used as a test
measurement device for a really fast movements (sometimes nearly as fast as
the speed of sound). Which colour is the best for this (in terms of more
sensitive, more response). And finally as far as I know it gives a very
small signal you can measure, so would you use a simple analogue amplifier
to attach it to the PIC?
unPIC -- The PIC Disassembler
Comparing the speed of operation to the speed of sound does not give enough information to determine speed requirements. Is this for a photo interruptor where an object breaks a beam of light? How big is this object traveling at Mach 1? How quickly must the detector respond when the beam is broken?
LED's can work backwards as photodiodes, albeit with quite low sensitivity. Is there a particular reason you want to use and LED rather than proper photodiode? If you have control of the wavelength of the illumination source then proper photodiodes are cheap, sensitive and fast and are available in the same package style as LED's.
A photodiode looks like a (very low) current source, the current direction is from cathode to anode. The best way to use them is with a transimpedance amplifer, e.g.
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Wouter van Ooijen
> Someone of you mentioned that an ordinary led can be used as
> a light sensor. How sensitive is that?
It generates a small voltage - you can measure it with a multimeter.
> How fast is it
AFAIK quite fast, compareable to an opto-transistor.
> can be it > used as a test
> measurement device for a really fast movements (sometimes
> nearly as fast as the speed of sound).
Unity error: you try to compare distance/time with time.
> And finally as far as I know it
> gives a very
> small signal you can measure, so would you use a simple
> analogue amplifier
> to attach it to the PIC?
comparator? (some PICs have one build-in) or use the bias-trick, check
ultrasound distance measurement with a PIC, there is some circuit out
there that does that without an amplifier.
Wouter van Ooijen
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu
Alan B. Pearce
>Someone of you mentioned that an ordinary led can be used as a light
Have a look at the second item in here
Note that the PDf file is loaded as an image in a web page, so don't right
click and do save as, you need to load the page in a browser.
Painful system I know ...
Tamas Rudnai wrote:
> Hi Everybody,
> Someone of you mentioned that an ordinary led can be used as a light sensor.
> How sensitive is that? How fast is it -- I mean can be it used as a test
> measurement device for a really fast movements (sometimes nearly as fast as
> the speed of sound). Which colour is the best for this (in terms of more
> sensitive, more response). And finally as far as I know it gives a very
> small signal you can measure, so would you use a simple analogue amplifier
> to attach it to the PIC?
I have not designed it commercially yet, but my tinkerings seem to
indicate the following:
1. The PIC ADC will work reliably to detect fast changes in incoming
light. But the LED
is NOT very sensitive
2. My experiments have covered green LED's used as sensors. I have used rear
light sources as well as front lighting sources. Room light that
normally reaches the
LED can be blocked by a finger in the obvious way. The output drops when the
finger blocks the light.
3. Another way is to illuminate the green LED is from the REAR, and when
touches the T1-3/4 surface, the finger/surface interface acts as a
reflector back into the
sensor. This is appealing because it offers a gentle GREEN glow,
marking where the
sensor is located. The output increases when the finger reflects the
light. But the sensor
is less sensitive to this, so the detection routine is more important.
4. The detection routine needs to accommodate slow changes due to
background light, so only quick changes are acted on.
Thanks everyone for the answers. It is a really small item, an airplane
model propeller. Basically I just wanted to make a simple rotation speed
measurement device and thought I just can put something together with my
existing components I have at home at the moment -- and also to learn
something from it.
So this prop rotating at 16k - 18k depending the size and sometimes the goal
is to keep the speed low enough to not overspeed the outer part of the
blades of the sound as it is not good to the performance and for my ear :-).
But mostly to set a proper speed that is healthy and worthy. There are quite
a few commercial products, however, wanted to build one by myself for my
entertainment basically and thought it could be something unique.
And no, there is no beam, just the measurements of the daylight (or normal
room lights) when the prop color differs from the background and where the
human eye could not even see it! (It is sometimes very dangerous when you
forget how big is your prop and move your fingers too close to it). The size
of the blade is few centimeters (1-2-3 inches) and if you have 2 blades you
can expect a 32k - 36k changes a minute which is just around 600Hz so not
really fast if you compare it with the PICs speed. And not need to measure
when the blade exactly passed the sensor, however, if there is a delay is
should be constant of course :-)
Also thought that later on I could play with other LED based sensors like a
a LED display that is brighter on sunlight (switch on for a while, then off
and put the pin to input, measure... etc.. or maybe I could use the
currently unused segments for that purpose). But this one is only because of
On 11/22/06, Jinx <clear.net.nz> wrote: joecolquitt
Timothy J. Weber
Tamas Rudnai wrote:
> Thanks everyone for the answers. It is a really small item, an airplane
> model propeller. Basically I just wanted to make a simple rotation speed
> measurement device and thought I just can put something together with my
> existing components I have at home at the moment -- and also to learn
> something from it.
I've fooled around with making optical "buttons" out of LEDs. My
experience is that, while LEDs can be used as optical sensors,
phototransistors and photodiodes are more sensitive. So, for a one-off
project, they'll be easier to get up and running, and the increased cost
Timothy J. Weber
You are trying to measure about 800Hz (=24krpm 2blades/rotation) with a fill
factor of about 1/15 (~7%) and a contrast of maybe 6dB. So the actual frequency
response should be computed for ~10-12kHz (the pulse width for each passing
blade should be about 100usec wide and these pulses should be cleaned and
applied to your counter. This is not so easy even with a normal photodetector
imho. Most photodetectors have problems with high frequency due to internal
capacitance combined with low photocurrent.
There's a good paper on this subject.... sorting.... digging.... here:
Very Low-Cost Sensingand Communication Using Bidirectional LEDs
PaulDietz, WilliamYerazunis, DarrenLeigh
They used it as a simple data port to send data, but other uses should be easy enough to work out.
William Chops Westfield
Thanks, that's a good artcle on this topic! It says different LEDs gives
different votage levels. Even 1.5 V could be achived! Wich leads to another
silly question: do we have to protect the pin of the PIC if it was driven
directly in a normal LED flashing app? So I put the device under the
sunlight, the LED is not currently switched on so there will be a 1.5 on the
PICs pin (well it's not that much or is it?)
On 11/22/06, William Chops Westfield <mac.com> wrote: westfw
> do we have to protect the pin of the PIC if it was driven directly in a
> normal LED flashing app? So I put the device under the sunlight, the
> LED is not currently switched on so there will be a 1.5 on the PICs
> pin (well it's not that much or is it?)
Do you mean the PIC is not powered up but there is voltage on
a pin from the sunlight ?
AIUI the voltage will go through the internal diode to Vcc. 1.5V-
Vdiode isn't enough to start the PIC, but it may affect a re-boot, ie
the PIC can't properly power down and reset. A series resistor
would limit the current available from the LED, but with LF and nW
parts there could still be "issues". It might be that you'd need an
active pass element (FET, 4066 etc) to enable the voltage from the
LED only if the PIC is powered
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