Practical case: Conveyor belt start system

Conveyor belt start system prototype (Maker Style)

Level: Medium. Design a safety interlock circuit that activates a conveyor belt only when the operator is present and a load is detected.

Objective and use case

In this practical case, you will build a safety logic circuit using a 74HC08 AND gate to control the activation of a DC motor via a relay. The system ensures the conveyor belt only runs when two distinct safety conditions are met simultaneously.

  • Real-world application: Industrial safety interlocks preventing machinery from starting without an operator at the controls.
  • Efficiency: Automated energy saving by ensuring the belt only runs when a product (load) is actually present on the line.
  • Machine protection: Preventing «dry runs» that might wear out mechanical components unnecessarily.

Expected outcome:
* Logic Output: The 74HC08 output pin goes HIGH (approx. 5V) only when both inputs are HIGH.
* Motor State: The DC motor turns ON only when the Operator Button is held AND the Optical Sensor detects an object.
* Current Drive: A transistor amplifies the weak logic signal to switch the 5V relay coil.
* Target Audience: Engineering students and maintenance technicians (Medium level).

Materials

  • V1: 5V DC Power Supply, function: Main circuit power.
  • U1: 74HC08 Quad 2-Input AND Gate, function: Safety logic processing.
  • S1: Push button (Normally Open), function: Simulates «Operator Presence».
  • S2: Switch (SPST) or Phototransistor module, function: Simulates «Optical Load Sensor» (Active High).
  • R1: 10 kΩ resistor, function: Pull-down for Operator input (S1).
  • R2: 10 kΩ resistor, function: Pull-down for Sensor input (S2).
  • R3: 1 kΩ resistor, function: Base current limiting for Q1.
  • Q1: 2N2222 NPN Transistor, function: Relay driver switch.
  • D1: 1N4007 Diode, function: Flyback protection for the relay coil.
  • K1: 5V Relay (SPDT), function: High-current switch for the motor.
  • M1: 5V DC Motor, function: Conveyor belt drive.
  • C1: 100 nF capacitor, function: Decoupling for U1 power supply.

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 Operator Button (S1)
2 1B Input B Connected to Optical Sensor (S2)
3 1Y Output Connected to Transistor Base Resistor (R3)
7 GND Ground Connected to 0V (GND)
14 VCC Power Connected to +5V (VCC)

Note: Pins 4-6 and 8-13 are unused in this single-gate application and should technically be tied to GND in a permanent noise-sensitive environment, but are left open for this basic prototype.

Wiring guide

Use the following nodes for your connections: VCC, 0 (Ground), OP_SIGNAL, LOAD_SIGNAL, LOGIC_OUT.

  • Power: Connect VCC to the positive rail of V1 and 0 to the negative rail.
  • Input S1 (Operator): Connect one side of S1 to VCC. Connect the other side to node OP_SIGNAL.
  • Pull-down R1: Connect R1 between OP_SIGNAL and 0.
  • Input S2 (Sensor): Connect one side of S2 to VCC. Connect the other side to node LOAD_SIGNAL.
  • Pull-down R2: Connect R2 between LOAD_SIGNAL and 0.
  • Logic U1:
    • Connect U1 Pin 14 to VCC and Pin 7 to 0.
    • Connect C1 between VCC and 0 near U1.
    • Connect OP_SIGNAL to U1 Pin 1 (Input 1A).
    • Connect LOAD_SIGNAL to U1 Pin 2 (Input 1B).
    • Connect U1 Pin 3 (Output 1Y) to node LOGIC_OUT.
  • Driver Stage:
    • Connect R3 between LOGIC_OUT and the Base of Q1.
    • Connect the Emitter of Q1 to 0.
    • Connect the Collector of Q1 to the Relay coil (K1 pin 1).
  • Relay & Motor:
    • Connect the other side of the Relay coil (K1 pin 2) to VCC.
    • Connect D1 across the Relay coil (Cathode to VCC, Anode to Q1 Collector).
    • Connect Relay Common (COM) to VCC.
    • Connect Relay Normally Open (NO) to the positive terminal of M1.
    • Connect the negative terminal of M1 to 0.

