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 NXP Semiconductors

  • Digital Code Lock

  • Created: Feb 10, 2014

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This circuit is a simple code lock circuit with an automatic reset facility. The main component of this circuit is the dual flip-flop integrated circuit CD4013. The 4013-IC, is a dual D-type flip-flop CMOS logic chip; for this circuit, two 4013-IC’s are used. In entering the number code, push buttons that correspond to a digit are used. A +12V DC supply is used to power the pushbutton switches. One end of each switch corresponding to the correct code is connected to the voltage source, while the other ends are connected to the clock input pins of the flip-flops. For the switches that are not included in the correct code, the ends are connected to the ground.

On the event that the correct code is entered, the relay coil will be activated; on the other hand, if a wrong sequence is entered, the lock will reset. In the sample circuit, the correct sequence is set to “2368”. Pressing the switch corresponding to “2” will trigger the first flip-flop and the value data from pin 9 of IC1 will be transmitted to the Q output pin 13. The value of pin 13 will become LOW since pin 9 will have the value of zero because it is grounded. The succeeding pressing of the rest of the code digits in correct order allows the “0” to reach the Q output pin 1 of the second flip-flop. This operation will switch the transistor ON and energize the relay. When the transistor is turned ON, The capacitor C2 charges and when the voltage across it becomes sufficient, the flip-flops will reset. This means that the lock will be open for a fixed time and it will then be locked automatically; the amount of time for this can be adjusted by changing the values of R11 and C2.

This circuit can be employed for access control and security purposes. Further improvements in this simple system will allow additional functionalities that would allow it to be used in the household or offices.