Grounding a wire using a chunk of metal?

Question

So I've made a complex LED grid and it works but has ghosting in some of the rows.

I've given up trying to work out the reason why, but have noticed that when I touch the SCLK wire, or connect the SCLK wire of the TLC5940 LED driver chip to a pair of scissors, or any big chunk of metal, it also fixes the ghosting (pretty much)

So I'm happy with this solution, but would like to understand what's going on. If i connect it to ground instead, it doesn't work. But i presume the chunk of metal has a grounding effect?

Try putting a 47pF capacitor from the wire to local 0 volts. I have a suspicion that the fast edges may be causing ringing and the ringing may cause ghosting. Putting a cap on the wire slows the edges and reduces the ringing. — Andy aka, Apr 11 '15 at 16:39

score 2 · Accepted Answer · edited Apr 13 '17 at 12:32

2

What the chunk of metal does is to capacitively couple your circuit to ground. Have a look at this post: Grounding the chassis with capacitor , where the poster notice the difference between connecting directly to ground and connecting via a capacitor.

edited Apr 13 '17 at 12:32

Community

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answered Apr 11 '15 at 16:14

Sredni Vashtar

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SCLK to capacitor to ground didn't work for me (i only tried in the nanofarad range), but it turned out to be my computer's USB power supply adding noise, so problem solved. – djb Apr 12 '15 at 16:29

score 0 · Answer 2 · answered Apr 12 '15 at 16:19

So just out of interest, the problem ended up being that i was testing my circuit, which is Arduino controlled, via USB to my computer, and for whatever reason, my computer introduced noise into the circuit. Plugging the USB into a battery instead fixed the ghosting issue. So in the end, I don't need a chunk of metal.

Grounding a wire using a chunk of metal?

2 Answers2