The primary problem with building things is that you need the correct parts to do so. Ah well, the best I can do is tell you were to buy them I guess.
LM393AN voltage comparotor or similar ----------- Jameco part # 212177 1.90$ for 10
cadmuim sulfide photoresistor ----------Jameco part 202411 1$ each
approximately 2.3 K ohm pot
0.3 K ohm resistor and 0.68 K ohm resistor (actuly I used two to make the 0.68 K ohm one)
a solonoid ---------- Jameco part # 1919191 10$ each
a 2-pole 2-thingy switch. aka one with two separate switches in one and three posts per switch
a cat laser. I got mine at wallgreens for 5$
a soldering board----------Radio Shack, but I don't remember the price
pnp amplification transistor. It doesn't really matter which one I think. i just found one lying around.
solderless breadbord for prototyping----------Radio Shack, butI I don't remember the price
hot glue, solder, elastic, a rubber band, batteries, Nerf darts or similar, 2 small nails, two peices of K'nex or a dowel or somthing, cardboard and wire.
I think that's it. Boy, it didn't seem like that many things when I was rooting though my box-o-junk for parts. Also this parts list is for one sentry bomb, and as we all know, the more the merrier! In case you don't know about Jameco, it's a really good online electronic supply store which can be found here.