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From: Kevin McMurtrie
Subject: Re: LED Pulsing - Apparent Brightness?
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Tue, 07 Jan 2003 21:04:56 GMT
NNTP-Posting-Date: Tue, 07 Jan 2003 13:04:56 PST
In article ,
email@example.com (Paul Mathews) wrote:
>Kevin McMurtrie wrote in message
>> In article ,
>> Mark Klinger wrote:
>> >I am experimenting with wireless communication via LEDs, and have a
>> >simple question of apparent brightness vs. actual light output.
>> >Keeping in mind that my receiver will be a photo detector (Fairchild
>> >L14N2) I'm trying to understand brightness.
>> >At DC, 20mA is the max drive for my LEDs and they are extremely
>> >bright. If, however, I put them in my circuit and pulse them with a
>> >width of around 1mS, they are barely even illuminated.
>> >Will the distance of the transmission suffer since they are
>> >"apparently" not as bright, or are they actually the same brightness
>> >as at DC to the photodetector, but my eyes just aren't seeing that?
>> >Ultimately what I am wondering is if what the photodetector sees is
>> >effected by the short pulse duration, or is it equally as bright at
>> >1mS as it is at DC?
>> The pulse durration matters. A reduced duty cycle makes the pulses
>> harder to see above noise. If bandwidth isn't an issue, you can make a
>> receiver that's narrowly tuned to those pulses. Over time it will be
>> able to find the signal.
>> Think of this as an analogy: If somebody tapped together drumsticks in
>> a crowd once every minute, it would take you a long time to notice it.
>You see the average power for short pulses. You can pulse most LEDs
>at higher currents than rated....easily 100 ma for 1ms pulses,
>provided that the average is within ratings. Peak intensity may even
>be slightly higher in the pulsed case if the LED is used with a
>heatsink and the average current is lower than 20 ma. This is because
>LEDs have higher output at lower temperatures.
>I don't agree at all with the 'drumsticks' analogy. If you 'tune'
>your receiver to be especially sensitive for the particular pulse
>shape you use, you can easily exceed the detection performance for DC
>excitation. This is because the ambient environment is mostly steady
>illumination...brief events do, in fact, stand out if you know how to
>tune for them.
>Many experimenters are using IR remote chips to do this type of
>detection. You can find a wealth of info by searching on phrases like
>"IR remote". Throw in a few more keywords like "schematic" or "robot"
>to narrow down the results. There's more info here:
I didn't mean to imply that DC was better. The sensor has to operate at
a frequency higher and narrower than other light sources. A 50% duty
cycle would be best.
(Funny story - I was once getting an interference pattern on my TV that
corresponded to the IR remote Power command. Every time I walked in
front of the TV, it turned off!)
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