Very simple project. You just need a 555 timer a couple of resistors, a capacitor, a couple of LEDs and a talented wife to provide a bat ;) I think she'll be adding a post on making the Halloween bats so I'll link to it once it's up.
I used the following online calculator to work out the values for R1, R2 and C1 for the standard astable 555 set up. I ended up with: R1 = 3K Ohm, R2 = 570 Ohm and C1 = 100uF (mainly because that was the combination of components that I had lying around and that gave an acceptable output wave). I didn't bother adding a switch to this one, I'm not sure why as it'd be very easy... here's the fully wired up project:
Did you notice the 9V battery clip onto an 2xAA holder? Weird eh? Useful though. I think I picked up the battery case at radioshack. You'll also have noticed that I decided to try a free-form (no perf-board) approach. It was a bit fiddly, but worked out ok. I still haven't found a combination of 3rd hand/clamp/foot/nose that works for me for these fiddly soldering projects...
Here's a couple of close-ups of the soldered 555 itself:
Most of the time I had the 555 held with a crocodile clip (on the 3rd hand). Unfortunately, I bought a really cheap 3rd hand from a market and the crocodile clips are not stable so everything kept moving around whilst I was trying to work on it... bah. At points I ended up jamming the chip into a corner of the 3rd hand base whilst pushing at it with the soldering iron. Not very elegant or skilled... any advice is greatly welcomed!
All that's left was to poke out some holes for the bats eyes and to mangle it all into the available space. I made the holes big enough for the dome of the LEDs to fit through but small enough to prevent the rest of the LED housing from pushing through.