(a) When a person's conduct would otherwise constitute an attempt to commit a crime under Code Section 16-4-1, it is an affirmative defense that he abandoned his effort to commit the crime or in any other manner prevented its commission under circumstances manifesting a voluntary and complete renunciation of his criminal purpose.

(b) A renunciation of criminal purpose is not voluntary and complete if it results from:

(1) A belief that circumstances exist which increase the probability of detection or apprehension of the person or which render more difficult the accomplishment of the criminal purpose; or

(2) A decision to postpone the criminal conduct until another time.