Here is a valuable Add on circuit to save electronic components in the assembled board from damage. It gives warning beep if there is a short circuit in the assembled board. Thus it helps to switch off the power supply immediately to save valuable components. If the circuit board is OK, Green LED lights indicating that power supply is normal. If there is a short in the PCB tracks or pins of components, Green LED turns off and Buzzer sounds indicating the short circuit.
The circuit uses two NPN transistors BC 547 to sense the short circuit. If the assembled board is normal, current flows from the power supply through polarity protecting diode D1. The Assembled board gets power and Green LED lights. At the same time T2 forward bias and its collector goes to ground potential. This makes T1 off due the absence of base bias. Buzzer and Red LED connected to the collector of T1 remains off. If there is a short circuit in the board under test, D2 reverse biases and T2 turns off. Now the base of T1 becomes high and it conducts. This turns on Buzzer and Red LED to indicate short circuit.
Connect the circuit to the output of the Power supply observing polarity.Output of the circuit is used to power the circuit board under test.