My 2007 Halloween Costume. I didn't document the build process enough for an instructable but I did take lots of pictures after it was done.
The Geiger counter, which I found while browsing The Electronic Goldmine inspired the costume. I started with the Geiger Counter which (as disclosed by EG) was broke to begin with. I added blue LEDs on the gauge in place of the needle and used a sound-to-light module to activate the LEDs when the sound from a record/playback module played a looped Geiger counter noise through a speaker.
I bought the gas mask from eBay ($9 I think) and wired in a green LED light bar from SparkFun to light my face. Also, a mic was wired inside the gas mask which ran to a voice changing module. The voice module was wired to a speaker that was attached to the outside of the suit, with it I could change the pitch of my voice from very deep to very high and robot style.
I used washer drain hose and attached it to one filter port of the gas mask to run the wires for the LED bar and the mic in to the control box. An additional hose was attached to the mask and then to a fan box to blow fresh air into the mask.
The Geiger counter was powered with 4 AA batteries for the record/playback module and a 9v battery for the LEDs. The mask LED bar and voice changer were powered by two 9v batteries (wired parallel - I was paranoid about the batteries dying). The suit fans and the fan light bars were powered by 12v sealed lead acid batteries (I used two wired in parallel - again paranoid about battery time, this was overkill). The sealed lead acid batteries were carried in a backpack that I wore inside the suit, the fan boxes and the speaker were velcroed to straps of the backpack to help support them.
There were two fans for the suit, one blew air into the mask via a hose and the other blew air directly into the suit. An unintended but welcome consequence of the fan blowing into the suit was that it inflated it adding a comic effect to the whole thing. I was somewhat surprised at first since it's only an 80mm case fan but it's a Panaflo high speed so it does move quite a bit of air. I can't stress enough that ventilation was very important, this was a real Level B chem suit, it's like being in a human size Ziploc - without the fans I would have passed out. In reality a suit like this would be used with a SCBA (self contained breathing apparatus) but since those are probably like a billion dollars I opted for the fans. All in all it worked out pretty well, it was hot and I sweet like a pig but I was able to stay in it as long as I needed to - then went home and took a shower.
Geiger counter - The Electronic Goldmine (I can't find them here anymore but you can find them on eBay)
Gas mask - [href="http://search.ebay.com/gas-mask_W0QQ_trksidZm37QQcatrefZC6QQfromZR10QQfsooZ1QQfsopZ1QQftrtZ1QQftrvZ1QQpqryZgasQ20maskrQQsabfmtsZ1QQsacatZQ2d1QQsaobfmtsZinsifQQsaprchiZQQsaprcloZQQsofocusZbs eBay]
Level B chem suit - Spill 911 (cheapest I could find)
Blue LEDs and Light bars - SparkFun
Voice changing module, sound-to-light module and record/playback module - Electronics123.com
SLA Batteries, wire, switches, project boxes and other miscellaneous items - All Electronics