Conceptual block diagram

Conceptual block diagram — 74HC08 AND gate

Schematic

[ INPUTS ]                       [ LOGIC ]                        [ OUTPUT STAGE ]

    (VCC)                                                                    (VCC)
      |                                                                        |
    [ S1: Operator ]--(OP_SIGNAL)-->+-------------+                       +----+----+
      |                             |  Pin 1 (A)  |                       | K1 Coil | (Parallel D1)
    [ R1: 10k ]                     |             |                       +----+----+
      |                             |   74HC08    |                            ^
    (GND)                           |     U1      |                            |
                                    |             |--(Pin 3)-->[ R3: 1k ]-->[ Q1: NPN ]
    (VCC)                           |             |          (LOGIC_OUT)       |
      |                             |             |                            v
    [ S2: Sensor ]--(LOAD_SIGNAL)-->|  Pin 2 (B)  |                          (GND)
      |                             |             |
    [ R2: 10k ]                     +-------------+                          (VCC)
      |                                    |                                   |
    (GND)                               [ C1 ]                           [ K1 Switch ]
                                           |                                   |
                                         (GND)                                 v
                                                                         [ M1: Motor ]
                                                                               |
                                                                             (GND)
Schematic (ASCII)

Truth table

This table represents the logic states required to start the motor.

Operator (S1) Load Detected (S2) U1 Output (Pin 3) Transistor Q1 Motor State
Low (0) Low (0) Low (0) OFF (Cut-off) STOP
Low (0) High (1) Low (0) OFF (Cut-off) STOP
High (1) Low (0) Low (0) OFF (Cut-off) STOP
High (1) High (1) High (1) ON (Sat) RUN

Measurements and tests

Validate the circuit operation using a multimeter:

  1. Input Verification: Measure voltage at OP_SIGNAL relative to GND. It should be 0V when S1 is open and 5V when pressed. Repeat for LOAD_SIGNAL (S2).
  2. Logic Output: With S1 and S2 active, measure voltage at LOGIC_OUT. It should be approximately equal to VCC (Logic High). If either is released, it should drop to ~0V.
  3. Base Current (I_b): Set your multimeter to Ammeter mode. Place it in series with R3. When logic is High, you should measure approximately 4.3mA (calculated as $(5V – 0.7V) / 1000\Omega$). This confirms the transistor is being driven hard enough to saturate.
  4. Relay Actuation: Listen for the «click» of the relay when both inputs are active. Measure voltage across the Motor terminals; it should read 5V.

SPICE netlist and simulation

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

* Title: Practical case: Conveyor belt start system

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

* --- Input S1: Operator Presence ---
* Component: Push button (NO) modeled as Voltage-Controlled Switch
* Wiring: VCC -> S1 -> OP_SIGNAL -> R1 -> 0
S1 VCC OP_SIGNAL CTRL_OP 0 SW_BTN
R1 OP_SIGNAL 0 10k
* Stimulus: Simulate button press (High) from t=1ms to t=4ms
V_ACT_S1 CTRL_OP 0 PULSE(0 5 1m 10u 10u 3m 10m)

* --- Input S2: Optical Load Sensor ---
* Component: Switch/Sensor modeled as Voltage-Controlled Switch
* Wiring: VCC -> S2 -> LOAD_SIGNAL -> R2 -> 0
S2 VCC LOAD_SIGNAL CTRL_LOAD 0 SW_BTN
R2 LOAD_SIGNAL 0 10k
* Stimulus: Simulate sensor active (High) from t=2ms to t=5ms
V_ACT_S2 CTRL_LOAD 0 PULSE(0 5 2m 10u 10u 3m 10m)

* --- Logic U1: 74HC08 Quad AND Gate ---
* Wiring: Pin 14=VCC, Pin 7=0, Pin 1=OP_SIGNAL, Pin 2=LOAD_SIGNAL, Pin 3=LOGIC_OUT
* Decoupling Capacitor C1
C1 VCC 0 100n
* Instantiation of Logic Gate Subcircuit
XU1 OP_SIGNAL LOAD_SIGNAL LOGIC_OUT VCC 0 74HC08_GATE

* --- Driver Stage ---
* ... (truncated in public view) ...

