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Practical case: Temperature and Pressure Monitoring

Temperature and Pressure Monitoring prototype (Maker Style)

Level: Medium. Implement an industrial safety circuit that activates an alarm only when both temperature and pressure sensors exceed critical safety limits.

Objective and use case

In this session, you will build a conditional logic circuit using an LM393 comparator to digitize analog sensor signals and a 74HC08 AND gate to process the safety logic.

  • Industrial Boiler Safety: Prevents catastrophic failure by detecting when a boiler is both overheating and over-pressurized.
  • Hydraulic Systems: Monitors fluid states to prevent pump damage or pipe bursts during high-stress operations.
  • Chemical Reactor Monitoring: Ensures reaction conditions remain within safe zones, triggering emergency cooling only when multiple critical variables spike.

Expected outcome:
* Safe State: LED remains OFF if only one or neither variable exceeds the limit.
* Critical State: Red LED turns ON (Logic High) only when Temp > Limit AND Pressure > Limit.
* Logic Level: The 74HC08 output shifts from ~0V to ~5V.
* Target Audience: Engineering students and hobbyists familiar with operational amplifiers/comparators and basic digital logic.

Materials

  • V1: 5 V DC supply
  • U1: 74HC08, function: Quad 2-Input AND Gate
  • U2: LM393, function: Dual Differential Comparator
  • RT1: 10 kΩ NTC thermistor, function: Temperature sensor
  • R1: 10 kΩ resistor, function: Voltage divider bottom for NTC
  • RP1: 10 kΩ linear potentiometer, function: Pressure sensor simulator
  • RP2: 10 kΩ potentiometer, function: Temperature reference threshold (V_REF_T)
  • RP3: 10 kΩ potentiometer, function: Pressure reference threshold (V_REF_P)
  • R2: 4.7 kΩ resistor, function: Pull-up for Comparator A output (required for LM393)
  • R3: 4.7 kΩ resistor, function: Pull-up for Comparator B output (required for LM393)
  • R4: 330 Ω resistor, function: LED current limiting
  • D1: Red LED, function: Critical Alert indicator

Pin-out of the IC used

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

PinNameLogic functionConnection in this case
11AInput AConnected to Temperature Comparator Output
21BInput BConnected to Pressure Comparator Output
31YOutputConnected to LED (via R4)
7GNDGroundConnected to 0V supply rail
14VCCPower SupplyConnected to +5V supply rail

Note: The LM393 Comparator is also used but the logic decision happens in the 74HC08.

Wiring guide

Construct the circuit using the following node connections:

  • Power Rail: Connect V1 positive terminal to node VCC and negative terminal to node 0 (GND). Connect pin 14 of U1 and pin 8 of U2 to VCC. Connect pin 7 of U1 and pin 4 of U2 to 0.
  • Temperature Sensor Input (V_TEMP): Connect RT1 between VCC and V_TEMP. Connect R1 between V_TEMP and 0. (As Temp rises, resistance drops, V_TEMP rises).
  • Pressure Sensor Input (V_PRESS): Connect the wiper of RP1 to node V_PRESS. Connect the outer legs of RP1 to VCC and 0.
  • Reference Thresholds: Connect the wiper of RP2 to node V_REF_T (Temp Limit). Connect the wiper of RP3 to node V_REF_P (Pressure Limit).
  • Comparator Stage (Digitization):
    • Connect V_TEMP to U2 pin 3 (Non-inverting input A).
    • Connect V_REF_T to U2 pin 2 (Inverting input A).
    • Connect V_PRESS to U2 pin 5 (Non-inverting input B).
    • Connect V_REF_P to U2 pin 6 (Inverting input B).
  • Comparator Outputs (LOGIC_T and LOGIC_P):
    • Connect U2 pin 1 (Output A) to node LOGIC_T. Connect pull-up resistor R2 between LOGIC_T and VCC.
    • Connect U2 pin 7 (Output B) to node LOGIC_P. Connect pull-up resistor R3 between LOGIC_P and VCC.
  • Logic Gate:
    • Connect LOGIC_T to U1 pin 1 (Input 1A).
    • Connect LOGIC_P to U1 pin 2 (Input 1B).
    • Connect U1 pin 3 (Output 1Y) to node ALERT.
  • Indicator: Connect R4 between ALERT and the anode of D1. Connect the cathode of D1 to 0.

