Using a MOSFET to generate a particular voltage
Since you're already using an arduino, you could use a closed loop control to get rid of all the external components except the mosfet. Basically, you would connect the analogwrite pin to the mosfet gate, and connect an analogin pin to the sensor, then adjust the analogwrite until you get the voltage you want. Of course, you would need to smooth the voltage either before or after the mosfet, depending on what your sensor is.







simulate this circuit &ndash; Schematic created using CircuitLab

This will not work quite as well as the opamp circuit suggested in the other answer here, and will require programming, but it means that you dont have to add another IC. If you know that the characteristics of your load and power supply wont change, then you could just find the correct values for 5V and 1.4V and hard code them without feedback. 

300mA at 5V means that your load resistance at that voltage can be as low as 15 Ohm. That means the maximum resistance of your mosfet should be around 0.1ohm. The mosfet specified above will actually be fine for that application, although thats actually an n type, whereas the circuit i showed uses a p type. To use that mosfet, all you would do is switch the position of the load and the mosfet.

Edit: Additional filtering on the sensing pin







simulate this circuit