From: Spehro Pefhany
Subject: Re: LED Pulsing - Apparent Brightness?
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Date: Tue, 07 Jan 2003 14:10:16 GMT
NNTP-Posting-Date: Tue, 07 Jan 2003 09:10:16 EST
On Tue, 07 Jan 2003 01:15:54 -0500, the renowned Mark Klinger
>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?
It depends on the response time of your photodetector. Your
phototransistor is quite fast if you use a 100R load as on the data
sheet and deal with the very low voltage that you get. If you put a
10K load resistor on there, it will become very slow.
The actual LED is probably up to full brightness within 100ns
of the current getting there, so you've got a nice square wave of
As others have said, your eye is average reading and you won't
perceive the bright flashes, just the low average intensity.
"it's the network..." "The Journey is the reward"
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