How do you control the time the kid watch TV, pad, computer and smartphones ?

Agree on the rules for screen time upfront with your child and get them to acknowledge them. Some parameters include:
 1) duration of screen time - eg. Maximum half hour a day on school days (regardless of which device) and maybe slightly more during weekends and school holidays.
 2). Set the conditions for using it eg. Must have completed homework etc
 3) The type of programs your child can have access to. If your child is young, set parental controls in the device on the content he can have access to.
 4) Screen time can also be interactive. Share or discuss something you watched together.

You should also discuss and ask your child to acknowledge the consequence if they do not follow the rules set. Eg. No screen time for the next 3 days. Or that you will confiscate the device. Enforce these rules. Be consistent in enforcing the rules.

To help them, you may want to give them advance reminders. Eg. That they have another 5 mins to go. This allows them to be prepared to ‘surrender’ the device and to switch it off.