Schematic and PCB layout tidied up to look more professional.
Open in Editor to see the changes.
Redrawn the schematic to make it more readable and to clarify signal and power flow and decoupling cap placements.
Note that the 2 pin output connector now has the unused pin grounded. Previously, this pin was floating.
It is a bad idea to have a single wire connecting the output to some other point especially as the output resistance is 10k. Such a a single wire wire connection forms a loop with the ground connection via the input and/or 12V supply connections. This loop acts as an inductor and allows noise to be injected into the signal being fed to the Arduino input. This in turn may cause measurement errors due to pulse miscounting.
Redesigned PCB layout to use thicker traces and to place decoupling caps and comparator feedback resistors closer to where they need to be and to make signal and power currents flow logically through the PCB. This also simplifies tracking and maximises effectiveness of copper areas.
Note that all tracking is now on the bottom layer except ground which is a single, continuous copper area on the top layer.
Check out the Schematic and PCB Design managers (Design button in the left hand panel)
Note: no DRC errors in the PCB.
PCB corners rounded.
Because we can.
And it costs no more.
Open the PCB up and play with it to see how the copper areas work and to see how net names help readability and checking using the Design Managers.