IC555 Fuse indicator cum pilot lamp

This is the circuit of IC555 Fuse indicator cum pilot lamp. You can use this circuit for mini project application in your school or college. The circuit is very simple to construct and uses simple trick that whenever, the external capacitor C1 is fully charged the output of the IC remains LOW, but when the fuse in the circuit is blown out, the indicator LED starts flashing. You can check the working of the circuit in the following video.

ic555 fuse indicator cum pilot lamp circuit
Practically tested circuit of IC555 fuse indicator cum pilot lamp. You can use it for mini project submission in your school or college


On the left side of the circuit there is a power supply battery of 9V. You can connect any supply from 3V to 18V, as the IC555 can work properly in this range. On right side of the circuit, there is one socket shown. You can connect any instrument or circuit to this socket to run on the input voltage of this circuit.

The circuit works in dual condition. When the fuse in the circuit is NOT BLOWN, the capacitor C1 remains charged through diode D1. So the output of the IC remains in LOW state and the LED lights up.

However, whenever the fuse BLOWS OUT, the capacitor C1 can no longer remain charged through D1, so it gets discharged through R3, into pin-7 of the IC. The output of the IC becomes HIGH during charging of the capacitor and becomes LOW during discharging of the capacitor.

So the LED starts flashing indicating that the fuse is blown out. In this way, the circuit serves the dual purpose, as an indicator or pilot lamp and also as fuse blown indicator.


To change the flashing rate of the LED, you can reduce the value R3 or C1 or even both. The rate at which the LED would flash can be calculated by using the following formula:

Remember that in calculations, you must use resistor values in ohms and capacitor value in Farad. To convert resistor value from kilo ohm to ohm, multiply by 103, and to convert value of capacitor from microfarad to farad, multiply by 10-6.

Note: In this circuit, the charging time t1 is slightly more than the discharging time. But it is negligible as the value of R1 is very small as compared to R3.

PCB Layout

The practically tested PCB layout of the above circuit is given below. The following image is NOT size proportional. If you want to get the PCB layout of this circuit for actual construction, then contact us. We shall send you the actual size PCB layout for free.

ic555 fuse indicator cum pilot lamp PCB
The practically tested PCB layout of IC555 fuse indicator cum pilot lamp PCB. For actual size PCB layout contact us

Video of actual working

ic555To understand the working using animation, you can download the Circuit Wizard file of the same project. You can run this circuit to actually see the voltage levels, current direction, etc. For complete details of IC555, refer to our most popular article on IC555.