Practical case: Production Line Fault Monitoring

Production Line Fault Monitoring prototype (Maker Style)

Level: Medium. Implement a safety system that stops a conveyor belt if either the temperature sensor OR the jam sensor detects an anomaly.

Objective and use case

You will build a logic control circuit using an OR gate to combine signals from two distinct safety sensors (Temperature and Optical Jam). When either sensor detects a fault (Logic High), the system will output an active signal to trigger an indicator or stop mechanism.

Why it is useful:
* Industrial Safety: Prevents machinery from operating under dangerous conditions.
* Equipment Protection: Stops motors immediately if they overheat to prevent permanent damage.
* Process Efficiency: Detects physical jams on conveyor belts automatically, reducing waste.
* Redundancy: Allows multiple different error types to trigger the same emergency stop routine.

Expected outcome:
* System Standby: When both sensors are Low (0V), the output LED is OFF.
* Temperature Fault: If the temperature sensor triggers (High/5V), the LED turns ON.
* Jam Fault: If the jam sensor triggers (High/5V), the LED turns ON.
* Critical Failure: If both sensors trigger simultaneously, the LED remains ON.

Target audience and level: Electronics students and hobbyists, Level Medium.

Materials

  • V1: 5 V DC power supply, function: Main circuit power.
  • U1: 74HC32, function: Quad 2-input OR gate IC.
  • S1: SPST Toggle Switch, function: Simulates Temperature Sensor (Open=Normal, Closed=Overheat).
  • S2: SPST Toggle Switch, function: Simulates Jam Sensor (Open=Clear, Closed=Jam).
  • R1: 10 kΩ resistor, function: Pull-down for Temperature Input.
  • R2: 10 kΩ resistor, function: Pull-down for Jam Input.
  • R3: 330 Ω resistor, function: Current limiting for indicator LED.
  • D1: Red LED, function: Visual Fault Indicator.

Pin-out of the IC used

Selected Chip: 74HC32 (Quad 2-Input OR Gate)

Pin Name Logic function Connection in this case
1 1A Input A Connected to Temperature Sensor (S1)
2 1B Input B Connected to Jam Sensor (S2)
3 1Y Output Connected to LED driver (R3 + D1)
7 GND Ground Connected to Power Supply Negative (0V)
14 VCC Power (+) Connected to Power Supply Positive (5V)

Wiring guide

  • VCC: Connect V1 positive terminal to U1 pin 14.
  • 0 (GND): Connect V1 negative terminal to U1 pin 7.
  • VA (Temp Signal): Connect S1 terminal 2 to U1 pin 1.
  • VA (Temp Signal): Connect R1 between U1 pin 1 and 0.
  • VCC: Connect S1 terminal 1 to VCC.
  • VB (Jam Signal): Connect S2 terminal 2 to U1 pin 2.
  • VB (Jam Signal): Connect R2 between U1 pin 2 and 0.
  • VCC: Connect S2 terminal 1 to VCC.
  • V_OUT: Connect U1 pin 3 to R3 terminal 1.
  • LED_NODE: Connect R3 terminal 2 to D1 Anode.
  • 0 (GND): Connect D1 Cathode to 0.

Conceptual block diagram

Conceptual block diagram — 74HC32 OR gate

Schematic

Title: Production Line Fault Monitoring (OR Logic)

      [ INPUT SENSORS ]                       [ LOGIC PROCESSING ]                 [ VISUAL OUTPUT ]

                                                 (Pin 14: VCC)
                                                       |
                                                       v
[ VCC ] --> [ S1: Temp Switch ] --+--(Pin 1)-->+---------------+
                                  |            |               |
                             [ R1: 10k ]       |   U1: 74HC32  |
                                  |            |   (OR Gate)   |--(Pin 3)--> [ R3: 330 ] --> [ D1: LED ] --> [ GND ]
                               [ GND ]         |               |
                                               |               |
[ VCC ] --> [ S2: Jam Switch  ] --+--(Pin 2)-->+---------------+
                                  |                    ^
                             [ R2: 10k ]               |
                                  |               (Pin 7: GND)
                               [ GND ]
Schematic (ASCII)

Truth table

This circuit utilizes positive logic (Active High).

Sensor A (Temp) Sensor B (Jam) Output (Fault Indicator) LED State
Low (0) Low (0) Low (0) OFF
Low (0) High (1) High (1) ON
High (1) Low (0) High (1) ON
High (1) High (1) High (1) ON

Measurements and tests

  1. Standby Check: Ensure both switches S1 and S2 are open. Measure voltage at U1 Pin 3 relative to GND. It should be ~0 V. LED should be OFF.
  2. Temperature Fault Simulation: Close S1 while keeping S2 open. Measure voltage at Pin 1 (Input A). It should be 5 V. The Output Pin 3 should go to High (~5 V) and the LED must light up.
  3. Jam Fault Simulation: Open S1 and close S2. Measure voltage at Pin 2 (Input B). It should be 5 V. The LED must light up.
  4. Simultaneous Fault: Close both S1 and S2. The LED must remain ON.

