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How does this circuit keep a constant 20ma going to the LED

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED blinker kit. All it does is alternately blink one led then the other. It seems pretty obvious how it does the blinking. But the kit says it will work on any where from 3V to 15V. What I can't understand, is how it can do that and still provide the same 20ma to the LEDs. Or maybe it doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just change out the resistor to get the correct current going to the higher voltage LED?

You can see the schematic here...

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Q1 and Q2 are 2N3904 R1 and R2 are 100 ohms R3 and R4 are 47k ohms C1 and C2 are 4.7uf

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Chris W
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Chris W
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You left out the most important component value: The LEDs. Unless they're special types, the current through a LED will rise to about 130mA at 15V. That's high, way too high for an ordinary 20mA LED. Even if you look only to the average, so 65mA, it's too high. If you don't blow them up at once, they will live short at that high a current. Now there are numerous extra-ordinary LEDs these days. Types that can handle over 100mA. But I don't expect that types to appear in a simple, lowcost kit like the one at hand seems to be.

petrus bitbyter

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petrus bitbyter

you assumptions are correct, the current will vary.

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Jamie 

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Hmm... 

Echoes are always a little late. ;)
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John Fields

John, you're a great philosopher :)

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Reply to
Jamie

Others have already answered - I'll address what may be an underlying assumption: Do you think 20 mA is "sacred" for LEDs ? It isn't.

20 mA is a "rule of thumb" kind of number for typical LEDs, but they will glow at a lot lower current. When you start to go above 20 mA, you may go out of the "safe" zone and cook them. And, there are a wide variety of LEDs available, so it is best to read the datasheet for the LEDs you intend to use.

Also, the current does not have to be constant for the LED to glow. If it varies a lot, the brightness of the LED will vary visibly which can be annoying. Less variation will be less annoying. Again, if it goes too high you can cook the LEDs.

Ed

Or maybe it

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ehsjr

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