This study design and construct a smoke detector. Smoke detector is made up to a smoke chamber, photodiode amplifier, comparator, multivibrator circuit and audio amplifier. The smoke chamber comprises mainly of a photodiode working in-conjunction with a red light emitting diode (LED) other stages are realized by using operational amplifiers, capacitors, and potentiometer. The input to the system is the photodiode amplifier actuates the system and an alarm comes on from the loud speaker through the audio amplifier the alarm will continue until the smoke varnished from the chamber. If the smoke collected by the chamber is not enough for it to operate there will be no alarm/sound. The system is simple and flexible because of the variable sensitivity and can used to detect any type of smoke. The smoke detector can be placed in the center of the laboratory or hallway; as it is impossible to predict the source of a fire. If it is necessary to place the alarm on a wall, always locate the tip of the smoke alarm 10-15cm (4-6 inches) from the ceiling. It can be placed as needed; compensate for closed doors and other obstacles that may interfere with the part of smoke to an alarm. They may also prevent occupants on one side of a closed door from hearing an alarm on the other side of the door. It is recommended that every home, industry and office be compelled to install smoke detecting units to alert any unforeseen danger of fire which might cause loss of fives and properties.