Question about Bill Marsden's "LEDs, 555s, Flashers, and Light Chasers"

Thread Starter

skinner89

Joined Jul 18, 2011
17


The forward dropping voltage, or Vf, of an individual LED is very stable. Go below this voltage and the LED stops conducting. This LED is assumed to be 2.5V, pretty standard for a modern red unit. The target current is 20ma. Going though the math (using Ohm's Law) the resistor is 325Ω. Since 330Ω is the nearest standard resistor value 330Ω it is.
I am very new to electronics. Trying to go through the math myself I am already confused. Using Ohm's Law E = IR I try to find the resistance using the known voltage (9V) and desired current (20mA):

9 = .02 * R or 9/.02 = R, 9/.02 = 450

So why wouldn't you want to go with a common 470 Ω resistor in this circuit?
 
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Thread Starter

skinner89

Joined Jul 18, 2011
17
Alright so the LED is not current limiting like the resistor, however, it does reduce the voltage or pressure of the circuit. Going back to the water example would this be analogous to puncturing a small hole in the pipeline? The same amount of water can flow through the pipe, however, some of the pressure is being lost on a small stream of water spraying out the hole (being converted to photons)?
 

strantor

Joined Oct 3, 2010
6,781
Alright so the LED is not current limiting like the resistor, however, it does reduce the voltage or pressure of the circuit. Going back to the water example would this be analogous to puncturing a small hole in the pipeline? The same amount of water can flow through the pipe, however, some of the pressure is being lost on a small stream of water spraying out the hole (being converted to photons)?
sure, sound good to me. :)
 
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