07-10-2017, 12:35 PM
Advantages and disadvantages of fire alarm circuit using LDR
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advantages and disadvantages of fire alarm circuit using LDR
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Advantages and disadvantages of fire alarm circuit using LDR
24-10-2017, 09:16 AM
When there is no smoke, the focus light will fall directly on the LDR. The LDR resistance will be low and therefore the voltage across it (below 0.6 V). The transistor will be off and nothing happens. When there is enough smoke to hide the light falling on LDR, the LDR resistance increases and also increases the voltage across it. Now the transistor will turn ON. This gives power to the IC1 and emits 5V. This activates the UM66 (IC2) IC tone generator for playing music. This music will be amplified by IC3 (TDA 2002) to drive the speaker. The resistor R6 is intended to protect the transistor when R4 is rotated towards low resistance values. The resistor R2 and R1 form a feedback network for the TDA2002 and C1 couples the feedback signal from the junction of R1 and R2 to the inverting input of the same IC. Diode D1 and D2 in combination falls 1.4 V to give nominal voltage (3.5V) to UM66. UM66 can not withstand more than 4V.
Circuit diagram with parts list:
![[Image: fire-alarm-circuit1.jpg]](http://www.circuitstoday.com/wp-content/uploads/2008/04/fire-alarm-circuit1.jpg)
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