## Wednesday, February 19, 2014

### Electronic Circuit Schematic Fire Alarm using Thermistor and IC NE555

Eltronicschool. - We think you must not loss many cost to provide fire alarm system for your house or your project. Although there are many fire alarm system circuit that developed to help people indicate fire when building on fire and others. With built this electronic circuit schematic Fire Alarm using thermistor and IC NE555 you can get fire alarm system that will useful with low cost.

In here we will show you simple Electronic Circuit Schematic Fire Alarm using Thermistor and IC NE555 that you can try implemented for your electronic project.

Circuit Schematic

 Figure 1. Fire Alarm using Thermistor and IC NE555 Circuit (Circuitstoday)

Component List

Please see all components of Electronic Circuit Schematic Fire Alarm using Thermistor and IC NE555 like in figure 1 above.

Description

To know more the electronic circuit schematic of Fire Alarm using Thermistor and IC NE 555 like in figure 1 please follow description below:
1. The IC1 (NE555) is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1.
2. The output (pin 3) of IC1 is couples to base of transistor T3 (SL100), which drives the speaker to generate alarm sound.
3. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2.
4. 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.
5. Capacitor C1 charges up to the positive supply voltage and increases the the time for which the alarm is ON.
6. 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).
7. 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.
Source : Circuitstoday blog

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