14 CFR 25.791: Passenger information signs and placards.

(a) Regarding “No Smoking” signs and placards:

(1) There must be at least one placard, or lighted sign, stating if smoking is prohibited. The placard or lighted sign must be legible to each person seated in the cabin.

(2) Lighted “No Smoking” signs must either be operable by a member of the flightcrew or be illuminated continuously during airplane operations. Illuminated signs must be legible under all probable conditions of cabin illumination to each person seated in the cabin.

(b) Signs that notify when seat belts should be fastened and that are installed to comply with the operating rules of this chapter must be operable by a member of the flightcrew and, when illuminated, must be legible under all probable conditions of cabin illumination to each person seated in the cabin.

(c) A placard must be located on or adjacent to the door of each receptacle used for the disposal of flammable waste materials to indicate that use of the receptacle for disposal of cigarettes, etc., is prohibited.

(d) Lavatories must have “No Smoking” or “No Smoking in Lavatory” placards conspicuously located on or adjacent to each side of the entry door.

(e) Symbols that clearly express the intent of the sign or placard may be used in lieu of letters.