LED circuit

Thread Starter


Joined Nov 9, 2011
I built one circuit and tested it for several months and it work great.

This week I built 20 circuits, all on the same perforated breadboard.
Each circuit works correctly, as per the attached diagram, during construction and testing.
When I turn the system full on there appears to be several types of interaction problems.

i.e. As an example, each group of four (from the same Digitrax DS-64) has some form of interaction. That is the LED does (or may) not change color and the other three lights flash or change colors.
i.e. It appears one circuit will work OK but when I add two or three circuits the panel will not function correctly.

The power supply I am using is an I.T.E. Power Supply model T481208003CT 12VDC 750 ma.



Joined Apr 5, 2008

I found this thread in the "completed projects" forum.
As it says it is for completed projects, where it will stay invisible until a mod approves or moves it to an other forum.
I did the last options, as you have problems with your project.

You are saying the circuit does not behave well.
Did you use decoupling capacitors at ALL chips?
Decoupling or Bypass Capacitors, Why?



Joined Jul 17, 2007
Bertus speaks the truth!

You must have an 0.1uF/100nF capacitor at each and every IC across its' power pins (in this case, pins 7 and 14), and you should also have a larger capacitor (anywhere between 100uF and 1,000uF) where power enters the board. Without those capacitors, you will have problems just like you are seeing.

It will be very easy to add them due to the way you built the circuit.

The leads for the capacitors should be as short as reasonably possible.


Joined Jul 17, 2007
I didn't think of this before, but something else that may be causing trouble is the "spike" that occurs when current flow through the solenoid coil (switch motor) is stopped. The spike can "ring" electrical connections like the dickens, and that could also causing the associated circuits to toggle states/change LED colors. It could also be that the current flow is being inductively coupled (like a transformer) between sets of wires.

If that's occurring, you may need to add small caps between your 10k resistors and the inputs to the NOR gates to ground. See what the power bypass caps do first.