The following circuit is a residential alarm the relay has t

The following circuit is a residential alarm, the relay has the following specifications: 12V - 30mA; X1 and X2 are sensors(wires) that when broken makes the transistor reach saturations. The available transistors are shown in the table. Project R(choosing the correct transistor) which make the relay works so the buzzer gets activate when the wires are broken.

Solution

Among all transistors given above Bd135 is high strength.The Bd 135 transistor will is a BJT(Bipolar Junction Transistor).

Its operating temperature is 150 degree celsius.

so,the withstanding capability is high.

so this transistor can work so the buzzer gets activate when the wires were broken .