A detonator is used to trigger bombs, shaped charges and other forms of explosive device[?]. Detonators are often attached to a timer[?] to ensure that the explosion takes place at the desired time, or when the person laying the explosives has reached a safe distance from the blast. Detonators can be chemical, mechanical, or a combination. Many detonators' primary (sensitive to heat and shock) explosive is a material called tetryl.