Hey all - Is it possible to build a blinking LED circuit using only capacitors and resistors? Would diodes help, I pulled some out of a TV but don't know their values...?

 

The short answer: no, unless the flasher is built into the LED itself.

Slightly longer answer: you need some kind of valve or semiconductor to sense when the voltage across the RC network has exceeded a certain threshold level, determined by the designer of the circuit. The valve or semiconductor then turns on the LED while draining the voltage from the RC network, until it has fallen below another threshold level.

As mentioned in the short answer, there ARE self-flashing LED's. These have a timer circuit and current-limiting features built in. You can actually use them to flash a pretty long string of LED's.

 

For use with scrap parts and two npn transistors, use a meter to figure out which parts will work. Learn how to read capacitors or get a meter that will read them.
http://xedox.de/blinker.htm

Simplest two I've seen that work with 1.5v
http://www.discovercircuits.com/DJ-Circuits/ac14fls.htm
Lasts six months

Vampires that cell
http://www.discovercircuits.com/DJ-Circuits/1vled3.htm
Lasts 12 months

LM3909 is obsolete and hard to get but the others circuits are cool.
http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#15flash.gif