SPICE netlist and simulation

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

* Practical case: Production Line Fault Monitoring

* --- Component Models ---
* Generic Red LED Model
.model DLED D (IS=1e-14 N=2 RS=10 BV=5 IBV=10u CJO=10p)

* --- Subcircuits ---
* 74HC32 Quad 2-input OR Gate
* Pinout: 1=InputA, 2=InputB, 3=Output, 7=GND, 14=VCC
* Implemented using a robust behavioral source with continuous functions
.subckt 74HC32 1 2 3 7 14
* Logic: Output = VCC if (A > 2.5V OR B > 2.5V)
* Using sigmoid function for smooth convergence: S(x) = 1/(1+exp(-k*(x-thresh)))
* max(V(1), V(2)) selects the higher voltage to compare against threshold (2.5V)
B_OR 3 7 V = V(14) * (1 / (1 + exp(-20 * (max(V(1), V(2)) - 2.5))))
.ends

* --- Main Power Supply ---
* V1: 5V DC Supply
* Wiring: Positive -> Node 14 (VCC), Negative -> Node 0 (GND)
V1 14 0 DC 5

* --- Input Sensors (Simulated Switches) ---
* S1: Temperature Sensor Switch
* Wiring: Connects VCC to VA (Pin 1). Modeled as Pulse Source to simulate toggling.
* Logic Sequence: High (Overheat) / Low (Normal)
VS1 VA 0 PULSE(0 5 0 1u 1u 200u 400u)

* S2: Jam Sensor Switch
* Wiring: Connects VCC to VB (Pin 2). Modeled as Pulse Source with faster period.
* ... (truncated in public view) ...

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

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* Practical case: Production Line Fault Monitoring

* --- Component Models ---
* Generic Red LED Model
.model DLED D (IS=1e-14 N=2 RS=10 BV=5 IBV=10u CJO=10p)

* --- Subcircuits ---
* 74HC32 Quad 2-input OR Gate
* Pinout: 1=InputA, 2=InputB, 3=Output, 7=GND, 14=VCC
* Implemented using a robust behavioral source with continuous functions
.subckt 74HC32 1 2 3 7 14
* Logic: Output = VCC if (A > 2.5V OR B > 2.5V)
* Using sigmoid function for smooth convergence: S(x) = 1/(1+exp(-k*(x-thresh)))
* max(V(1), V(2)) selects the higher voltage to compare against threshold (2.5V)
B_OR 3 7 V = V(14) * (1 / (1 + exp(-20 * (max(V(1), V(2)) - 2.5))))
.ends

* --- Main Power Supply ---
* V1: 5V DC Supply
* Wiring: Positive -> Node 14 (VCC), Negative -> Node 0 (GND)
V1 14 0 DC 5

* --- Input Sensors (Simulated Switches) ---
* S1: Temperature Sensor Switch
* Wiring: Connects VCC to VA (Pin 1). Modeled as Pulse Source to simulate toggling.
* Logic Sequence: High (Overheat) / Low (Normal)
VS1 VA 0 PULSE(0 5 0 1u 1u 200u 400u)

* S2: Jam Sensor Switch
* Wiring: Connects VCC to VB (Pin 2). Modeled as Pulse Source with faster period.
* Logic Sequence: High (Jam) / Low (Clear)
VS2 VB 0 PULSE(0 5 0 1u 1u 100u 200u)

* --- Pull-down Resistors ---
* R1: 10k Pull-down for Temp Input
R1 VA 0 10k
* R2: 10k Pull-down for Jam Input
R2 VB 0 10k

* --- Logic IC U1 ---
* U1: 74HC32 Quad OR Gate
* Connections per wiring guide:
* Pin 1 (A) -> VA
* Pin 2 (B) -> VB
* Pin 3 (Y) -> V_OUT
* Pin 7 (GND) -> 0
* Pin 14 (VCC) -> 14
XU1 VA VB V_OUT 0 14 74HC32

* --- Output Indicator ---
* R3: 330 Ohm Current Limiting Resistor
R3 V_OUT LED_NODE 330

* D1: Red LED Visual Indicator
* Anode -> LED_NODE, Cathode -> GND
D1 LED_NODE 0 DLED

* --- Analysis Directives ---
* Transient analysis to capture truth table states (00, 01, 10, 11)
.tran 1u 400u

* Print required voltages for verification
.print tran V(VA) V(VB) V(V_OUT) V(LED_NODE)

