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)

Pin Name Logic function Connection in this case
1 1A Input A Connected to Vehicle Sensor (S1)
2 1B Input B Connected to Ticket Validator (S2)
3 1Y Output Connected to Barrier Indicator (LED)
7 GND Ground Connected to Supply Ground (0 V)
14 VCC Power Supply Connected 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 V Closed (OFF)
Low (0) High (1) ~0 V Closed (OFF)
High (1) Low (0) ~0 V Closed (OFF)
High (1) High (1) ~5 V Open (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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