This is a design for fire alarm circuit. This circuit is based on a LDR and lamp pair for sensing the fire. The alarm will works by sensing the smoke produced during fire. The circuit produces an audible alarm when the fire breaks out with smoke. This is the circuit of the circuit.
For operation the circuit will work 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 .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 do the voltage across it. Now the transistor will switch to 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. The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V) to UM66. UM 66 cannot withstand more than 4V.
The speaker that can be used for this circuit is a 8 ohm tweeter. The POT R4 can be used to adjust the sensitivity of the alarm circuit. The POT R3 can be used for varying the volume of the alarm. There are general purpose NPN transistor (like BC548, BC148, 2N222) can be used for Q1. The circuit can be powered from a 9V battery or a 9V DC power supply. Instead of bulb you can use a bright LED with a 1K resistor series to it.
For operation the circuit will work 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 .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 do the voltage across it. Now the transistor will switch to 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. The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V) to UM66. UM 66 cannot withstand more than 4V.
The speaker that can be used for this circuit is a 8 ohm tweeter. The POT R4 can be used to adjust the sensitivity of the alarm circuit. The POT R3 can be used for varying the volume of the alarm. There are general purpose NPN transistor (like BC548, BC148, 2N222) can be used for Q1. The circuit can be powered from a 9V battery or a 9V DC power supply. Instead of bulb you can use a bright LED with a 1K resistor series to it.
