Design and Construction of Fire Detector and Alarm System

Abstract

In this study, self monitoring fire detector and alarm system device was designed and constructed. The device comprises of four modules: Power supply module, fire and fault detector module, alarm sounder module and system reset module. The power supply module produces 27 VDC regulated output from 24V step-down transformer, bridge rectifier and smoothing capacitor of 4700uf/63V. There are two 12V batteries which serve as power back up. The fire and fault detector module incorporates by design; smoke detector which is made up of a phototransistor and an infrared LED. Under normal condition the detector sends a low current signal to the fire and fault circuit, upon detection of fire it sends a high current signal to the same circuit while during a fault condition the detector sends no current signal to the circuit causing an open circuit. The fire and fault detector circuit is implemented with three transistors-T4, T5 and T6, the latter two are connected as master and slave transistors. T4 is used to trigger the fire alarm by monitoring the high current signal of the detector while T5 and T6 are used for monitoring the low current signal and open circuit signals from the detector and therefore monitor both the normal and fault condition of the detector respectively. The sounder circuit was implemented using an LM555 TIMER IC and frequency elements-resistor and capacitor. The sounder circuit generates two distinguishable tones, low frequency tone of about 0.48Hz for the fault alarm and higher frequency tone of about 75Hz for the fire alarm. Finally the system reset circuit comprises of press button switch, a relay and a diode used to reset the fire detector and alarm circuit to normal working condition after an activated alarm condition have been rectified. The device has been tested and the result was very successful.