He waved his hand as a sinal for help.

他挥手作为求助的信号。

The smoke was used as a sinal to indicate danger.

烟雾被用作表示危险的信号。

The traffic light turned green, giving a sinal to go.

交通灯变绿，给出了前进的信号。

The flashing light is a sinal that the battery is low.

闪烁的灯光是电池电量低的信号。

The referee blew the whistle as a sinal to start the game.

裁判吹哨作为开始比赛的信号。