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Cairo University Faculty of Engineering Biomedical Engineering DPT Mohamed Ashref Oransa sec(2) Ahmed Magdy Ahmed sec(1) Ayman Mohamed Anwar sec(1) Ahmed Tarek Reda sec(1) Description. The circuit is as simple and straight forward so that, it can be easily implemented. The thermistor offers a low resistance at high temperature and high resistance at low temperature. 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. Circuit diagram with Parts list. Notes. The circuit can be powered from a 6V battery or a 6V power supply. The thermistor can be mounted on a heat resistant material like mica to prevent it from damage due to excessive heat. The LED acts as an indication when the power supply is switched ON. Components: 1. Ic1 NE555 2. Kit-v-NTC10k 3. BC 548 4. BC 558 5. R 470 Ω ¼ W 6. R 33 K Ω ¼ W 7. R 2.2 K Ω ¼ W 8. R 560 Ω ¼ W 9. R 47 K Ω ¼ W 10. C-10 UF -50 V 11. C-00.01 UF -50 V 12. C-0.047 UF-100 V 13. DI-1N4005 14. LED 5M RED 15. Speaker 8 Ω 0.5 W 16. BD 135 Application: It can be used as a Fire alarm Data sheet : Ic1 NE555: Rev. 1.0.2 Features: • High Current Drive Capability (200mA) • Adjustable Duty Cycle • Temperature Stability of 0.005%/C • Timing From Sec to Hours • Turn off Time Less Than 2Sec Applications: • Precision Timing • Pulse Generation • Time Delay Generation • Sequential Timing Description: The LM555/NE555/SA555 is a highly stable controller capable of producing accurate timing pulses. With monostable operation, the time delay is controlled by one external resistor and one capacitor. With astable operation, the frequency and duty cycle are accurately controlled with two external resistors and one capacitor. 555 is a very commonly used IC for generating accurate timing pulses. It is an 8pin timer IC and has mainly two modes of operation: monostable and astable. In monostable mode time delay of the pulses can be precisely controlled by an external resistor and a capacitor whereas in astable mode the frequency & duty cycle are controlled by two external resistors and a capacitor. 555 is very commonly used for generating time delays and pulses. Pin Description: : Pin No 1 2 3 4 Function Name Ground (0V) Voltage below 1/3 Vcc to trigger the pulse Pulsating output Active low; interrupts the timing interval at Output Ground Trigger Output Reset 5 6 7 8 Provides access to the internal voltage divider; default 2/3 Vcc The pulse ends when the voltage is greater than Control Open collector output; to discharge the capacitor Supply voltage; 5V (4.5V 16 V) Control Voltage Threshold Discharge Vcc