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A special sensor or transducer called a thermistor is used to detect an increase in the temperature of the surrounding air whenever smoke is produced by a fire. A different type of sensor is used to detect an increase in the size and density of particles in the surrounding air after it has been contaminated by smoke from a fire.
Either sensor gives an output in the form of an increase in voltage whenever it detects an increased temperature and/or smoke particles. That increase in voltage is fed to the base of a transistor. When the voltage to its base increases, the current from the transistor's emitter to collector (or from its collector to emitter) increases and that increase in current operates the alarm. In other words, in a fire alarm circuit, the transistor acts as a switch which turns on the alarm whenever a sensor or transducer detects the presence of a fire, else it remains off.
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Why to use pnp transistor in fire alarm circuit?
You can use a PNP or an NPN transistor in a fire alarm circuit. Both will work. It all depends on how you want to energize the alarm, and with what polarity of signal.
What is the main function of resistances in fire alarm circuit?
Heat changes the resistance of a circuit. The change is detected by the system, and activates the alarm.
What is a function of a resistor in a fire alarm circuit?
A resistor in a sensor circuit is used as a simple way to monitor the function of the circuit. The resistor has a calibrated and known voltage drop and current while the circuit is "normal". If there is a short-circuit, or an open-circuit, the voltage and current will change and trigger the trouble alarm.
Fire alarm using ic 555 and germanium diode dr 25?
A germanium diode controls the movement of electricity; arrows on the circuit depict which way, the 555 timer IC a unified chip capable of alarm and multivibratior functions. A fire alarm can built using these two parts plus resistors, capacitors, transistor, speaker, printed circuit board and 9v battery.
Does a fire alarm circuit have to be in a separate conduit?
yes because of it having a signals, if it mix with power cable, fire alarm circuit get distributed.
Will manually turning off a shunt trip breaker set off the fire alarm?
Manually turning off a shunt trip breaker should not set off the fire alarm. The only way that the breaker could set off the fire alarm when turned to the off position is if there is an auxiliary contact on the breaker connected to the fire alarm circuit. This is not usually done because the breaker should be able to be turned off when doing maintenance on the breakers circuit. If the breaker is a mandatory feed breaker to a piece of fire alarm equipment and shouldn't be left in the off position it might have a trouble circuit connected to the fire alarm panel. A trouble alarm on a fire alarm panel is different from an alarm circuit alarm.
Does a fire alarm control panel need a dedicated branch circuit?
Under the NFPA Fire Alarm Code, the control panel must have its own dedicated circuit, the circuit must be labeled "Fire Alarm Circuit" and the handle of the circuit breaker must be colored red. Other rules apply in jurisdictions that adopt other standards, or for other types of fire alarms. For example, under New Hampshire Fire Code, you are PROHIBITED from installing self-contained smoke alarms on a dedicated circuit.
What does nacs mean referencing fire alarm?
Notification Appliance Circuit
How is a circuit for a fire alarm marked in the panel?
In the US, NFPA 72 (2007) 4.4.1.4.2.2: "Circuit disconnecting means shall have a red marking, shall be accessible only to authorized personnel, and shall be identified as 'FIRE ALARM CIRCUIT'". Other codes, or earlier versions, may have slightly different requirements, such as marking it "FACP" for "fire alarm control panel."
What are the advantages and disadvantages of open circuit fire alarm system?
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A bimetallic strip can be used as a thermostst in a simple fire alarm system Explain in detail how a fire would cause the alarm to sound And describe how it could be activated at a lower temperature.?
a fire would bend the strip which would complete the circuit for an alarm. When cooled, it might complete another circuit, answering your second question.
How a fire alarm would be raised?
The fire alarm device is usually equipped with an Amercium-241 that is alpha emitter source together with a battery derived electric circuit. In the normal situation the alpha particles ionize the air causing circuit connection. However when there is a smoke the circuit is disconnected due to the smoke that acts as an insulator and hence the alarm is actuated.
