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> I'm wondering if anyone is aware of a replacement for the LM3909 
> chip. This
> is a very handy LED flashing chip that will run off a 1.5V battery 
> and is
> sadly no longer available. Does a good substitute exist?

The attached circuit, which I "invented" based on a vvv old flasher 
cct, may be useful depending on how you want to use it. All I did was 
add the L - I assume many others have dome the same. It works but can 
be finnicky. There's more to getting it right than meets the eye. As 
shown it has minimum component count but an extra R or two can help.

I posted this about 2.5 years ago so there will be discussion in the 
archives back then.

LED2 load position adds Vbattery to Vflyback but may not fully 
visually extinguish the LED during off times depending on LEd 
characteristics.

LED1 position uses only flyback energy. You may need to invert your 
brain to understand how it lights with the polarity shown :-). (People 
who already have inverted brains will have no problem).

Play with components. Add extra components as desired. eg a resistor 
in series with cap will alter discharge time.
R2 must be high enough that base current x Bet1 x Beta2 will not hold 
load on at end of flash cycle.
Q1b-Q2c needs a resistor if Vbat > 1 cell and may benefit even with 1 
cell.




        RM

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