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Practical case: Vehicle access control with barrier

Vehicle access control with barrier prototype (Maker Style)

Level: Medium — Design a safety logic circuit that lifts a barrier only when vehicle presence and valid ticket verification occur simultaneously.

Objective and use case

In this practical case, you will build a digital control circuit using a 74HC08 AND gate to simulate the logic of an automated parking barrier. The barrier (represented by an LED) will only activate when a vehicle presence sensor and a ticket validation system trigger simultaneously.

Why it is useful:
* Parking lots: Ensures the gate does not open for pedestrians or if a ticket is invalid.
* Toll booths: Synchronizes payment confirmation with physical vehicle presence.
* Industrial safety: Prevents machinery operation unless a guard is in place and a start command is issued.
* Secure access: Requires dual authentication factors in physical security systems.

Expected outcome:
* State 0 (Rest): LED remains OFF when no buttons are pressed (0 V output).
* State 1 (Partial): LED remains OFF if only the vehicle is detected or only the ticket is validated.
* State 2 (Active): LED turns ON (approx. 5 V / High logic) ONLY when both inputs are active simultaneously.
* Logic Verification: Confirmation of the standard Boolean AND operation ($Y = A \cdot B$).

Target audience and level: Electronics students and hobbyists / Medium.

Materials

  • V1: 5 V DC power supply, function: Main circuit power
  • U1: 74HC08, function: Quad 2-Input AND Gate IC
  • S1: Push-button (NO), function: Simulates «Vehicle Presence Sensor»
  • S2: Push-button (NO), function: Simulates «Ticket Validation Signal»
  • R1: 10 kΩ resistor, function: Pull-down for Vehicle input
  • R2: 10 kΩ resistor, function: Pull-down for Ticket input
  • R3: 330 Ω resistor, function: LED current limiting
  • D1: Green LED, function: Simulates «Barrier Motor/Open Signal»
  • Breadboard and jumper wires

Pin-out of the IC used

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

PinNameLogic functionConnection in this case
11AInput AConnected to Vehicle Sensor (S1)
21BInput BConnected to Ticket Validator (S2)
31YOutputConnected to Barrier Indicator (LED)
7GNDGroundConnected to Supply Ground (0 V)
14VCCPower SupplyConnected to +5 V Supply

Note: Pins 4, 5, 6, 8, 9, 10, 11, 12, and 13 are unused in this specific circuit.

Wiring guide

  • Power Supply:
    • V1 (+) connects to node VCC.
    • V1 (-) connects to node 0 (GND).
  • IC Power:
    • U1 Pin 14 connects to node VCC.
    • U1 Pin 7 connects to node 0.
  • Input Stage (Vehicle Sensor):
    • S1 connects between node VCC and node VEHICLE_IN.
    • R1 connects between node VEHICLE_IN and node 0 (active-high configuration).
    • U1 Pin 1 connects to node VEHICLE_IN.
  • Input Stage (Ticket Validator):
    • S2 connects between node VCC and node TICKET_IN.
    • R2 connects between node TICKET_IN and node 0 (active-high configuration).
    • U1 Pin 2 connects to node TICKET_IN.
  • Output Stage (Barrier Actuator):
    • U1 Pin 3 connects to node LOGIC_OUT.
    • R3 connects between node LOGIC_OUT and node LED_ANODE.
    • D1 (Anode) connects to node LED_ANODE.
    • D1 (Cathode) connects to node 0.

Conceptual block diagram

Conceptual block diagram — 74HC08 AND gate

Schematic

[ INPUT SENSORS ]                     [ LOGIC PROCESSING ]                  [ ACTUATOR OUTPUT ]

 [ VCC ]
    |
 [ S1: Vehicle ]
    |
    +----(Node: VEHICLE_IN)----(Pin 1)-->+----------------------+
    |                                    |                      |
 [ R1: 10k ]                             |      U1: 74HC08      |
    |                                    |      (AND Gate)      |
 [ GND ]                                 |                      |--(Pin 3)--> [ R3: 330 ] --> [ D1: Green LED ] --> [ GND ]
                                         |  (Pin 14: VCC)       |
 [ VCC ]                                 |  (Pin 7:  GND)       |
    |                                    |                      |
 [ S2: Ticket ]                          |                      |
    |                                    |                      |
    +----(Node: TICKET_IN)-----(Pin 2)-->+----------------------+
    |
 [ R2: 10k ]
    |
 [ GND ]
Schematic (ASCII)