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* Title: Practical case: Conveyor belt start system

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

* --- Input S1: Operator Presence ---
* Component: Push button (NO) modeled as Voltage-Controlled Switch
* Wiring: VCC -> S1 -> OP_SIGNAL -> R1 -> 0
S1 VCC OP_SIGNAL CTRL_OP 0 SW_BTN
R1 OP_SIGNAL 0 10k
* Stimulus: Simulate button press (High) from t=1ms to t=4ms
V_ACT_S1 CTRL_OP 0 PULSE(0 5 1m 10u 10u 3m 10m)

* --- Input S2: Optical Load Sensor ---
* Component: Switch/Sensor modeled as Voltage-Controlled Switch
* Wiring: VCC -> S2 -> LOAD_SIGNAL -> R2 -> 0
S2 VCC LOAD_SIGNAL CTRL_LOAD 0 SW_BTN
R2 LOAD_SIGNAL 0 10k
* Stimulus: Simulate sensor active (High) from t=2ms to t=5ms
V_ACT_S2 CTRL_LOAD 0 PULSE(0 5 2m 10u 10u 3m 10m)

* --- Logic U1: 74HC08 Quad AND Gate ---
* Wiring: Pin 14=VCC, Pin 7=0, Pin 1=OP_SIGNAL, Pin 2=LOAD_SIGNAL, Pin 3=LOGIC_OUT
* Decoupling Capacitor C1
C1 VCC 0 100n
* Instantiation of Logic Gate Subcircuit
XU1 OP_SIGNAL LOAD_SIGNAL LOGIC_OUT VCC 0 74HC08_GATE

* --- Driver Stage ---
* Wiring: LOGIC_OUT -> R3 -> Q1 Base
R3 LOGIC_OUT Q1_BASE 1k
* Wiring: Q1 Collector -> Relay Coil, Emitter -> 0
Q1 RELAY_COIL_LOW Q1_BASE 0 2N2222MOD

* --- Relay K1 ---
* Wiring: VCC -> Coil -> Q1 Collector (RELAY_COIL_LOW)
* Coil modeled as Inductance + Resistance
L_K1 VCC K1_INT 10m
R_K1 K1_INT RELAY_COIL_LOW 100

* Flyback Diode D1
* Wiring: Cathode to VCC, Anode to Q1 Collector
D1 RELAY_COIL_LOW VCC 1N4007MOD

* Relay Contact (Switch)
* Wiring: COM (VCC) -> NO (MOTOR_POS)
* Controlled by voltage across the coil: V(VCC) - V(RELAY_COIL_LOW)
* FIXED: Connected negative control node to Ground (0) to fix Singular Matrix error
E_K1_SENSE K1_CTRL_P 0 VOL = 'V(VCC) - V(RELAY_COIL_LOW)'
S_K1 VCC MOTOR_POS K1_CTRL_P 0 SW_RELAY

* --- Motor M1 ---
* Wiring: MOTOR_POS -> Motor -> 0
* Modeled as an inductive load
R_M1 MOTOR_POS M1_INT 10
L_M1 M1_INT 0 1m

* --- Models & Subcircuits ---

* Button/Sensor Switch Model
.model SW_BTN SW(Vt=2.5 Vh=0.1 Ron=0.1 Roff=10Meg)

* Relay Contact Switch Model (Activates when coil voltage > 3.5V)
.model SW_RELAY SW(Vt=3.5 Vh=0.5 Ron=0.05 Roff=100Meg)

* Transistor Model
.model 2N2222MOD NPN(IS=1E-14 BF=200 VAF=100 IKF=0.3 XTB=1.5 BR=3 CJC=8p CJE=25p)

* Diode Model
.model 1N4007MOD D(IS=7n RS=0.03 N=1.2 BV=1000 IBV=5u CJO=10p TT=100n)

* 74HC08 AND Gate Behavioral Model
* Pins: A B Y VCC GND
.subckt 74HC08_GATE A B Y VCC GND
* Continuous Sigmoid function for convergence: 5V * sigmoid(A) * sigmoid(B)
B_AND Y GND V = V(VCC) * (1 / (1 + exp(-50*(V(A)-2.5)))) * (1 / (1 + exp(-50*(V(B)-2.5))))
.ends

