LED dimming issue

Components & Accessories LEDs
1

So I’m quite confused with what’s happening here, my LED bar is dimming but lights up fully when I touch the bed of my breadboard… the video explains it best. It will also stop dimming if I put my finger on the shift register.

Videos:

https://photos.app.goo.gl/yx85PCBvYJxE5Y1x7

https://photos.app.goo.gl/msYmNL43oxzq9r3G9

I’d like to know what is happening, is there some electrical term for it? Is it because the power cable for the board doesn’t have a ground? Any ideas what I can test?

Details and testing:

I’m running a 3.3V to the shift register power with 165 ohms in series so 20 mA. The shift register inputs have the 3.3V digital output signals with 330 ohms in series so 40 mA. I’ve played with the current by changing the resisters, I’ve tried a different breadboard, different power supplies (usb and barrel), and a few other things… I’m stumped

Products:

Board - https://www.sparkfun.com/products/13321

Shift Register - https://www.sparkfun.com/products/13699

LED bar - https://www.sparkfun.com/products/9936

Circuit pictures:

https://photos.app.goo.gl/rCXDusvdP1HCJJcW6

I can post the code if needed but this seems like an electrical issue, not a code issue

2

Double check to make sure all of your grounds have good continuity throughout your circuit. Sometimes those long power bus bars on breadboards are split in the middle and require a jumper.

3

darrellg raises a good point about the power rails, additionally I’d like to add a couple of things.

  • - You need a resistor per LED on your bar graph, otherwise you risk damaging your LEDs and the 74HC595 with excess current. Individual resistors would work, but it might be easier to use a pair of resistor packs like COM-10855. (https://www.sparkfun.com/products/10855)

  • - I'd also recommend adding a 0.1uF decoupling capacitor across the VCC and GND pins (8 &16) on your shift register. Logic chips like these can sometimes do weird things without a decoupling cap in place and adding a cap should fix that. Your finger touching the base plate might be adding *just* enough capacitance so that the chip works, but to make things reliable, I'd just add a cap. ([https://www.sparkfun.com/products/8375](https://www.sparkfun.com/products/8375))

    • If you do these two things, everything should work for you. :slight_smile: