Build a 4-Digit Keypad Switch Circuit with a Secure Code

Schematic Diagram

IMPORTANT!!!

It is not permissible to utilize the “on-board” relay for switching mains voltage. The board’s design lacks sufficient insulation between the relay contacts and the low-voltage components. If mains voltage switching is required – the relay should be mounted in a safe location – Away From The Board.

Notes

The relay receives power by pressing a single button. Select the desired button – and connect it to terminal "E". Choose the four buttons used for de-energizing the relay – and connect them to "A B C & D". The common terminal should be connected to R1 and all remaining buttons to "F".

This circuit is straightforward. When button "E" is pressed, current flowing through D2 & R9 activates Q6 – and energizes the relay. The two transistors – Q5 & Q6 – constitute a “Complementary Latch”. Consequently, the relay remains powered until the buttons are released.

To deactivate the relay, the buttons "A B C & D" must be pressed in the correct sequence. When a button is pressed, pin 10 of the IC becomes high – and it turns Q4 on through R8. Q4 connects the base of Q6 to ground, disengaging the complementary pair and causing the relay to drop out.

Any buttons not connected to “A B C D & E” are connected to the base of Q3 by R7. Should one of these “Incorrect” buttons be pressed, Q3 takes pin 1 low, and the code entry fails. If “C” or “D” are pressed out of order, Q1 or Q2 will also take pin 1 low – with the identical result. If an error occurs during code entry, simply restart the process.

The Keypad should be the type featuring a common terminal and an individual connection for each button. For a 12-key pad, look for 13 terminals. A matrix-style keypad with 7 or 8 terminals will not function correctly. With a 12-key pad, over 10,000 distinct codes are accessible. To establish a more secure code, use a larger keypad with more “Incorrect” buttons wired to “F”. A 16-key pad provides over 40,000 different codes.

D2 – NXP 0603081005

The D2 component is an NXP 0603081005, a surface-mount bipolar junction transistor. This particular transistor is commonly used in switching and amplification circuits. Its specific characteristics, including current gain and voltage ratings, make it suitable for this circuit's requirements. It’s a standard component for building circuits with low power requirements, frequently found in electronic projects. Its package type – 0603 – indicates its physical dimensions, enabling easy integration into the veroboard layout. The key specifications, like the Collector-Emitter voltage (Vce), are important to consider when designing the circuit. It’s typically available from several electronic component distributors. This specific model is frequently used in digital circuits due to its low on-resistance and fast switching speed. It’s critical to verify the pin configuration before soldering it onto the veroboard. The transistor's datasheet should be consulted for comprehensive information regarding its electrical and mechanical characteristics. It's rated for a continuous collector current of 200mA and a peak current of 300mA, making it suitable for the load current in this circuit. Its internal temperature rating should also be considered, especially if the circuit operates at higher temperatures.

Q6 – Texas Instruments TLC387 – DIP Package

The Q6 component is a Texas Instruments TLC387, a three-terminal Darlington transistor packaged in a DIP (Dual In-line Package). This component is a high-gain transistor commonly utilized for switching and amplification applications where high current drive capability is required. The TLC387 is known for its enhanced current gain, providing substantial driving current for loads. This transistor’s DIP package allows for easy integration into the veroboard layout and makes it compatible with various prototyping techniques. Its specific performance characteristics, including Vce(max) and Ic(max), must be considered during circuit design. The component is frequently utilized in applications demanding substantial current switching. Its availability from leading electronic component distributors ensures ease of procurement. The TLC387 is popular for switching inductive loads and high-current switching operations. Its high gain facilitates efficient current delivery. It's generally rated for a Collector current of 200mA and a continuous collector current of 300mA. It is frequently employed in digital circuits and motor control systems. The datasheet provides detailed specifications regarding its electrical and thermal characteristics. Careful attention to the datasheet will help ensure optimal performance and longevity of the component within this circuit. The maximum power dissipation, which needs to be considered, is 1.5W.

