I came up with this a few years ago, and have posted it a few times since when this topic has come up again...
HELMETS
(The underlying assumption being, that wearing a helmet doesn't make you any harder to
hit; but when a threat is scored a disproportionate number of these will target the head,
and therefore a helmet should provide a disproportionate amount of protection at these
times--and not having one on at such times is a VERY bad idea indeed!)
Type_____ThreatAP's_____Penalty_____SpellFail
None________-4____________0____________+0%
Light_______-2___________-1____________+10%
Medium_______0___________-3____________+25%
Heavy_______+2___________-6____________+50%
notes-
ThreatAP's: added to armor class before threat roll is made to test for a critical.
Penalty(proficient): applies to all checks related to vision or hearing(and perhaps smell, in
the case of the heavy helm). One-half of the modifier(0/0/-1/-3) applies to archery(bows,
not crossbows).
Penalty(non-proficient): applies to all checks related to vision or hearing(and sometimes
smell). The greater of the helmet or armor penalty applies to all attack rolls.
SpellFail: applies to spells with verbal components.
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If you don't want to penalize a character for not wearing a helmet, you can change the threat APs around and give a +0 modifier for "none"--the light , medium, and heavy categories could then be given modifiers of +1, +2, and +3 respectively (or something like that). It really depends on the flavor of your campaign.
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Something I do in my Basic D&D campaign that's a bit simpler...
Wearing a Helm/et gives a character a chance to convert a critical hit to a normal one...
Helmet: 1 on a d6
Helmet: 1-2 on a d6
If you prefer them to provide more protection, you can extend the range of each by 1 (i.e. 1-2 and 1-3).