Truth table

The 74HC08 follows the standard AND logic verification:

Vehicle Detected (S1)Ticket Validated (S2)Output Voltage (Pin 3)Barrier State (LED)
Low (0)Low (0)~0 VClosed (OFF)
Low (0)High (1)~0 VClosed (OFF)
High (1)Low (0)~0 VClosed (OFF)
High (1)High (1)~5 VOpen (ON)

Measurements and tests

  1. Idle Check: Ensure neither S1 nor S2 is pressed. Measure voltage at U1 Pin 1 and Pin 2 relative to GND. It should read 0 V (Logic Low). The LED must be OFF.
  2. Single Input Test: Press S1 (Vehicle) only. Measure voltage at Pin 1 (5 V) and Pin 3 (0 V). LED must remain OFF. Repeat for S2 (Ticket).
  3. Activation Test: Press both S1 and S2 simultaneously. Measure voltage at U1 Pin 3. It should read close to 5 V (Logic High).
  4. Load Check: Observe the LED turning ON brightly when both buttons are held. This confirms the barrier would lift.

SPICE netlist and simulation

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

* TITLE: Practical case: Vehicle access control with barrier
* Ngspice Netlist
* Implements a 74HC08 AND gate circuit with push-button inputs and LED output

* --- Component Models ---
* Switch Model: Voltage Controlled Switch for Push-buttons
* Vt=2.5V (Threshold), Ron=1 ohm (Closed), Roff=100Meg (Open)
.model SW_PUSH SW(Vt=2.5 Ron=1 Roff=100Meg)

* LED Model: Generic Green LED
.model D_GREEN D(Is=1e-22 Rs=5 N=1.5 Cjo=10p BV=5)

* --- Power Supply ---
* V1: 5V DC Main Power Supply
V1 VCC 0 DC 5

* --- Dynamic Stimuli (User Button Presses) ---
* These sources actuate the switches S1 and S2 to simulate user interaction.
* They are not part of the physical circuit but provide the mechanical "push".
* Sequence designed to test Truth Table: 00 -> 10 -> 01 -> 11
* Time unit: microseconds (us)

* S1 Actuator (Vehicle Sensor): Toggles every 200us (starts at 100us)
V_ACT_S1 S1_CTRL 0 PULSE(0 5 100u 1u 1u 100u 200u)

* S2 Actuator (Ticket Validator): Toggles every 400us (starts at 200us)
V_ACT_S2 S2_CTRL 0 PULSE(0 5 200u 1u 1u 200u 400u)

* --- Input Stage: Vehicle Sensor ---
* S1: Push-button connecting VCC to VEHICLE_IN when pressed
* ... (truncated in public view) ...

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

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* TITLE: Practical case: Vehicle access control with barrier
* Ngspice Netlist
* Implements a 74HC08 AND gate circuit with push-button inputs and LED output

* --- Component Models ---
* Switch Model: Voltage Controlled Switch for Push-buttons
* Vt=2.5V (Threshold), Ron=1 ohm (Closed), Roff=100Meg (Open)
.model SW_PUSH SW(Vt=2.5 Ron=1 Roff=100Meg)

* LED Model: Generic Green LED
.model D_GREEN D(Is=1e-22 Rs=5 N=1.5 Cjo=10p BV=5)

* --- Power Supply ---
* V1: 5V DC Main Power Supply
V1 VCC 0 DC 5

* --- Dynamic Stimuli (User Button Presses) ---
* These sources actuate the switches S1 and S2 to simulate user interaction.
* They are not part of the physical circuit but provide the mechanical "push".
* Sequence designed to test Truth Table: 00 -> 10 -> 01 -> 11
* Time unit: microseconds (us)

* S1 Actuator (Vehicle Sensor): Toggles every 200us (starts at 100us)
V_ACT_S1 S1_CTRL 0 PULSE(0 5 100u 1u 1u 100u 200u)

* S2 Actuator (Ticket Validator): Toggles every 400us (starts at 200us)
V_ACT_S2 S2_CTRL 0 PULSE(0 5 200u 1u 1u 200u 400u)

* --- Input Stage: Vehicle Sensor ---
* S1: Push-button connecting VCC to VEHICLE_IN when pressed
S1 VCC VEHICLE_IN S1_CTRL 0 SW_PUSH
* R1: 10k Pull-down resistor for Vehicle input
R1 VEHICLE_IN 0 10k

