Troubleshooting LED Running Lights!

I found this LED running lights circuit diagram from elonics.org. It is a well explained and simple project to make and the author has provided detailed step by step instructions. But as is usual when you try to make something on your own, when I assembled the project on a breadboard, the lights would not run! The lights were just flickering. By previous experiences, I was sure that I had gone wrong somewhere and switched to troubleshooting mode.

This was the LED Running Lights circuit assembled on a breadboard before connecting the power. There are 10 LEDs, a 555 IC, a 4017 IC, few resistors and a capacitor. My first thought was that the capacitor might be defective or of wrong value. Took it out and checked with my LCR meter. It was functioning well and of correct value.

Checking resistors in LED running lights circuit

Then I remembered the author’s description of the LED running lights mentioning that this 47K resistor in the circuit could be replaced by a potentiometer to change the speed of running lights. When I checked the resistor in this position in the circuit which I had assembled, it was just 1K. That explained the fast flickering of the lights. This circuit functions by the switching action of 555 IC and decadic counter function of 4017 IC. Each time 555 IC output changes, the 4017 IC switches output from one pin to the next, switching on the LEDs sequentially. So faster switching by 555 IC with lower value of resistor explained the flickering lights.

As soon as I replaced the 1K resistor with the correct value of 47K, I was quite happy to see the running lights functioning as expected. Time and again, troubleshooting involves just checking each component for correct placement. If it had been a faulty IC it would have been more difficult for me to troubleshoot. In fact I had purchased four pieces each of the ICs, in case one was defective. These ICs are easily available and quite inexpensive in the local market.

Trying out higher value of resistor

Just for a curiosity I then replaced the 47K resistor with a 100K resistor to check the response of the LED running lights. I did have 47K and 100K potentiometers in store. But using them would mean more jumper wires. I would also need crocodile clips to connect the potentiometer.

As expected, the ‘running’ speed of LED light came down when I replaced the 47K resistor with 100K resistor. That was a good learning experience and let me thank the author for detailed explanation of circuit function. You can easily try out the circuit and I have placed the link to author’s article in the description box.