Posted on Apr 21, 2012 7236
As you can see, the 3x4 keypad uses only 4 I/O ports. To determine what key is pressed, I read the discharge time of each capacitor. This is a new technique that I developed since two years ago but finally I was able to test it successfully. The only inconvenience of this method is the reading from the keypad is affected by the voltage of the circuit. Anyway, the circuit needs exactly +5V because the LCD. The switch labeled as "Toggle" and "Normal" defines the function of the security keypad. With the "Toggle" setting, the "Enable Signal" will toggle each time the correct password is entered. With the "Normal" setting, each time when the correct password is typed, the "Enable signal" will be ON for only four seconds. Activate an electric or electronic device enable by the signal. Example: A car or any electric machine cannot be turned on until the correct password is provided. To enable or disable a central alarm. Keep a circuit ON or OFF that requieres authorization. (Maybe to keep children without TV until the homework is donde). Security vault - Input the code and an electromagnet will be enable during 4 seconds allowing to open the door. Security entry port - Only authorized employees can input the code enabling the electromagnetic door lock for 4 seconds. Enable a circuit with one pulse to toggle it.