Set this kit in an inconspicuous place with light shining on it, and it remains perfectly silent. However, as soon as the light is turned off, the kit gives a high-pitched, irritating sound. When students turn the light on to look for the culprit, the alarm goes silent again. Easy to build and lots of fun. Operates using a 9 V battery, sold separately. Complete with all parts, PC board, and instructions. Size of board: 2.9" x 1.8".