An operator controllable machine is retrofitted to be operator controlled or to be automatically controlled. An electrical joystick output is provided by a manually actuated joystick control, including a joystick, to a machine control in the manually controlled machine. An interface is added between the joystick control and the machine control to permit an external source, such as a GPS receiver or a laser receiver, to apply an electrical automatic control output to the machine control, as well. The interface includes a number of optical isolation circuits and a microprocessor. The isolated microprocessor output is supplied to the machine control, which produces valve control signals to control the hydraulic valves of the machine. The application of hydraulic fluid to the hydraulic cylinders of the machine is then controlled by either the joystick control output or the automatic control output. In an intermediate range of joystick movement, the hydraulic cylinders of the machine are controlled by both outputs, resulting in a smooth transition between manual and automatic operation.