How would you convert an imperative sentence from an active voice to passive voice in English?

“Let” is used to convert the active voice sentence into a passive voice sentence. 

"Let " is followed by The subject ( i.e the object of an imperative sentence in Active voice becomes the subject of passive voice). The subject is followed by "be". The "be" is followed by the past participle form of the given verb in the sentence. Ex:  Post the letter- Let the letter be posted.