- I saw him reading a book.
The main verb, "saw", becomes "was seen"; the object "him" becomes the subject "he", and the subject "I" becomes "by me". So the result is: He was seen reading a book by me.
- Dr. Yunus was awarded the Nobel Prize for micro-credit system.
This is already in the passive voice. If we want to make it active, then we change "was awarded" to "awarded", make "Dr. Yunus" the object, and add a subject: Someone awarded Dr. Yunus the Nobel Prize for micro-credit system.
- He was expected to be faithful.
This is also already in the passive voice, so change "was expected" to "expected", make "he" the object, and add a subject: Someone expected him to be faithful.
- It is time to close the door.
This sentence can't be put in the passive voice because the main verb is "is", which is a form of "to be", and the verb "to be" cannot be used in the passive voice.
However, as helen's answer points out, instead of putting the entire sentence in the passive voice, it's possible to just put the phrase "to close the door" in the passive voice.
- Let me do it.
helen's answer describes how to put this in the passive voice.