# Short Circuit Indicator for Bread board. Design Note 38

Here is a solution to protect your circuit assembly on Bread board. Usually we test the working of a circuit first on bread board before making the PCB assembly. Sometimes due to mistake in wiring, valuable components may destroy through short circuits. With the power on , we still continue the trouble shooting which may further destroy many components. Only after sensing heat or burning, we realize the problem. So here is the simple solution to assemble the circuit on bread board safely. If there is any short circuit in the bread board, the power to the board will cut off and Red LED lights. So immediately we can detach the board from power supply.

The circuit shows the following indications.
1.  Green LED On and Red LED Off– Out put power available and the bread board assembly is perfect.

2. Green LED Off and Red LED On – Board shorted and No output power.

Hook up this circuit in the bread board and give input power from power supply. Ideal input voltage is 5- 12 volts at 500 mA. Take its output and connect to the bread board. That is all to do.

How it works?

It is the trick of two complementary transistors T1 and T2. T1 is the general purpose NPN transistor BC 547 and T2 is the medium power PNP transistor SK 100. It is also available as CK 100 and BEL 100 P. At power on, T1 conducts and pulls the base of T2 to ground potential. Since T2 is a PNP transistor, it conducts when its base becomes negative. So full power is available at the collector T2 which is used to power the bread board. Green LED lights at this time to indicate the availability of output power and no short in the bread board.SK 100 can deliver maximum 800 mA current so almost full supply voltage ( less 0 .6 to 0.8 volts due to D1 and T2) is available in the board.

If the bread board power is ok, Red LED remains off since its cathode is connected to the output and in a high state. When a short  circuit occurs in the board, Green LED extinguishes and Red LED lights. Short circuit in the output blocks the collector current of T2 so the cathode of Red LED gets the current path and it lights. Green LED remains off due to the absence of current from T2.

A Red Flash LED gives more visual attraction so use it if available. You can also connect an Yellow LED with 1K resistor at the right end of bread board across the supply rails to get the indication of continuity of power. Use a heat sink for T2 since it becomes hot during short circuit.

Circuit assembled and tested on bread board

Circuit attached to bread board and showing normal and shorted conditions