A16v3 low voltage inputs

[twostar]

The LD2410 has a TTL output (or at least a 3.3V output) meant for interfacing to Arduinos, it's not meant to be fed into a dry-contact input which relies on pulling the input down to GND to light the optocoupler LEDs.  You could do something like creating a battery of MOSEFT switches to perform the conversion, but that'd require custom circuitry.  Another option which would require surgery on the KC868 is to bypass the optocouplers and feed the LD2410 output directly into the PCF8574 inputs, but then you're losing the protection on the inputs that the optocouplers give you.

You could try asking on somewhere like EEVBlog to see if anyone has any ideas.