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Before, I asked some ways to solve the EMI problem and I got some useful answer from here: Really occasional malfunction of the Schmitt-trigger input - Really occasional malfunction of the Schmitt-trigger input However, another problem has risen.

My board has some adjacent LEDs near the 'trouble making area', and I revealed that sometimes they are powered off by my hand(Of course, I didn't touch the LEDs, I just brought my hand near the lights). For the 74HC14 problem, I added some capacitors to reduce the effect of the EMI and that was really successful. But, what can I do for these LEDs? LEDs are just connected by the MCU output pin, series connection with a resistor. For example, the part of the LED circuit will be MCU PIN->Resistor->LED->GND.

Ironically, this output pin indicates whether the network is on or off and even if the LED is off, the WiFi chip keeps connected to the server. So I think it is just output-related issue.

Would just adding some capacitors near the output pin going to GND be okay? If this is applicable, could I apply to similar situation?

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  • This is the part of the circuit. The board is perforated and there are so many adjacent connections(especially for 74HC14, near the LED).

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These are somewhat more general circuit(showing nearly located circuits).

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    \$\begingroup\$ No, adding capacitors that way won't do anything. This is a bit of a shot in the dark but what if you put a high value resistor (maybe 10K) in parallel with the LED? \$\endgroup\$ – DKNguyen Feb 27 '19 at 0:14
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    \$\begingroup\$ You can't just hold a resistor to the LED pins on the board to test it out? If it fixes it you could just solder it straight on top of the LED. Or is this surface mount? \$\endgroup\$ – DKNguyen Feb 27 '19 at 0:18
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    \$\begingroup\$ A capacitor might also work across the LED since it would short out EMI across the leads that might be turning it on. \$\endgroup\$ – DKNguyen Feb 27 '19 at 0:34
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    \$\begingroup\$ What LED specifically, are you using? I've soldered similar SMD packages directly on top of each other before so if it matches an SMD component size you could do that. Put a cap or resistor right under the LED. But just test it out by holding a through hole resistor or capacitor to the LED and see if it helps. As I said before it's a shot in the dark so I don't even know if it will work. \$\endgroup\$ – DKNguyen Feb 27 '19 at 0:42
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    \$\begingroup\$ I don't suppose you have 1k+ 2012 resistors as well do you? Or capacitors? Probably not since 2012s are really expensive. LED pin wire? I thought these are SMD. Post a photo of the circuit. \$\endgroup\$ – DKNguyen Feb 27 '19 at 0:51

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