Clock & Timer Circuit DiagramsElectronic Keys & LocksLCD-LED Display

Hard-Wired Code Lock Schematic Circuit Diagram

Securing Restricted Access: Introduction

In installations with limited access, safeguarding through an access code is common practice. For relatively straightforward code modification needs, a static design with the access code hardcoded into the hardware suffices. This code is set via wire links inserted into the IC socket illustrated on the left in the schematic diagram. The diagram displays the configuration of the code as ‘0280,’ which users input using buttons S1-S10.

Key Components and Setup

The primary components pivotal to this circuit include four NAND gates (4011 CMOS IC) and a counter featuring ten decoded outputs (4017 CMOS IC). Resistors R1-R4 are instrumental in maintaining the four pushbutton inputs at ground level when no button is pressed. Interestingly, no special measures are necessary to debounce the switch signals, as the circuit inherently disregards repeated button presses. In its resting state, a ‘1’ is applied to the reset input (pin 15) of IC2, causing output Q0 (pin 3) of IC2 to be the sole output at a ‘1’ level. Simultaneously, all other counter outputs remain at the ‘0’ level.

Hard-Wired Code Lock Schematic Circuit Diagram

Indicating Code Entry Readiness with Dual LED D6

Dual LED D6 serves as an indicator, illuminating in red to signify that a code can be input. When the button corresponding to the first digit of the preset code is pressed (for instance, the ‘0’ button), IC1a’s output switches to ‘0,’ pulling the reset input of IC2 to ‘0’ via D1 and D5. Upon button release, a rising edge at IC2’s clock input increments the counter by 1.

Setting the Time Limit and Waiting for Button Press

R6 and C1 create a delay of about 10 seconds, holding IC2’s reset input low. Output Q1 (pin 2) registers as ‘1’ due to the clock pulse, with IC1b awaiting the press of the second button (‘2’) within this timeframe. Failure to do so discharges C1 through R5 and R6, resetting IC2. The dual LED returns to red, prompting the user to re-enter the code.

Successful Code Entry and Output Activation

Correct pressing of the remaining digits within 10 seconds leads Q4 output (pin 10) to ‘1,’ changing the dual LED to green. After another 10 seconds, C1 discharges, reverting the dual LED back to red. Q4 output can be employed to activate devices like an electronic door latch.

Code Modification and Input Validation

Changing the code merely involves altering the configuration of wire jumpers in the IC socket. The combination of diodes D1–D4 and R5 functions as an AND gate. If any of the four NAND gate outputs in IC1 are ‘0’ (indicating a correct button press), a ‘0’ is applied to IC2’s clock input while the button remains pressed. For those desiring a shorter delay than 10 seconds, adjusting the values of R6 or C1 can achieve the desired timing, which is approximately R6 multiplied by C1 (R6 x C1).


Related Articles

Leave a Reply

Your email address will not be published.

Back to top button