I've got a pretty basic circuit, running an 10-segment LED bar graph through two TI HC595 shift registers. The very strange thing is that when I run the whole setup, and turn on a few of the LEDs, they only light up if I touch either the legs off the arduino that run it. Here's the circuit:

enter image description here

I'm assuming it has something to do with my hands' capacitance, but i don't understand what to do about it.

  • \$\begingroup\$ Is your symbol for your powerplug correct? ie +ve is thus connected to GND \$\endgroup\$
    – user16222
    Apr 26, 2014 at 21:18
  • \$\begingroup\$ that's not a plus sign, it's the symbol "handle" for moving things around in eagle. the polarity is correct. it's a center-positive plug. \$\endgroup\$
    – kolosy
    Apr 26, 2014 at 21:21
  • \$\begingroup\$ What are: The chip on the lower right, the SOL_Gn and PSU_LN signals? \$\endgroup\$
    – gwideman
    Apr 26, 2014 at 21:34
  • 1
    \$\begingroup\$ Where are your decoupling caps? \$\endgroup\$
    – Passerby
    Apr 26, 2014 at 21:41

1 Answer 1


In general this sort of symptom happens when you have a floating input. This suggests that there's some input, probably to the 595's, that you think is connected but isn't. Could be on the Arduino, if it's something the Arduino code is waiting for. Could even be a wire that's connected, but the Arduino code forgot to set that pin to output.

  • \$\begingroup\$ whew. thanks. left the serial clear line floating, which caused all this havoc. \$\endgroup\$
    – kolosy
    Apr 26, 2014 at 22:02

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