Diet Conscience$14. Available from Art Adventures, 888 madison ave., New York, N.Y. 10021. Send payment by check or major credit card.212-628-2267.

A New Year's scenario: You know you're a compulsive nibbler, but this New Year's Eve you resolve to reform once and for all. Well, you think as you detour through the kitchen one evening, one thin silver of mincemeat pie never ruined anyone's diet. On the brink of surrender, you inch closer and closer to the refrigerator. Somehow, uncontrollably, your hand flings open the door, grabs for its plunder, but is halted by a voice from the refrigerator shelf that scolds: "Are you eating again? Shame on you! No wonder you look the way you do. (Mortifying laughter.) You'll be sorry, fatty. Do yourself a favor. Shut the door!" Finally. That nagging little mental voice called conscience or will power that most dieters never can summon when they need it the most. When the psyche won't trigger it, leave it to imaginative inventors to create a gadget that will. Battery-powered "Diet Conscience" will deliver its recorded insults from a fefrigerator shelf or a drawer. CAPTION: Illustration, no caption