This be circuit warn when be become dark, or Light sensitive switch. By use electronics part the base is important , be the transistor 2N2926 and AC128. For the light sensor also LDR (light dependent resistor) or a resister quickly the light. When have no the light makes Q1 , stop work. The Q2 and Q3 then work, It then send current give buzzer utter loud immediately. By circuit arrangement of Q1,Q2 be the character of Schmitt trigger help testify modify analog signal to digital 2 the level well. And help decrease the condition disturbs all with. The detail is other , see in the circuit.
Circuit Light Sensitive Switch with LDR + 2N2926