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Instructables user momo! writes:

I decided that it would be helpful to make some sort of light doohickey that I can attach to my dog's collar when I walk him at night. His black fur has a nice sheen, but it sure makes it difficult to see where he is and *what* he's chewing on at night (ewww) at the not-so-well-lit dog park near my house. The end result is a nifty mod resin clip that shines a semi-diffused LED light operated on 2 AA batteries through a switch. I used a silicone mold and clear resin to make the body (it was my first time casting!). Of course, a lot of these steps can be modified to personal preference. But the most important thing is: make sure that the simple circuit is safe and secured, and that the resin body + battery case is not too heavy! You might need to adjust what battery source and circuit you will need based on the size of your pet. Cisco is 90+ pounds.

Learn how to make your own at the instructable!