* --- Simulation Directives ---
.op
* Transient analysis: 10us step, 8ms total time
.tran 10u 8m
* Print required voltages
.print tran V(OP_SIGNAL) V(LOAD_SIGNAL) V(LOGIC_OUT) V(RELAY_COIL_LOW) V(MOTOR_POS)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (7686 rows) 
Index   time            v(op_signal)    v(load_signal)  v(logic_out)
0	0.000000e+00	4.995005e-03	4.995005e-03	2.199277e-108
1	1.000000e-07	4.995005e-03	4.995005e-03	2.199277e-108
2	2.000000e-07	4.995005e-03	4.995005e-03	2.199277e-108
3	4.000000e-07	4.995005e-03	4.995005e-03	2.199277e-108
4	8.000000e-07	4.995005e-03	4.995005e-03	2.199277e-108
5	1.600000e-06	4.995005e-03	4.995005e-03	2.199277e-108
6	3.200000e-06	4.995005e-03	4.995005e-03	2.199277e-108
7	6.400000e-06	4.995005e-03	4.995005e-03	2.199277e-108
8	1.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
9	2.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
10	3.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
11	4.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
12	5.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
13	6.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
14	7.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
15	8.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
16	9.280000e-05	4.995005e-03	4.995005e-03	2.199277e-108
17	1.028000e-04	4.995005e-03	4.995005e-03	2.199277e-108
18	1.128000e-04	4.995005e-03	4.995005e-03	2.199277e-108
19	1.228000e-04	4.995005e-03	4.995005e-03	2.199277e-108
20	1.328000e-04	4.995005e-03	4.995005e-03	2.199277e-108
21	1.428000e-04	4.995005e-03	4.995005e-03	2.199277e-108
22	1.528000e-04	4.995005e-03	4.995005e-03	2.199277e-108
23	1.628000e-04	4.995005e-03	4.995005e-03	2.199277e-108
... (7662 more rows) ...

Common mistakes and how to avoid them

  • Directly driving the motor: Students often connect the motor directly to the 74HC08 output. The chip can only source ~20mA, while a motor needs hundreds of mA. Solution: Always use a transistor (Q1) and relay interface.
  • Floating Inputs: Forgetting resistors R1 and R2 causes the inputs to «float,» leading to erratic motor behavior triggered by static electricity. Solution: Ensure pull-down resistors are firmly connected to Ground.
  • Missing Flyback Diode: Omitting D1 allows high-voltage spikes from the relay coil to destroy the transistor Q1 when it turns off. Solution: Install D1 in parallel with the coil, cathode pointing to VCC.

Troubleshooting

  • Motor does not run: Check if the relay clicks. If no click, check voltage at U1 Pin 3 (Logic Out). If Logic Out is 5V but relay doesn’t click, check Q1 orientation.
  • Logic Output always High: Check if R1 or R2 are disconnected (floating inputs often read as High in some logic families, though 74HC usually floats random). Verify S1/S2 wiring.
  • Chip gets hot: Check if U1 is wired backwards (Pin 14 must be VCC, Pin 7 GND). Ensure outputs are not shorted to ground.

Possible improvements and extensions

  1. Self-Latching Circuit: Replace the logic with a latch or add a feedback loop so the operator can press a «Start» button once, and the belt keeps running until «Stop» is pressed or the load is removed.
  2. Emergency Stop: Add a 74HC04 NOT gate or use a NAND configuration to include a «Normally Closed» Emergency Stop button that immediately cuts power to the relay regardless of other inputs.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary function of the 74HC08 integrated circuit in this project?




Question 2: Which two conditions must be met simultaneously for the conveyor belt to activate?




Question 3: What component is typically used to simulate the 'Operator Presence' in this type of circuit?




Question 4: What is the specific purpose of the transistor (e.g., 2N2222) in this circuit?




Question 5: What logic level does the 74HC08 output pin produce when both inputs are HIGH?




Question 6: Why are pull-down resistors typically used on the logic gate inputs in this circuit?




Question 7: What is the function of the base resistor connected to the transistor?




Question 8: Which real-world application is explicitly mentioned for this safety interlock circuit?




Question 9: How does this circuit contribute to machine protection?




Question 10: What component would likely simulate the 'Optical Load Sensor' in a basic prototype of this circuit?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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