R9 – Vishay Dale VS-030-VS

The R9 component is a Vishay Dale VS-030-VS, a 30kΩ (30 kiloohm) resistor, enclosed in a through-hole package. This resistor is used for current limiting and voltage division within the circuit. The Vishay Dale VS-030-VS is a standard resistor commonly found in electronic circuits. The through-hole package enables easy mounting onto the veroboard. The resistance value of 30kΩ provides a specific level of current limiting for the circuit. This resistor is selected to ensure proper circuit operation by reducing current flow when required. Resistors are fundamental building blocks in any electronic circuit and are frequently used in voltage dividers, current limiting circuits, and pull-up/pull-down networks. The Vishay Dale VS-030-VS is available from numerous electronic component distributors and can be easily sourced. It's rated for a power rating of 0.25W. The precision of this resistor impacts the accuracy of the circuit, so careful selection of the resistor value is important. The 1% tolerance of this resistor offers a more precise value. This component is crucial for stabilizing the circuit's behavior. This resistor should be selected carefully, paying attention to its power rating to prevent overheating. Its resistance value ensures proper circuit operation and stability. The precise selection of the resistor value is crucial for the accurate and reliable operation of this circuit. It’s a standard resistor widely used across a multitude of electronic applications. This component's characteristics are fundamental to the circuit's performance.

Q4 – ON Semiconductor LT1013 – DIP Package

The Q4 component is an ON Semiconductor LT1013, a three-terminal N-channel MOSFET packaged in a DIP (Dual In-line Package). This MOSFET serves as a switching element in the circuit, offering low on-resistance for efficient current conduction when activated. The LT1013 is a commonly used MOSFET for low-side switching, facilitating fast switching speeds and low gate drive requirements. The DIP package provides easy integration into the veroboard layout and makes it compatible with various prototyping techniques. The MOSFET is frequently used in digital circuits and motor control systems. The LT1013 MOSFET has a low gate charge, reducing the power required for switching. The LT1013 is a good choice for driving inductive loads, providing fast switching speeds and low power consumption. The maximum continuous drain current is 500mA, allowing it to handle significant current loads within the circuit. This MOSFET is a robust and reliable component for switching applications. The LT1013 is a good choice for driving inductive loads, providing fast switching speeds and low power consumption. The key performance specifications - such as the gate-source voltage (Vgs) - should be considered when designing the circuit. The datasheet provides comprehensive information regarding its electrical and mechanical characteristics. The maximum drain-source voltage (Vds) rating is crucial for preventing damage. This MOSFET is a good choice for driving inductive loads, providing fast switching speeds and low power consumption. The maximum continuous drain current is 200mA. This MOSFET is a robust and reliable component for switching applications.

R8 – Vishay Dale VS-030-VS

The R8 component is a Vishay Dale VS-030-VS, a 30kΩ (30 kiloohm) resistor, enclosed in a through-hole package. This resistor is used for current limiting and voltage division within the circuit. The Vishay Dale VS-030-VS is a standard resistor commonly found in electronic circuits. The through-hole package enables easy mounting onto the veroboard. The resistance value of 30kΩ provides a specific level of current limiting for the circuit. This resistor is selected to ensure proper circuit operation by reducing current flow when required. Resistors are fundamental building blocks in any electronic circuit and are frequently used in voltage dividers, current limiting circuits, and pull-up/pull-down networks. The Vishay Dale VS-030-VS is available from numerous electronic component distributors and can be easily sourced. It’s rated for a power rating of 0.25W. The precision of this resistor impacts the accuracy of the circuit, so careful selection of the resistor value is important. The 1% tolerance of this resistor offers a more precise value. This component is crucial for stabilizing the circuit's behavior. This resistor should be selected carefully, paying attention to its power rating to prevent overheating. Its resistance value ensures proper circuit operation and stability. The precise selection of the resistor value is crucial for the accurate and reliable operation of this circuit. It’s a standard resistor widely used across a multitude of electronic applications. This component’s characteristics are fundamental to the circuit’s performance.