Here the simple file alarm circuit based timer ID NE555. The works is simple, the lamp give light to the LDR (Light Depending Resistor) as light sensor. When the light from the lamp covered with smoke then the LDR will change its resistance value and then activated the alarm.Simple fire alarm circuit using IC timer NE555The thermistor offers a low resistance at high temperature and high resistance at low imperature. This phenomenon is employed here for sensing the fire.The IC1 (NE555) is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output(pin 3) of IC1 is couples to base of transistor T3(SL100), which drives the speaker to generate alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2.When thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor T1 through diode D1 and resistance R2.Capacitor C1 charges up to the positive supply voltage and increases the the time for which the alarm is ON. The larger the value of C1, the larger the positive bias applied to the base of transistor T1 (BC548). As the collector of T1 is coupled to the base of transistor T2, the transistor T2 provides a positive voltage to pin 4 (reset) of IC1 (NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the absence of the positive voltage. Diode D1 stops discharging of capacitor C1 when the thermistor is in connection with the positive supply voltage cools out and provides a high resistance path. It also inhibits the forward biasing of transistor T1.summaryAt times we forget to switch off our heating devices and eventually they get damaged. Therefore there is always a need of a device which can alert by sounding an alarm if the device temperature goes beyond a particular value. Same concept can be used in fire alarm. If a building catches fire then it will raise an alarm and people could evacuate the building.DescriptionThis circuit is based around LM35 which is a temperature sensor and could be used to switch an alarm when temperature goes beyond a preset value. In this circuit the output of LM35 is fed to the negative pin of comparator of LM339. The positive input is connected to a preset VR1 of value 10K. This preset is used to set the reference temperature. If output of LM 35 is less than V reference than the output of comparator is low. The IC LM339 has four inbuilt comparators.The output of comparator is fed to the base of transistor T1(BC547). As the output is low, T1 is in the cut off state and behaves like an open switch. As a result point A is at high potential. This in turn biases the transistor T2, which behaves like a closed switch. The potential of point B is low, which in turn gives a low signal to the reset pin 4 of the IC 555. When the reset pin is low, there is no output at the output pin 3 and hence the buzzer does not sound.When the temperature exceeds the reference temperature, the output of the comparator becomes high and T1 gets biased. The point A comes at a low potential thereby making the point B at higher potential. Since the point B is connected to the reset pin, it also becomes high and the 555 timer which is configured in the astable mode, starts producing pulses and the buzzer makes a sound to alert the rise in temperature. Notification appliances: This component uses energy supplied from the fire alarm system or other stored energy source, to inform the proximate persons of the need to take action, usually to evacuate. This is done by means of a flashing light, strobe light, electromechanical horn, speaker, or a combination of these devices.Notification appliances:Main article: Fire alarm notification applianceA Honeywell speaker and a Space Age Electronics V33 remote light. * Audible, visible, tactile, textual or even olfactory stimuli (odorizer).[1] to alert the occupants. Evacuation signals may consist of audible or visible appliances with a distinct audible tone or speakers to deliver live or pre-recorded instructions to the occupants. * In the United States, fire alarm evacuation signals generally consist of a standardized temporal code 3 audible tone, with visual notification in all public and common use areas. Emergency signals are intended to be distinct and understandable to avoid confusion with other signals.Other methods include: * Audible textual appliances, which are employed as part of a fire alarm system that includes Emergency Voice Alarm Communications (EVAC) capabilities. High reliability speakers are used to notify the occupants of the need for action in connection with a fire or other emergency. These speakers are employed in large facilities where general undirected evacuation is considered impracticable or undesirable. The signals from the speakers are used to direct the occupant's response. The system may be controlled from one or more locations within the building known as Fire Wardens Stations, or from a single location designated as the building Fire Command Center. Speakers are automatically actuated by the fire alarm system in a fire event, and following a pre-alert tone, selected groups of speakers may transmit one or more prerecorded messages directing the occupants to safety. These messages may be repeated in one or more languages. Trained personnel activating and speaking into a dedicated microphone can suppress the replay of automated messages in order to initiate or relay real time voice instructions.[2]