How to convert a three wire hall effect sensor to a 2 wire sensor?
A two wire sensor surely implies that the device becomes one that operates as a current device. The current would vary between two levels depending upon the magnetic field sensed by the Hall device. You have not indicated if the sensor is an on/off type detector of if it is an analogue device intended to show the relative strength of the magnetic field detected. In either case your target implementation could be designed to work as a 4-20mA type device. For the on/off type design the circuit could simply switch between 4mA and 20mA. The analogue implementation would vary continuously over the 4 to 20mA range. You can find various 4-20mA circuit designs on the web but the easiest is to deploy an IC chip that performs the 4-20mA driver function. Two possibilities to use for this are the Analog Devices AD5750 or the Texas Instruments XTR117.