How Fire Alarm Circuit Works

Date:  2021-03-30 21:44:47
2 pages  (501 words)
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The functionality of the fire alarm circuit is based on the switching property of the transistor. The thermistor and resistor R1 form a potential divider network that drives the transistor. The thermistor is the center of the circuit. Luckily enough, thermistors are inexpensive, readily available temperature sensors and can be used for relatively simple temperature measurements. An increase in temperature leads to a decrease in the resistance offered by a thermistor while a decrease in temperature leads to an increase in the resistance of a thermistor. The effect of temperature on resistance is a presence in all resistors. However, the semiconductor material used for thermistor makes them more sensitive to temperature changes compared to the normal resistors. An average drop in voltage across the resistor R1results in turning on of the transistor. The decrease in voltage across the resistor R1 is due to reduce resistance as a result of the opposite effect of elevated temperatures. According to ohms law V (voltage) = I (current) x R (resistance). This depicts direct proportionality between resistance and voltage; a decrease in resistance leads to a decrease in voltage while an increase in resistance leads to an increase in voltage in a circuit (Jee, Lee, Kim, Lee, & Kim, 2012).

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Practically, in atmospheric room temperature is usually about 25oC during which the resistance of the thermistor is high, and hence the voltage across the thermistor is also high. When a heat source is placed close to the thermistor, rise in temperature cause a drop in the resistance and ultimately a drop in the voltage across R1 resulting in turning on the transistor. The threshold voltage can be varied using the potentiometer. Activation of the transistor eventually leads to current flow to the LED resulting in the emission of flashing lights as signs of a fire alarm (Jaseem, 2016).

If the simple alarm system were to be compared with advanced fire detection and alarm system, the LED would be the signaling device. The thermistor works like the heat detectors hence they form initiating devices. The potentiometer and resistor can be compared to the FAPC. Hence, it is evident that the two systems work is similar manner.


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Jee, S., Lee, C., Kim, S., Lee, J., & Kim, P. (2012). Development of a Traceable Fire Alarm System Based on the Conventional Fire Alarm System. Fire Technology, 50(3), 805-822.

Kang, M., Tung, T., & Kim, J. (2013). Efficient video-equipped fire detection approach for automatic fire alarm systems. Optical Engineering, 52(1), 017002.

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