After seeing the bristlebot and many variants I figured I needed to make one, but felt like they needed a little more character. With Halloween approaching, I figured it out, I would an an LDR and put a small plastic rat over the whole thing. Leave a few in a room with the lights off and as someone walks in and turns the light on, they would all scoot around.
Well, I knew it would be a relatively simple project, although turned out to be a little more time consuming than i expected to get everything together and working. Unfortunately, I don't have pictures as I was making it because I just made it and only thought about this afterwards... maybe I'll remember to take pictures when I make the next ones.
There are plenty of instructions (and instructables) for making a bristlebot (e.g. http://www.evilmadscientist.com/2007/bristlebot-a-tiny-directional-vibrobot/ ), so I won't put too much time into that, basically I just made a bristlebot, but used 2 button cells instead of the coin cell so they would fit inside the rats. I also wired a simple switch in series (to turn them off and save the battery) and the plan was an LDR in series as well so it would appear that they would run away from the light.
My son had a lot of fun playing with the one I made, but the batteries did not last too long. (I might have shorted them when I was making it, which would not have helped.) I tried to take some pictures while I took the thing apart since I would have to dismantle the entire thing to replace the batteries, but the pictures were all blurry and useless.
It moved slower than I would have liked, but I'm definitely going to make more. Unfortunately I have a couple of other projects which take priority and need to be completed before Halloween (in 3 days) so I might not have time this year. This is part of the reason for writing up the project so I remember what happened so hopefully for next Halloween I can make a whole swarm of these rats.
small vibrating motors: ~84c each (ebay - $8.36/10) LDRs: ~5c each (ebay $0.99/20) Simple SPDT switches, ~10c each. (ebay $1.86/20) Toothbrushes : ~17c each (dollar store $1.00/6) 2xButton cells: 25c each (dollar store $1.00/4) And creepy crawly to go on top......
Originally I didn't know exactly what I would use, I was at the dollar store for the batteries and toothbrushes and initially picked up a bag of "creepy crawlies" that had roaches, millipedes, spiders etc in various colours. The problem was that they were very cheap (a pack of 32 for a dollar) but looked cheap and nasty. They would be big enough from above that they would cover the bristlebot, but they were flat and would look like they were an inch off the floor, and then when you looked even slightly from the side you would see the bristles and everything else.
Then I found these rubber rats and they were perfect, they are hollow inside and I figured if I used 2 button cells instead of a coin cell in each one, I could hide everything in the hollow and they would be even creepier.
Rats: 13c each (dollar store $1.00/8)
A total of about $1.80 per rat (or it would be if any of these could be bought individually or in similar size packs)