electrical

sensors
The sensor component of our design is relatively simple. Four Fairchild semiconductor photointerrupter sensors (QVA11234) were used to count the number of coins passing each stage of our mechanical system. Each sensor is supplied with a 5 Volt power source in series with a 300 Ohm resistor. The signal obtained from each sensor is debounced with a Schmitt Trigger (chip 74LS14). The debounced signals are then fed into the Handyboard digital inputs.

motor control
Because the motor of our system encountered high amounts of friction which in turn caused the motor draw high amounts of current, a power supply larger than what the Handyboard could provide was needed. To do this we used a relay (275-241 from RadioShack.) The motor was connected to a 12 Volt power source from the Breadboard with the relay coil controlled by the motor outputs of the Handyboard.

keypad
We decided to use a matrix-encoded style keypad with our project due to its simplicity of use and low cost. Unlike many other keypad styles, matrix-encoded keypads require inputs and outputs and some extra code to make it function. A diagram of the keypad circuit is shown below:

A 4x4 keypad requires 4 inputs and 4 outputs. We designated the rows to be our inputs and the columns to be our outputs but it could be done the other way just as well. A key press is determined by activating a row and then checking to see which column is active. We activate a row by setting the pin for that row to ground and setting all the other row pins to 5 Volts. We use ground to designate the active row because of the Handyboard's active low digital inputs. With one row active the keypad circuit becomes a set of 4 switches in parallel. A closed switch will send ground to the corresponding column output pin.