Baxter Stockman with the Sarnath-o-meter
The Sarnath-o-meter was a device created by Donatello to track the fragments of the Eye of Sarnath during season 2 in the 1987 TV series. It was the second tracking device of its type that Donatello had made, and was seemingly made out of the remnants of the original Crystal Converger.
Donatello lost the Sarnath-o-meter just as Shredder manifested the power of the eye, and it was taken by Baxter Stockman, who used it to track the Eye. After gaining Shredder's helmet, he discarded the Sarnath-o-meter, which was acquired by Blodgett and used in turn to track down Baxter. Finally the device found its way into Shredder's hands, but that would ultimately be disastrous for him. In addition to locating the Eye, the Sarnath-o-meter was designed to trigger a massive explosion in the Eye when the two objects came in contact. Shredder flung them both into the river, where they exploded.