Light-sensitive and functioning fire alarm circuit
The circuit diagram of the light-sensitive fire alarm is shown in Fig. 1. The L14F1 (T1), transistor BC547 (T2) and the NE555 (IC1) timer are constructed around the photo transistor L14F1.
T1 can detect infrared light and visible light. NE555 based on a multi stable vibrator is used for audible warning. The resistors R3 and R4 and the capacitor C1 determine the frequency. Here, the frequency is around 272Hz, which can be changed by changing the values of R3, R4 or C1.
When T1 detects light, T2 stops driving. As a result, the reset pin 4 of IC1 remains high. This causes the multivibrator to operate and an alarm to sound on the LS1 speaker.
Construction and testing
A single-size light-sensitive fire alarm PCB is shown in Fig. 2 and its arrangement of components in Fig. 3. After mounting the circuit on a PCB, enclose it in a suitable plastic box.