MOSFET switch replacement not working
Measure the MOSFET source (connected to scanner) when the MOSFET is ON.
You'll find it does not pull down to ground as the MOSFET needs a voltage of about Vgsth to just start to turn on. You are probably getting 1+ Volts source to ground. source-

Driving an NChannel MOSFET , source to ground, drain to scanner should work. The drive sense will be inverted. This can be fixed if needs be. 

If you wish to use the P Channel FET try a 100k scanner in to ground to see if it turns on. If not try lower R till it works. Then pull scanner input UP with MOSFET, source to V+, drain to scanner in. Again, polarity reversed.



Magic :-): 

This circuit converts 


A High / Ground signal at the MOSFET source   


into   


An open_circuit / ground output at the MOSFET drain.. 


The external input voltage swing and current sinking ability has not been specified.
The circuit below will convert a 0 to V1 signal to 0 to V2 where V2 > V1, OR to a 0 /Open circuit output.

In this case when Vin is high, when (Vdd-Vin) &lt; Vgsth_FET the FET will be off.
When Vin = 0, if Vgsth_FET &lt; Vdd then FET will be on.
The input line needs to be able to sink the current from S1 - which is said to be low show it should work.

N Channel FET.
Switching polarity as original.  Input low = on.
Gate to V+, drain to scanner, source to external input.







simulate this circuit &ndash; Schematic created using CircuitLab