This code lock provides a high degree of security wholst being a very simle design

This code lock provides a high degree of security wholst being a very simle design. At the heart of the circuit is the type 4022 octal counter. In the non-active state, C2 is charged via R5, so that the reset ( r ) input of the counter is kept logic high. This causes output Q0 to be actuated, while all other outputs are logic low. When S1 is pressed, T1 is switched on via debouncing network R2-C1 and IC1 receives a clock pulse, Also, C2 is discharged via R4-D1, ending the reset state of the counter and enabling it to advance. The time requiredfor R5 to rechafdrge C2, i.e, to reset the counter, is the maximum time that can lapse before the next key is pressed. The above cycle is therefore repeated only when S7 at the Q1 output is pressed in time. When all keys have been pressed in time and in the correct order, Q7 goes high for about 4 seconds to enable driving the unlock circuitry, e.g. a relay driver for an automatic door opener. The code for the lock shown in the circuit diagram is 1704570 : this is but an examlpe, however, and the combination code is readily altered by swapping connections between the counter outputs and the switches. When the 7 digit code os considered too simple to crack, the 4022 can be replaced by an 4017, which makes it possible to add two keys. The means that the number of combination is 10^9 instead of 10^7. The quiescent current consumption of the code lock is negligible at 0.5 uA, so that battery operatio is feasible. The circuit works well from any supply between 6 and 15V.

The accompying photograph shows that the code lock can be built as a very compact unit thanks to the use of a printed circuit board that holds the 10 keys also.Description.This is a simple but effective code lock circuit that has an automatic reset facility. The circuit is made around the dual flip-flop IC CD4013.Two CD 4013 ICs are used here. Push button switches are used for entering the code number. One side of all the push button switches are connected to +12V DC. The remaining end of push buttons 2,3,6,8 is connected to clock input pins of the filp-flops. The remaining end of other push button switches are shorted and connected to the set pin of the filp-flops.The relay coil will be activated only if the code is entered in correct sequence and if there is any variation, the lock will be resetted. Here is correct code is 2368.When you press 2 the first flip flop(IC1a) will be triggered and the value at the data in (pin9) will be transferred to the Q output (pin13).Since pin 9 is grounded the value is 0 and so the pin 13 becomes low. For the subsequent pressing of the remaining code digits in the correct sequence the 0 will reach the Q output (pin1) of the last flip flop (IC2b).This makes the transistor ON and the relay is energised.The automatic reset facility is achieved by the resistor R11 and capacitor C2.The positive end of capacitor C2 is connected to the set pin of the filp-flops.When the transistor is switched ON, the capacitor C2 begins to charge and when the voltage across it becomes sufficient the flip-flops are resetted. This makes the lock open for a fixed amount of time and then it locks automatically. The time delay can be adjusted by varying the values of R11 and C2.

Circuit diagram with Parts list.

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