Longevity of the LM3909 LED flasher

The LM3909 [1] is rather clever as it only requires a 1.5V supply, which is too low to light an LED, however, the chip works this 'miracle' by charging up a capacitor and then putting this stored voltage in series with the battery supply to create a brief 3V pulse. This flashes the led; the rate depends on the capacitor value and the pin 1 and 8 connections [1]. As the battery runs down it takes longer to build-up the capacitor voltage which makes it flash more slowly conserving the dwindling power – so it's very efficient. Other 1.5V led flasher circuits have appeared over the years [2] but to my eyes none seem to give such a satisfying bright flash as the good old LM3909 circuit. I have run electronics workshops [3,4] for many years, and I love to show people this little circuit. In the 2000's I used to tell the children, to their delight and wonder, that "this has been flashing as long as you have been alive!". Who knows one day I might even say "... it's been flashing as long as you, your parents and your grandparents have been alive!"

Over the years I have wired this circuit to all sorts of 1.5V cells and hidden them in various places as personal time capsules. I made my first one around 1979 and it ran till the 2000's, when I had to replace the capacitor. The second (shown) has been going continuously with about one flash per second since 1992, and since 1996 it has run on just three consecutive HP2 D cell 1.5V Duracell batteries. That's about 12 years a battery ... and it's still going strong!

It would be an interesting project to consider how you might design the set up (e.g. the battery and capacitor etc.) so that it could run continuously for 50 or perhaps even 100 years.

References and links
[1] search for 'LM3909 data sheet + PDF'
[2] alternative 1.5V circuit
[3] Transistor workshop
[4] Demonstrating the wonderful amplifying action of a transistor, J. P. Hare, IOP press, Journal of Physics Education, March 2004, p.128-131