-	-		-	-
Character Classes	-	-	-	DMG

Simple mechanical traps can be set by thieves || assassins.
The chance to do so successfully is equal to that of the chance shown for detecting such traps,
    but in this case the assassin operates at an ability level equal to two levels above his or her own
    and exactly as if he or she were a thief, e.g. a 5th level assassin has the same chance of setting a trap as a 7th level thief does.

<the above is not well-worded: i would recommend using the example as the rule. e.g. L1 assassin sets traps as a L3 thief, L2 assassin sets traps as a L4 thief, L3 assassin sets traps as a L5 thief, etc, etc.>

Simple traps are those which involve mechanical components which the character setting them has normal access to,
    such as arrow traps, trip wires, and spring-propelled missiles.
Special devices such as poisoned needles, scything blades,
    and any similar traps with special mechanical components will also require the efforts of one or more specialists
-- those required to manufacture the component parts.

Whenever a thief or assassin character desires to set a trap,
    require him or her to furnish you a simple drawing to illustrate how the trap will function.
If the chance to successfully set the trap results in failure,
    there is a chance of causing injury to the trap setter,
    just as if he or she had set such a trap off.
    This chance is rolled for separately and is the obverse of the chance for successful setting of a trap.
    The drawing of the trap will modify the chances for injury in cases where failure results.
        Modification can be upwards or downwards according to the complexity and danger of the trap.
Note that even with a prepared mechanism for a poison needle, for example, the trap must be set, and failure can result.
Gloves or protective handwear cannot be worn when setting such traps.

Finally, failure on the first attempt to set a trap does not mean that the thief or assassin can never set the trap.
Unlike other similar thief functions, repeated attempts are permissible.