Noise is a serious environmental problem that affects us in our daily life. There is scientific evidence supporting that noise exposure can cause hearing loss, hypertension, heart disease, annoyance, sleep disturbance and decreased performance in schools. Sound-level indicators like the one presented here can help address this problem. This sound-level indicator monitors the sound level and indicates through an LED when the level is above the preset value. Circuit and working Fig. 1 shows the circuit of noise-level alarm . The circuit is built around three BC550 npn general-purpose transistors (T1 through T3), electret microphone (MIC1), two LEDs (LED1 and LED2) and a few other components. Simple Noise-Level Alarm Circuit Diagram Fig. 1: Circuit of noise-level alarm The sound is captured by microphone MIC1 and amplified by first-stage high-gain transistor T1. Trimmer potentiometer VR1 is used to adjust the threshold level. the signal is amplified again with second-stage transisto...