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I'm kinda new to PCB design, but here we go:

I've created a simple sub-pump water level monitor and it uses a constant 5V from a MOSFET. In the end, it's all monitored by a raspberry pi and I know if I put 5V straight into a GPIO pin as input, that's going to kill the pi very fast so i'm wondering if anybody knows an appropriate resistor to use for this.

PCB Schematic

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  • \$\begingroup\$ Can you provide a schematic? CTRL+M when you edit post for schematic editor. From the sounds of it, you need a clamp diode and resistor, not just a resistor. \$\endgroup\$ – DKNguyen Feb 26 '19 at 3:14
  • \$\begingroup\$ Sure but just letting you know it's a Eagle File \$\endgroup\$ – Nathan Johnson Feb 26 '19 at 3:16
  • \$\begingroup\$ Use the Windows Snipping tool it then and copy paste to the post. \$\endgroup\$ – DKNguyen Feb 26 '19 at 3:17
  • \$\begingroup\$ It's a very simple design as you can tell \$\endgroup\$ – Nathan Johnson Feb 26 '19 at 3:21
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    \$\begingroup\$ Oh, the Pi pins aren't the npn base. You are using the NPNs as open collector outputs to feed the pins? Your NPNs are upside down and your LED resistors are too high. You need base resistors for your float switch too. Then we can worry about 5V on the pins. \$\endgroup\$ – DKNguyen Feb 26 '19 at 3:30
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Just some minor corrections

enter image description here

Murphy's Law.

If anything can be backwards, it will be.

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    \$\begingroup\$ Not really clear where you would hook up the MCU to this. It is a board using 3.3V, not 5V. And the LED forward voltage could be anything. Using a LED in series with the driving voltage is fishy overall. I can't make any sense of the original schematic. \$\endgroup\$ – Lundin Apr 17 '19 at 13:00
  • \$\begingroup\$ The input can be any logic level and output DC should not much more than LED so it could be a 12V string or a 3 LEDs +R or 5V 1 LED+R .. Just make Base current > 5% of LED current by choosing base R. THe question lacks experience \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 17 '19 at 13:03

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