Driving the stepper motors
Each stepper motor has six wires coming out of it: two brown wires and blue, orange, white and red wires. In order to drive the motor the brown wires need be connected to the power supply and the other four have to be excited in sequence. In order to find the appropriate voltage for the power supply, we found out that the stepper motors work properly with voltages above DC 8 volts and since we preferred for the motors not to get very hot, we used DC 12 volts as the power supply for the motors. To drive the stepper motors with appropriate voltage, we used TIP31C NPN configured as in ULN 2003 IC, figure below.
In order to stamp, we used two solenoids to have enough power to put the stamp on the envelope and then bring the stamp to its initial position as quick as possible. The proper power was achieved when exciting the stamp with 10.5 - 12 V DC. For this application we also used the same driving system as we used for the stepper motors. (As in ULN 2003)
The brain of the system is the Atmel 8515 micro-controller. The stepper motors, LCD, keypad and the stamps are connected to input/output ports of the MCU. The schematic of the circuit and connections is as in below figure.
The user interface was done through an LCD and a keypad connected to PORT A and PORT C of the MCU respectively. The schematic of the connections is as below.