Electronic Number lock opens only when the user types in the correct code set earlier. Unauthorized access is impossible since no one can enter without the knowledge of the code. The entry of a wrong code more than twice sets off a buzzer that informs the owner or neighbours of an intruder. Since the locking mechanism is placed inside the house, tampering with the lock is not possible. Multiple access is possible since anyone possessing the code can open the lock. This is not possible with the conventional lock-and-key mechanism unless each user has his own key. The code number to open the lock can be easily changed by the user whenever he wishes. This lock can be used in homes, cars, safes or anywhere where there is a locking requirement. Thus, this innovative project provides a safe and tamper-proof locking solution.

The Electronic number lock has four main parts- a DTMF keypad, a set of thumbwheel switches, a logic circuit and an electromagnetic locking mechanism. The keypad is installed outside the house along with a switch for opening the lock (‘Open’) and a switch for clearing the memory (‘Clear/Lock’). The thumbwheel switches, logic circuit and the electromagnetic locking mechanism are installed inside the house. Two switches – one for opening the lock and another for clearing the memory are also installed inside. The user can set the three-digit code number using the thumbwheel switches. Anyone trying to open the door should, first type the correct code through the keypad. Once the correct code is set he can use the ‘Open’ switch to open the lock whenever he wishes. An ‘Open’ LED (Green) informs the user that the correct code has been keyed in. When the user wants to clear the memory of the correct code, he may use ‘Clear/Lock’ switches either from inside or outside. When this is done, a ‘Locked’ LED (Red) lights up. Hence, when the user leaves the house, he merely has to press the ‘Clear/Lock’ switch to lock the door. Locking from inside is done in the same way using the ‘Clear/Lock’ switch installed inside. Attempts to enter the wrong code more than two times will sets off a buzzer installed in a hidden location. This alerts the owners/neighbours of the presence of the intruder. Only typing the correct code can reset this buzzer. The user can open the lock from inside by just pressing the ‘Open’ switch installed inside.

The keypad circuit consists of a telephone dialler keypad, a dialler IC (UM91214) and a decoder IC (KT3170). The principle of DTMF (Dual Tone Multi-Frequency) is used to generate the BCD code of the pressed digit. Each digit in the DTMF code corresponds to a combination of two discrete frequencies one each from a low and a high group of frequencies, which are generated when any switch on the dialler keypad is pressed. Figure shows such a keypad along with the frequencies associated with each row and column. The keypad is used in conjunction with the dialler IC UM91214 to generate the pair of frequencies. The DTMF signals are decoded by IC KT3170 to give the BCD code of the pressed digit.

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