Practical case: Double Key Security System

Double Key Security System prototype (Maker Style)

Level: Basic – Build a logic circuit that activates an alarm only when two security keys are turned simultaneously.

Objective and use case

In this project, you will build a digital safety interlock circuit using a 74HC08 Quad 2-Input AND Gate. The system mimics a high-security protocol where a mechanism (represented by an LED) activates only if two separate inputs (switches) are triggered at the exact same time.

  • Real-world applications:

    • Industrial machinery: Safety presses requiring the operator to place both hands on separate buttons to prevent injury.
    • Bank vaults: Dual-key requirements where two managers must be present to open a safe.
    • Aerospace: Launch control systems requiring dual confirmation commands.
    • Home Automation: «Smart» lock logic where biometric data and a PIN code must both match.

  • Expected outcome:

    • Idle State: LED remains completely OFF (Logic Low, < 0.1 V) when switches are open.
    • Single Activation: LED remains OFF if only Switch A or only Switch B is closed.
    • Active State: LED turns ON (Logic High, > 3.5 V) exclusively when Switch A AND Switch B are closed.
    • Visual: A clear, stable light signal indicating «Access Granted.»

  • Target audience and level: Students exploring digital logic basics and the 7400 IC family.

Materials

  • U1: 74HC08 Quad 2-Input AND Gate IC.
  • S1: SPST toggle switch or push button, function: Security Key A.
  • S2: SPST toggle switch or push button, function: Security Key B.
  • R1: 10 kΩ resistor, function: pull-down resistor for Input A.
  • R2: 10 kΩ resistor, function: pull-down resistor for Input B.
  • R3: 330 Ω resistor, function: LED current limiting.
  • D1: Red LED, function: System Status Indicator.
  • V1: 5 V DC power supply.

Pin-out of the IC used

Selected Chip: 74HC08 (Quad 2-Input AND Gate)

Pin Name Logic Function Connection in this case
1 1A Input A (Gate 1) Connected to S1 node (VA)
2 1B Input B (Gate 1) Connected to S2 node (VB)
3 1Y Output (Gate 1) Connected to LED driver node (VOUT)
7 GND Ground Connected to 0 V
14 VCC Power Supply Connected to +5 V

(Note: Pins 4, 5, 6, 8, 9, 10, 11, 12, 13 are unused in this single-gate implementation but inputs should technically be grounded in a permanent PCB design to prevent noise.)

Wiring guide

Follow this node-based connection guide to assemble the circuit on your breadboard.

  • Power Rail Connections:

    • Connect V1 positive terminal to node VCC.
    • Connect V1 negative terminal to node 0 (GND).
    • Connect U1 Pin 14 to VCC.
    • Connect U1 Pin 7 to 0.

  • Input Stage (Switch A):

    • Connect S1 between node VCC and node VA.
    • Connect R1 between node VA and node 0 (this ensures VA is Low when S1 is open).
    • Connect U1 Pin 1 to node VA.

  • Input Stage (Switch B):

    • Connect S2 between node VCC and node VB.
    • Connect R2 between node VB and node 0 (this ensures VB is Low when S2 is open).
    • Connect U1 Pin 2 to node VB.

  • Output Stage:

    • Connect U1 Pin 3 to node VOUT.
    • Connect R3 between node VOUT and the Anode of D1.
    • Connect the Cathode of D1 to node 0.

Conceptual block diagram

Conceptual block diagram — 74HC08 Quad AND gate

Schematic

[ INPUTS ]                                [ LOGIC ]                         [ OUTPUT ]

 [ VCC ] -> [ S1: Key A ] --+--(Node VA)-->+-------------------+
                            |              |  Pin 1            |
                       [ R1: 10k ]         |                   |
                            v              |    U1: 74HC08     |
                         [ GND ]           |    (AND Gate)     |--(Pin 3)--> [ R3: 330 Ω ] --> [ D1: LED ] --> [ GND ]
                                           |                   |
 [ VCC ] -> [ S2: Key B ] --+--(Node VB)-->+-------------------+
                            |                 Pin 2
                       [ R2: 10k ]
                            v
                         [ GND ]
Schematic (ASCII)

Truth table

The 74HC08 follows standard positive boolean logic (A AND B).

