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Alarms - Electronic Security System

2 (Ee Project) Alarms - Electronic Security System | CIRCUIT IDEAS ELECTRONIC SECURITY SYSTEM K. BHARATHAN This reliable and easy-to-operate electronic security system can be used in banks factories commercial establishments houses etc. The system comprises a monitoring system and several sensing zones. Each sensing zone is provided with a closed-loop switch known as sense switch. Sense switches are fixed on the doors of premises under security and connected to the monitoring system. As long as the doors are closed sense switches are also closed. The monitoring system can be installed at a convenient central place for easy operation. Fig. 1 shows the monitoring circuit only for zone 1 along with the common alarm circuit. For other zones the monitoring circuit is identical with only the prefixes of components changing as per zone number. Encircled points A B and C of each zone monitoring circuit need to be joined to the corresponding points of the alarm circuit upper half of Fig. 1 . When zone 1 sensing switch S11 zone on off slide switch S12 and system on off switch S1 are all on pnp transistor T12 reverse biases to go in cut-off condition with its collector at around 0 volt. When the door fitted with sensor switch S11 is opened transistor T12 gets forward biased and it conducts. Its collector voltage goes high which forward biases transistor T10 via resistor R10 to turn it on. Capacitor C10 serves as a filter capacitor. As a result the collector voltage of transistor T10 falls to forward bias transistor T11 which conducts and its collector voltage is sustained at a high level. Under this latched condition sensor switch S11 and the state of transistor T12 have no effect. In this state red LED11 of the zone remains lit. Simultaneously the high-level voltage from the collector of transistor T11 via diode D10 is applied to VDD pin 5 of siren sound generator IC1 UM3561 whose pin 2 is grounded. Resistor R3 connected across pins 7 and 8 of IC1 determines the frequency of the in-built oscillator. As a result IC1 starts .