Principle of Operation: At the beginning, green led light on the upper half, then flows to the ... half like sand. ... Principle of Operation:At the beginning, green led light on the upper half, then flows to the lower ... 3, 2, 1”, then screen turn to red. Like the green color, at the last 2 seconds, the screen turns to ... adjustable.3. The operation is completed by key-press. LED sandy clock traffic light