Re: Simple circuit, stupid problem

On 21 Aug 2005 11:34:39 -0700, "Eriswerks" <eriswerks@xxxxxxxxx>
wrote:

>I'm trying to build a two transistor LED flasher, as shown at
>http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
>be perfectly simple, but I must be making some mistake that I'm just
>not seeing yet.
>
>When I build this circuit, both LEDs light continuously rather than
>flashing. I've tried varying the values of the resistors and caps that
>should control the timing, but it doesn't make a difference. I can
>actually pluck the capacitors right out of the circuit and both LEDs
>are still lit. I've built this thing a dozen times now from several
>different people's instructions, and I get the same result every time.
>
>The transistors I have on hand are a little mixed... I've tried
>2N3904s, 3906s, and 4424, any of which ought to work as they're all
>NPN.
>
>Does anyone have any suggestions as to what stupid mistake I'm making
>here? I am starting to get sick of looking at this thing. Thanks in
>advance..!

---
You're not making any mistakes, it's a stupid circuit.

Try this:


+V>----+------------------------+
| |
[470] [470]
|A |A
[LED] [LED]
| |
+-[+10µF]--+ +-[10µF+]-+
| \ / |
| X |
| / \ |
C / \ C
NPN B-------+ +------B NPN
E | | E
| [39K] [39K] |
| | | |
GND>---+---------+-----+--------+

If it won't start, change one of the 39k's to 47k


--
John Fields
Professional Circuit Designer
.

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