I was assigned to choose an existing project and design a custom PCB for it. My project involves LEDs arranged in a circle that light up sequentially, while the LED in the center is supposed to stay on continuously. After soldering everything together, the circuit didn’t work. When I measured the operation of each multivibrator and the CMOS 4017 counter with an oscilloscope, everything seemed to function correctly, but the LEDs didn’t light up.

When I accidentally shorted the cathode of LED 8 to the negative terminal of capacitor C2, the circular LEDs started working correctly, but the center LED still didn’t light up. What could I have done wrong, and where might the problem be?

Im adding in the files the original project documentation, my LTspice simulation, and my eagle project.

Thats my board:


All help would be greatly appreciated

 

I would start by checking that the ground is actually common to all the common points in the circuit.

 

Perhaps when you shorted the cap you created the imbalance needed to start the oscillators.

As far as the center LED not lighting, I would consider it might be shorted somehow or there is another path to ground shunting it.

I wouldn't let the circuit run without that LED being lit, as you risk the others.

 

I would start by checking that the ground is actually common to all the common points in the circuit.

I got all of my elements grounded the same way as in the original project, just checked

 

Perhaps when you shorted the cap you created the imbalance needed to start the oscillators.

As far as the center LED not lighting, I would consider it might be shorted somehow or there is another path to ground shunting it.

I wouldn't let the circuit run without that LED being lit, as you risk the others.

The problem is i need to keep shorting the cap, for it to work. When i stop doing that it stops working.

 

Another thing that i forgot to add is that when I measured the voltage and current on the LEDs, the issue I noticed was that there is voltage present, but it's negative, and no current flows at all.

 

Okay, so i shorted the kathode of the middle diode D9 with the ground, and then the whole circuit works properly exepct diode D2, which doesnt turn on at all. Is it possible that my GND route is broken somewhere, and that D2 diode is broken?

 

With a quick look, I see 2 possible problems:

● Looks like D9 (the center one) is connected backwards. Its common connection is reversed compared to all the other LEDs.

● I don't see any connection to power for the LED common connection.

 

With a quick look, I see 2 possible problems:

● Looks like D9 (the center one) is connected backwards. Its common connection is reversed compared to all the other LEDs.

● I don't see any connection to power for the LED common connection.

View attachment 348986

The middle one is reversed, because it's supposed to be turn on all the time. The diode routes are good like in the original project.

 

Oh, I see the schematic now. That's kind of bizarre.

 

R11 equals zero ohms.
oh boy! No current limiting for the LEDs, and relying solely on the 4017 maximum Ioh?