Key A (S1) Key B (S2) Input A (Pin 1) Input B (Pin 2) Output Y (Pin 3) LED Status
Open Open 0 (Low) 0 (Low) 0 (Low) OFF
Open Closed 0 (Low) 1 (High) 0 (Low) OFF
Closed Open 1 (High) 0 (Low) 0 (Low) OFF
Closed Closed 1 (High) 1 (High) 1 (High) ON

Measurements and tests

  1. Supply Check: Use a multimeter to verify 5 V between VCC and 0 on the breadboard rails.
  2. Input Verification:
    • Keep S1 open: Measure voltage at VA. It should be 0 V.
    • Close S1: Measure voltage at VA. It should be ~5 V.
    • Repeat for S2 and VB.
  3. Logic Logic Verification:
    • Close S1 only. Measure VOUT at Pin 3. Expected: ~0 V.
    • Close S2 only. Measure VOUT at Pin 3. Expected: ~0 V.
    • Close both S1 and S2. Measure VOUT. Expected: > 3.5 V (High Logic).
  4. Current Draw (Optional): Measure the current through R3 when the LED is ON. It should be approximately 8–10 mA.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Double Key Security System

* --- Power Supply ---
* V1: 5V DC Power Supply connected to VCC and GND (0)
V1 VCC 0 DC 5

* --- Input Stage (Switch A) ---
* User actuation simulation for Switch A (Control Signal)
* Generates a pulse sequence to test logic states. 
* Logic sequence plan: 00 -> 01 -> 10 -> 11
* Actuation A: Low for 100us, High for 100us.
V_ACT_A ACT_A 0 PULSE(0 5 100u 1u 1u 99u 200u)

* S1: SPST Switch connecting VCC to VA when actuated
S1 VCC VA ACT_A 0 SW_PUSHBUTTON

* R1: 10k Pull-down resistor for Input A
R1 VA 0 10k

* --- Input Stage (Switch B) ---
* User actuation simulation for Switch B (Control Signal)
* Actuation B: Toggles every 50us.
V_ACT_B ACT_B 0 PULSE(0 5 50u 1u 1u 49u 100u)

* S2: SPST Switch connecting VCC to VB when actuated
S2 VCC VB ACT_B 0 SW_PUSHBUTTON

* R2: 10k Pull-down resistor for Input B
R2 VB 0 10k

* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans 

* Practical case: Double Key Security System

* --- Power Supply ---
* V1: 5V DC Power Supply connected to VCC and GND (0)
V1 VCC 0 DC 5

* --- Input Stage (Switch A) ---
* User actuation simulation for Switch A (Control Signal)
* Generates a pulse sequence to test logic states. 
* Logic sequence plan: 00 -> 01 -> 10 -> 11
* Actuation A: Low for 100us, High for 100us.
V_ACT_A ACT_A 0 PULSE(0 5 100u 1u 1u 99u 200u)

* S1: SPST Switch connecting VCC to VA when actuated
S1 VCC VA ACT_A 0 SW_PUSHBUTTON

* R1: 10k Pull-down resistor for Input A
R1 VA 0 10k

* --- Input Stage (Switch B) ---
* User actuation simulation for Switch B (Control Signal)
* Actuation B: Toggles every 50us.
V_ACT_B ACT_B 0 PULSE(0 5 50u 1u 1u 49u 100u)

* S2: SPST Switch connecting VCC to VB when actuated
S2 VCC VB ACT_B 0 SW_PUSHBUTTON

* R2: 10k Pull-down resistor for Input B
R2 VB 0 10k

* --- Logic Stage (U1: 74HC08) ---
* Quad 2-Input AND Gate. Using 1 gate (Pins 1, 2, 3).
* Connections: Pin1=VA, Pin2=VB, Pin3=VOUT, Pin7=GND, Pin14=VCC
XU1 VA VB VOUT 0 VCC 74HC08

* --- Output Stage ---
* R3: 330 Ohm Current Limiting Resistor
R3 VOUT LED_ANODE 330

* D1: Red LED System Status Indicator
D1 LED_ANODE 0 DLED

* --- Models ---

* Switch Model (Voltage Controlled Switch)
* Vt=2.5V threshold, Low resistance when ON, High when OFF
.model SW_PUSHBUTTON SW(Vt=2.5 Ron=1 Roff=100Meg)

* LED Model
.model DLED D(IS=1e-14 N=2 RS=10 BV=5)

* 74HC08 Subcircuit Model (Behavioral AND Gate)
* Implements Vout = VCC * AND(A, B) using continuous sigmoid functions for convergence
* Pins: 1=A, 2=B, 3=Y, 7=GND, 14=VCC
.subckt 74HC08 P1 P2 P3 P7 P14
* Behavioral Source B1: Logic AND function
* Sigmoid function: 1 / (1 + exp(-k*(V-Vth)))
* k=50 provides sharp transition, Vth=2.5V
B1 P3 P7 V = V(P14, P7) * (1 / (1 + exp(-50 * (V(P1, P7) - 2.5)))) * (1 / (1 + exp(-50 * (V(P2, P7) - 2.5))))
.ends

