Fire Alarm Circuit
Fire Alarm circuit using LDR: This is a simple fire alarm circuit based on a Light Dependent Resistor (LDR) and lamp pair for sensing the fire. The alarm works by sensing the smoke produced during the fire. The circuit produces an audible alarm when the fire breaks out with smoke.
Fire Alarm circuit
Working on Fire Alarm circuit using LDR
When there is no smoke the light from the bulb will be directly falling on the LDR. The LDR resistance will be low and so the voltage across it (below 0.6V). The transistor will be OFF and nothing happens. When there is sufficient smoke to mask the light from falling on LDR, the LDR resistance increases and so does the voltage across it. Now the transistor will switch ON.
This gives power to the IC1 and it outputs 5V. This powers the tone generator IC UM66 (IC2) to play a music. This music will be amplified by IC3 (TDA 2002) to drive the speaker. Resistor R6 is meant for protecting the transistor when R4 is turned towards low resistance values. Resistor R2 and R1 form a feedback network for the TDA2002 and C1 couples the feedback signal from the junction of R1 & R2 to the inverting input of the same IC.