Tom Carroll
Preston Linser
Project Overview
• Use keypad to receive 4 numbers from a user.
• Compare these 4 numbers to a combination defined in the code. • If the combo is correct, light an LED.
• If incorrect, play a tone through the speaker. • After a 4th incorrect combo, play a warning siren.
Keypad
• Sparkfun COM‐08653
• 12 keys: 3 columns x 4 rows
• Keys are momentary switches
•
Make a connection between a row
and a column
• Requires current-limiting
resistors and pull-up resistors.
Keypad Issues
• Mechanical switches bounce
• Causes signal inconsistencies for a short period after switch is depressed
• Can be addressed with hardware or software.
• Set‐reset latch, CMOS gate debouncer, Integrating RC circuit debouncer, etc.
• A short software delay (5‐20 ms)
Keypad Issues
• Holding down a key should only be interpreted as one keystroke.
Create a variable to store keystroke data.
Store the value of the last key that was pressed.
• If no key was pressed, store a zero.
• Once the key is released, return the variable to zero.
• This tests for both depression and release of the key.
•
•
Speaker
• Use Pulse‐Width Modulation
• Define a period corresponding to a desired tone frequency.
• Set output pin high for half the period, then low for the remaining half.
• Loop for desired duration of sound.
Circuit Schematic
Pseudocode – Main Loop
Pseudocode – Key Test Pseudocode – Wrong Combo and Beep
Pseudocode ‐ Alarm
Alternate/Further Options
• Use a distinct state machine structure.
• After the 4th incorrect combo, lock the user out.
• Implement a servo motor to control an actual lock. • Use HCS12 Pulse‐Width Modulation hardware.
Questions?