You have been requested by your Senior Engineer to design a lift control unit (LCU) and prepare a report detailing your proposed solution. The client requires a microcontroller based control unit to automate the operation of a passenger lift that services a three story building. The existing equipment including the winch, lift, cabling, doors and electrical switch gear has recently been refurbished, but the old electromechanical lift controller unit  is  to be replaced. The new LCU is also required to provide a display showing the location of the lift as a floor number. 

The existing  lift equipment includes the lift itself, the doors, the winch, a position sensor, a door ajar sensor, 4 call buttons on three floors and 3 in-lift floor select buttons. The detail for the electrical interface to equipment is shown in Figure 1, and explained below:

• The winch controls are used to drive the lift up(1) or down(0); using run(1) and stop(0) 
• The door control is used to open(1) or close(0) the lift doors.
• The position sensor measures the location of the lift. The range is 0m=0V to 6.4m=5V.
• The door ajar sensor (E) indicates if the doors are ajar(1) or properly closed(0).
• The 4 call buttons (A-D) allow passengers to call the lift to their floor. (Up only on floor 1, up and down on floor 2, and down only on floor 3) 
• The 3 level buttons (F-H) in the lift allow passengers to select the floor they wish to travel to.