... sand. ... Principle of Operation:At the beginning, green led light on the upper half, then flows to the lower half ... 3 seconds, the screen will turn to yellow, the sandy clock will show “3, 2, 1”, then screen turn to ... the screen turns to yellow, the cycle will go on…Function of control system:1. Control time of the funnel is ... 3. The operation is completed by key-press. LED sandy clock traffic light