* --- Simulation Commands ---
* Transient analysis for 250us to cover all logic states (00, 01, 10, 11)
.tran 1u 250u

* Print directives for logging
.print tran V(VA) V(VB) V(VOUT)

* Calculate DC operating point
.op

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (322 rows) 
Index   time            v(va)           v(vb)           v(vout)
0	0.000000e+00	4.999500e-04	4.999500e-04	1.403014e-108
1	1.000000e-08	4.999500e-04	4.999500e-04	1.403014e-108
2	2.000000e-08	4.999500e-04	4.999500e-04	1.403014e-108
3	4.000000e-08	4.999500e-04	4.999500e-04	1.403014e-108
4	8.000000e-08	4.999500e-04	4.999500e-04	1.403014e-108
5	1.600000e-07	4.999500e-04	4.999500e-04	1.403014e-108
6	3.200000e-07	4.999500e-04	4.999500e-04	1.403014e-108
7	6.400000e-07	4.999500e-04	4.999500e-04	1.403014e-108
8	1.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
9	2.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
10	3.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
11	4.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
12	5.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
13	6.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
14	7.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
15	8.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
16	9.280000e-06	4.999500e-04	4.999500e-04	1.403014e-108
17	1.028000e-05	4.999500e-04	4.999500e-04	1.403014e-108
18	1.128000e-05	4.999500e-04	4.999500e-04	1.403014e-108
19	1.228000e-05	4.999500e-04	4.999500e-04	1.403014e-108
20	1.328000e-05	4.999500e-04	4.999500e-04	1.403014e-108
21	1.428000e-05	4.999500e-04	4.999500e-04	1.403014e-108
22	1.528000e-05	4.999500e-04	4.999500e-04	1.403014e-108
23	1.628000e-05	4.999500e-04	4.999500e-04	1.403014e-108
... (298 more rows) ...

Common mistakes and how to avoid them

  1. Floating Inputs: Forgetting resistors R1 or R2. Without them, the inputs «float» and may pick up static noise, causing the LED to flicker randomly even when switches are open.
  2. Missing Power to IC: Forgetting to connect Pin 14 to VCC and Pin 7 to GND. The chip will not function and may overheat if inputs are driven while the chip is unpowered.
  3. LED Polarity: Inserting D1 backward (Anode to Ground). The LED will never light up, even if the logic is correct.

Troubleshooting

  • Symptom: LED is always ON, regardless of switch position.

    • Cause: Input resistors (R1/R2) might be connected to VCC instead of GND, or the switch is wired incorrectly (shorting VCC to Input directly).
    • Fix: Check that R1 and R2 connect the inputs to Ground (Pull-down configuration).

  • Symptom: LED flickers when I touch the wires.

    • Cause: Floating input pin.
    • Fix: Ensure the pull-down resistors are firmly seated in the breadboard and making contact.

  • Symptom: LED is very dim when both switches are pressed.

    • Cause: R3 value is too high (e.g., 10 kΩ instead of 330 Ω) or the supply voltage is too low.
    • Fix: Replace R3 with a 220 Ω or 330 Ω resistor.

Possible improvements and extensions

  1. Triple Security: Replace the 74HC08 with a 74HC11 (Triple 3-Input AND Gate) to require three simultaneous keys.
  2. High Power Output: Connect the output VOUT to an NPN transistor (like 2N2222) or a Relay Module to drive a loud siren or a 12V motor instead of a small LED.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Quick Quiz

Question 1: What is the primary function of the logic circuit described in this project?




Question 2: Which specific Integrated Circuit (IC) is used to create the logic for this project?




Question 3: Which of the following is a real-world application mentioned for this specific type of logic?




Question 4: What is the expected state of the LED when the circuit is in its 'Idle State'?




Question 5: According to the text, what voltage level represents a Logic High output in this context?




Question 6: What condition must be met for the output to be Logic High (LED ON)?




Question 7: What component is used to visually represent the 'System Status' or mechanism activation?




Question 8: Which industry is cited as using this logic for launch control systems?




Question 9: In the context of Home Automation, what example is given for this dual-input logic?




Question 10: Who is the primary target audience for this project?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Related Posts

Scroll to Top