Electronic Keys & Locks

Lock code unit with auto reset

Design and Operation of the Code Unit for Electromagnetic Door Lock

The designed code unit serves the purpose of controlling an electromagnetic door lock using a four-digit code, which automatically resets after a brief period.

Key Assignments and Bistable Connections

In the schematic, the keys responsible for setting the code are labeled S1-S4. Each key corresponds to a bistable (flip-flop) in IC1. When key S1 is pressed, the set input of the first bistable (pin 4) goes high, causing the associated Q-output at pin 2 to become high as well. Subsequently, pressing key S2 results in the input of the second bistable (pin 6) going high, along with its associated Q-output (pin 9). The same pattern applies to keys S3 and S4.

If these keys are pressed in the correct sequence, a logic high level appears at the Q-output (pin 1) of the fourth bistable. If any of the other keys (S5-S10) are pressed during this process, the set bistables are reset through R9, adding an extra layer of complexity to the code, making it more challenging to decipher. When pin 1 of IC1 is high, transistor Ti is activated, leading to the energization of the relay responsible for operating the door lock.

Lock code unit with auto reset Schematic diagram

Setting Up the Initial State for Key S1 Operation

Before key S1 is activated, the Q-output of the first bistable is low, maintaining its reset input at a low level through R1, D1, and R5. This configuration ensures that the potential across C1 is nearly equal to the supply voltage. Upon pressing S1, pin 2 goes high. Due to the presence of D1, capacitor C1 discharges slowly through R5 over approximately seven seconds. During this period, the reset inputs (pins 8) of all four bistables transition to a logic high state. If the relay is activated during this interval, it will deactivate. If the entire code hasn’t been inputted, the process needs to be restarted. Lengthening the reset time is possible by increasing the capacitor C1’s value.

Customizing the Keyboard Configuration

The keyboard configuration can be adjusted to include more or fewer switches. However, having fewer than six switches makes the code vulnerable to being easily broken. Conversely, if there are more than 10 switches, the additional ones can be wired in parallel with S5-S10. To expand the code to eight digits, a second 4043 can be connected in series with ICI. Pin 1 of the second 4043 is linked to key S5, and the subsequent three keys are connected to the second 4043 in the same manner as S2-S4 are linked to IC1.

Addressing a Potential Issue

It’s important to note a minor issue: if switches S1-S4 are pressed simultaneously, the lock will open. This situation can be prevented by designating one of the keys from S5-S10 as a reset switch and ensuring that adjacent keys are not selected for S1-S4.

In quiescent operation, the circuit draws a current_ of about 10 mA.

The relay should be a 12 V type whose draws a current not exceeding 100 mA.


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