* --- Input Stage: Ticket Validator ---
* S2: Push-button connecting VCC to TICKET_IN when pressed
S2 VCC TICKET_IN S2_CTRL 0 SW_PUSH
* R2: 10k Pull-down resistor for Ticket input
R2 TICKET_IN 0 10k

* --- Logic Stage: U1 (74HC08 Quad 2-Input AND Gate) ---
* Subcircuit representing one gate of the 74HC08 IC
* Pins mapped: 1(A), 2(B), 3(Y), 7(GND), 14(VCC)
.subckt 74HC08_GATE PIN1 PIN2 PIN3 PIN7 PIN14
    * Behavioral AND logic using continuous sigmoid functions for convergence
    * Y = VCC if (A > 2.5V) AND (B > 2.5V)
    B_LOGIC PIN3 PIN7 V = V(PIN14) * (1 / (1 + exp(-50*(V(PIN1)-2.5)))) * (1 / (1 + exp(-50*(V(PIN2)-2.5))))
.ends

* Instantiate U1 connected according to Wiring Guide
* Pin 1->VEHICLE_IN, Pin 2->TICKET_IN, Pin 3->LOGIC_OUT, Pin 7->0, Pin 14->VCC
XU1 VEHICLE_IN TICKET_IN LOGIC_OUT 0 VCC 74HC08_GATE

* --- Output Stage: Barrier Actuator ---
* R3: 330 ohm current limiting resistor
R3 LOGIC_OUT LED_ANODE 330
* D1: Green LED (Anode to R3, Cathode to GND)
D1 LED_ANODE 0 D_GREEN

* --- Simulation Directives ---
* Transient analysis for 500us to capture full sequence
.tran 1u 500u
.op

* Print signals to verify logic: 
* Expect LOGIC_OUT to be High (~5V) only when both Inputs are High (300us-400us)
.print tran V(VEHICLE_IN) V(TICKET_IN) V(LOGIC_OUT) V(LED_ANODE)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (1254 rows)
Index   time            v(vehicle_in)   v(ticket_in)    v(logic_out)
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
... (1230 more rows) ...

Common mistakes and how to avoid them

  1. Floating Inputs: Failing to include resistors R1 and R2 causes the inputs to «float,» leading to erratic LED flickering even when buttons are not pressed. Always use pull-down resistors with 74HC series logic.
  2. Missing Power Connections: Forgetting to connect Pin 14 (VCC) and Pin 7 (GND) is a classic error. Logic chips will not function without power, even if inputs are wired correctly.
  3. LED Without Resistor: Connecting the LED directly to the logic output (Pin 3) without R3 can damage the LED or the 74HC08 output stage due to excessive current.

Troubleshooting

  • Symptom: The LED is always ON, even when buttons are released.
    • Cause: Missing pull-down resistors or inputs connected directly to VCC.
    • Fix: Ensure R1 and R2 are installed correctly between the inputs and GND.
  • Symptom: The LED does not light up when both buttons are pressed.
    • Cause: LED polarity reversed or IC not powered.
    • Fix: Check D1 orientation (flat side is cathode/GND) and measure 5 V across Pins 14 and 7.
  • Symptom: LED is very dim when active.
    • Cause: Current limiting resistor (R3) value is too high.
    • Fix: Ensure R3 is 330 Ω (orange-orange-brown). If it is 10 kΩ or higher, the LED will be barely visible.

Possible improvements and extensions

  1. Emergency Stop: Introduce a third input using a 3-input AND gate (74HC11) or cascading another 74HC08 gate, connected to a «Stop» switch that overrides the open command.
  2. Motor Driver Interface: Replace the LED with an NPN transistor (e.g., 2N2222) and a relay to drive a real DC motor or solenoid, simulating a heavy-duty barrier mechanism.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary objective of the circuit described in the text?




Question 2: Which specific logic gate IC is utilized to control the barrier's logic?




Question 3: Under which specific condition will the LED (representing the barrier) turn ON?




Question 4: What is the role of the pull-down resistors (typically 10 kΩ) connected to the inputs?




Question 5: What voltage level corresponds to the 'Active' state (State 2) where the LED is ON?




Question 6: Which Boolean logic expression represents the operation of this circuit?




Question 7: Which component is typically used in this simulation to represent the 'Vehicle Presence Sensor'?




Question 8: How does this logic apply to industrial safety scenarios?




Question 9: What is the status of the LED during 'State 1 (Partial)'?




Question 10: What is the purpose of the current-limiting resistor (e.g., 330 Ω) connected to the LED?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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