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

Chris W wrote:
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.


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 data*** 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
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...

http://hp15c.org/BL1.gif

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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