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

On Wed, 28 Nov 2007 10:16:19 -0600, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 28 Nov 2007 01:03:17 -0600, Chris W <1qazse4@xxxxxxx> 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.

---
It doesn't.
---

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?

---
Sure, but why not get rid of the resistors altogether and feed the
blinker from a constant current source, like this:

<snip>

---
Even simpler, since the two LEDs are driven out of phase with each
other, you could get rid of one resistor and do this:

Version 4
*** 1 880 708
WIRE -800 -224 -896 -224
WIRE -640 -224 -720 -224
WIRE -16 -224 -640 -224
WIRE -896 -144 -896 -224
WIRE -496 -144 -896 -144
WIRE -160 -144 -496 -144
WIRE -640 -96 -640 -224
WIRE -496 -96 -496 -144
WIRE -160 -96 -160 -144
WIRE -16 -96 -16 -224
WIRE -640 32 -640 -32
WIRE -368 32 -640 32
WIRE -16 32 -16 -32
WIRE -16 32 -288 32
WIRE -896 80 -896 -144
WIRE -640 80 -640 32
WIRE -16 80 -16 32
WIRE -496 128 -496 -16
WIRE -496 128 -576 128
WIRE -464 128 -496 128
WIRE -368 128 -288 32
WIRE -368 128 -400 128
WIRE -288 128 -368 32
WIRE -256 128 -288 128
WIRE -160 128 -160 -16
WIRE -160 128 -192 128
WIRE -80 128 -160 128
WIRE -896 256 -896 160
WIRE -640 256 -640 176
WIRE -640 256 -896 256
WIRE -16 256 -16 176
WIRE -16 256 -640 256
WIRE -896 320 -896 256
FLAG -896 320 0
SYMBOL npn -80 80 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn -576 80 M0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -512 -112 R0
SYMATTR InstName R3
SYMATTR Value 100k
SYMBOL res -176 -112 R0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap -400 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 10e-6
SYMBOL cap -192 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10e-6
SYMBOL voltage -896 64 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value 15
SYMATTR InstName V2
SYMBOL LED -656 -96 R0
WINDOW 0 -29 -1 Left 0
WINDOW 3 -135 70 Left 0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
SYMBOL LED -32 -96 R0
SYMATTR InstName D2
SYMATTR Value NSPW500BS
SYMBOL res -704 -240 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 620
TEXT -616 288 Left 0 !.tran 0 3 2 uic


--
JF
.