* Calculate DC operating point
.op

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (906 rows) 
Index   time            v(va)           v(vb)           v(v_out)
0	0.000000e+00	0.000000e+00	0.000000e+00	9.643749e-22
1	1.000000e-08	5.000000e-02	5.000000e-02	1.928750e-21
2	2.000000e-08	1.000000e-01	1.000000e-01	5.242886e-21
3	4.000000e-08	2.000000e-01	2.000000e-01	2.137746e-20
4	8.000000e-08	4.000000e-01	4.000000e-01	2.632654e-19
5	1.600000e-07	8.000000e-01	8.000000e-01	2.587285e-17
6	3.200000e-07	1.600000e+00	1.600000e+00	7.614990e-08
7	4.700575e-07	2.350288e+00	2.350288e+00	2.384318e-01
8	6.126008e-07	3.063004e+00	3.063004e+00	4.999936e+00
9	7.041960e-07	3.520980e+00	3.520980e+00	5.000000e+00
10	7.932149e-07	3.966074e+00	3.966074e+00	5.000000e+00
11	9.007723e-07	4.503862e+00	4.503862e+00	5.000000e+00
12	1.000000e-06	5.000000e+00	5.000000e+00	5.000000e+00
13	1.021511e-06	5.000000e+00	5.000000e+00	5.000000e+00
14	1.064534e-06	5.000000e+00	5.000000e+00	5.000000e+00
15	1.150580e-06	5.000000e+00	5.000000e+00	5.000000e+00
16	1.322672e-06	5.000000e+00	5.000000e+00	5.000000e+00
17	1.666856e-06	5.000000e+00	5.000000e+00	5.000000e+00
18	2.355224e-06	5.000000e+00	5.000000e+00	5.000000e+00
19	3.355224e-06	5.000000e+00	5.000000e+00	5.000000e+00
20	4.355224e-06	5.000000e+00	5.000000e+00	5.000000e+00
21	5.355224e-06	5.000000e+00	5.000000e+00	5.000000e+00
22	6.355224e-06	5.000000e+00	5.000000e+00	5.000000e+00
23	7.355224e-06	5.000000e+00	5.000000e+00	5.000000e+00
... (882 more rows) ...

Common mistakes and how to avoid them

  1. Leaving Inputs Floating: Failing to install pull-down resistors (R1, R2) causes the inputs to «float» and pick up noise, causing the LED to flicker or stay ON randomly. Solution: Always use 10kΩ pull-down resistors on CMOS inputs connected to switches.
  2. Missing Current Limiting Resistor: Connecting the LED directly to the 74HC32 output pin without R3. Solution: Ensure R3 (330Ω) is in series with the LED to prevent burning out the IC or the LED.
  3. Confusing Pinout: Treating the 74HC32 like a different logic chip (e.g., 74HC02 NOR) due to similar package shape. Solution: Always verify the datasheet pin diagram; Pin 3 is output for the first gate on the 74HC32.

Troubleshooting

  • LED is always ON: Check if pull-down resistors R1 and R2 are connected to Ground. If inputs are disconnected, they float High.
  • LED is very dim: The resistor R3 might be too high (e.g., 10kΩ instead of 330Ω) or the power supply voltage is below 3V.
  • Nothing happens when switches close: Verify that U1 Pin 14 is connected to 5V and Pin 7 is connected to GND. Check switch continuity.
  • Logic is inverted (LED OFF when fault occurs): You may have accidentally used a NOR gate or wired the LED active-low (Anode to VCC, Cathode to Output).

Possible improvements and extensions

  1. Latching Alarm: Add an SR Flip-Flop or a feedback loop so that once a fault is detected, the alarm stays ON until a manual «Reset» button is pressed, even if the sensor returns to normal.
  2. Audible Alert: Connect a transistor driver and a 5V active buzzer in parallel with the LED to provide an audio warning for noisy factory environments.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary logic gate used in this safety system circuit?




Question 2: What happens to the output LED when both the temperature sensor and the jam sensor are Low (0V)?




Question 3: Which component is typically used to simulate the Temperature Sensor in a basic prototype of this project?




Question 4: What is the specific function of the 74HC32 IC in this circuit?




Question 5: Why are pull-down resistors typically used on the input switches in this logic circuit?




Question 6: If only the Jam Sensor triggers (High/5V), what is the expected state of the LED?




Question 7: What is the primary purpose of a resistor placed in series with the output LED?




Question 8: Which of the following is listed as a benefit of this system for 'Equipment Protection'?




Question 9: What is the standard logic voltage level (High) used for the sensors in this description?




Question 10: How does the system behave during a 'Critical Failure' where both sensors trigger simultaneously?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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