I am working on an invention that is 99% mechanical in nature. However, it requires a closed loop servo, consisting of a linear Hall Effect sensor, whose output is amplified to drive a voice coil. I have come up with the circuit shown below but can't quite figure out how to make it perform correctly. The output of the Hall Effect sensor varies from 0V to 2.5V and in response to that I would like to see R7 (which represents the voice coil) to conduct from 20mA to 100mA. Which resistor values do I have to change to accomplish that? https://www.circuitlab.com/circuit/42gc63byzs44/complete/
**Edit in response to Neil, UK:**The output of the Hall Effect sensor is controlled by a magnet attached to a mechanical slide. The voice coil acts as a mechanical forcer. The voice coil consists of a re-purposed actuator arm from a hard drive. Currently I am controlling the voice coil with a potentiometer and it requires 20mA to 100mA to work correctly.