How to change the direction of dc current in a circuit?

Is there any way that i can automatically change the direction of current in a DC circuit after a predefined time.
Update: that means i will have to use H bridge is this device ready made available or i have to make it

also cant understand how to use electronic timer so it reversed direction after lets say 20 secs
Update 2: I am just using a simple motor for which i want to change its direction automatically after 20 sec is there any IC available to do that
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