The first Emerald Detector to appear was called the Jewel Reader (ジュエルレーダー,Juerurēdā?, lit. "Jewel Radar"). Developed by Tails, it detected a reaction from the Master Emerald after the Death Egg landed on Angel Island, prompting Sonic and Tails to head out and investigate.
In Sonic Riders, an Emerald Detector was used by Team Sonic. This one resembled a stopwatch with a radar screen, a button display on the bottom and a small antenna on top. Its screen could display the surroundings in 3D, allowing for better location of the Emeralds.
Team Sonic used the Emerald Detector to track a Chaos Emerald to Metal City. While experiencing interferences, the Emerald Detector pinpointed an Emerald in the possession of the Babylon Rogues.
Sonic Rivals 2
In Sonic Rivals 2, Dr. Eggman Nega had a Ring-powered Emerald Detector. The Master Emerald, which Nega had stolen, was inserted into this device. This device was taken from Nega by Knuckles and Rouge who used it to find Emeralds as well; Knuckles sought the Master Emerald while Rouge looked for the Chaos Emeralds. At the end of their hunt, where Rouge found the Chaos Emeralds thanks to the Emerald Detector, Knuckles discovered the Master Emerald inside it, much to his dismay.
In Sonic Runners, an Emerald Detector, also called a Master Emerald detector, was used by Dr. Eggman to track down the Emerald Shards of the Master Emerald. This device was a rectangular red pad with a round green radar screen, buttons on the edges and an antenna on top.
Early on, the Emerald Detector was used by Dr. Eggman to track Team Sonic's movements, who were also hunting Emerald Shards, but when Eggman confronted them, he lost the Emerald Detector to the heroes, who used it to find the remaining Emerald Shards.
They are both used to track down the prominent objects in their respective series; the Dragon Radar locates the Dragon Balls, and the Emerald Detector locates emeralds with Chaos energy.
The Emerald Detector's design is very similar to the Dragon Radar; in Sonic Riders its design looked nearly identical to the Dragon Radar, in Sonic Rivals 2 it would beep when near the sought out objects, and in Sonic Rivals its radar display was nearly identical to the Dragon Radar's.