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Old 07-25-2008, 02:56 PM
hary536 hary536 is offline
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Default how to use RC time constant to run an LED in my circuit?

I am trying to design a circuit, where i am trying to light up an LED long enough based on my pulse signal.
Here is what i am trying to do.

a 10us pulse(every second) --->74HCT04 buffer---> (RC circuit)----> LED.

since this is a 10us pulse, i want to make sure to light up the led for long enough time, so that one can see it. So i am trying to use RC time constant to add delay so that the LED remains ON for a longer time. I am not able to figure out the exact circuit for that and how i should design the RC circuit and how and where to place it with/before/after the LED.
Also i am not able to figure out how to calculate R and C.

It would be great, if someone can help me out.
Any kind of input is appreciated.
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Old 07-25-2008, 04:24 PM
Audioguru Audioguru is offline
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Location: Ontario, Canada
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Instead of powering the LED from a huge capacitor that will never charge, use a small capacitor that is charged through a diode by the pulse, then the voltage in the capacitor drives the very low input current of the 74HCT04 buffer that drives the LED through a current-limiting resistor. A discharge resistor in parallel with the capacitor determines the duration of each blink.
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Old 07-25-2008, 06:39 PM
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Norfindel Norfindel is offline
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Another option could be to use a latch, instead of a buffer, at least if the latch's output current is enough to drive the led(s). Would be steady, but depends on how many leds you have, and if some of them get refreshed at the same time, or not. There are addressable latches, also, if none of the leds get refreshed at the same time.
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