Conceptual block diagram

Conceptual block diagram — 74HC08 AND gate

Schematic

[ ANALOG INPUTS ]                  [ COMPARATORS ]                  [ LOGIC GATE ]               [ OUTPUT ]

[ Temp Sensor (RT1/R1) ] --(V_TEMP)---->+------------------+
                                        | U2: Comparator A |
                                        | (LM393)          |--(LOGIC_T)-->+
[ Temp Ref Pot (RP2)   ] --(V_REF_T)--->| w/ Pull-up R2    |              |
                                        +------------------+              |
                                                                          v
                                                                   +----------------+
                                                                   | U1: AND Gate   |
                                                                   | (74HC08)       |--(ALERT)--> [ Resistor R4 ] --> [ LED D1 ] --> GND
                                                                   +----------------+
                                                                          ^
                                        +------------------+              |
[ Press Sensor (RP1)   ] --(V_PRESS)--->| U2: Comparator B |              |
                                        | (LM393)          |--(LOGIC_P)-->+
[ Press Ref Pot (RP3)  ] --(V_REF_P)--->| w/ Pull-up R3    |
                                        +------------------+
Schematic (ASCII)

Truth table

This table represents the logic states at the inputs of the 74HC08 (after the comparator stage) and the final output.

Sensor: TemperatureSensor: PressureInput 1A (Temp Alert)Input 1B (Press Alert)Output 1Y (System Alarm)LED State
Low (< Ref)Low (< Ref)000OFF
Low (< Ref)High (> Ref)010OFF
High (> Ref)Low (< Ref)100OFF
High (> Ref)High (> Ref)111ON

Measurements and tests

  1. Calibrate Thresholds: Use a voltmeter to set V_REF_T (at RP2 wiper) to 3.0V and V_REF_P (at RP3 wiper) to 3.0V.
  2. Test Temperature Logic: Heat RT1 (or simulate by shorting R1 slightly) until V_TEMP > 3.0V. Measure LOGIC_T; it should be High (~5V). Verify LED is OFF (since Pressure is Low).
  3. Test Pressure Logic: Turn RP1 until V_PRESS > 3.0V. Measure LOGIC_P; it should be High (~5V).
  4. System Alert Test: Create a condition where V_TEMP > 3.0V AND V_PRESS > 3.0V simultaneously.
    • Measure Voltage at ALERT (U1 Pin 3): Expected ~5V.
    • Visual: The Red LED D1 must turn ON.

SPICE netlist and simulation

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

* Practical case: Temperature and Pressure Monitoring

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

* --- Sensors and Inputs ---
* Temperature Sensor (RT1 NTC + R1 Divider)
* RT1: 10 kΩ NTC thermistor (Modeled as R_RT1)
* Connected between VCC and V_TEMP
R_RT1 VCC V_TEMP 10k
* R1: 10 kΩ resistor (Voltage divider bottom)
* Connected between V_TEMP and 0 (GND)
R1 V_TEMP 0 10k

* Pressure Sensor (RP1 Potentiometer)
* RP1: 10 kΩ linear potentiometer
* Modeled as two resistors (Top/Bot) representing the wiper position.
* Outer legs to VCC and 0, wiper to V_PRESS.
R_RP1_TOP VCC V_PRESS 5k
R_RP1_BOT V_PRESS 0 5k

* --- Dynamic Stimuli (Simulation) ---
* These voltage sources drive the sensor nodes to simulate physical changes
* over time, verifying the logic thresholds (sweeping 1V to 4V).
* They effectively override the static resistor dividers for transient analysis.
V_TEMP_STIM V_TEMP 0 PULSE(1 4 0.5m 100u 100u 1m 3m)
V_PRESS_STIM V_PRESS 0 PULSE(1 4 1m 100u 100u 1.5m 4m)

* --- Reference Thresholds ---
* ... (truncated in public view) ...

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* Practical case: Temperature and Pressure Monitoring

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

* --- Sensors and Inputs ---
* Temperature Sensor (RT1 NTC + R1 Divider)
* RT1: 10 kΩ NTC thermistor (Modeled as R_RT1)
* Connected between VCC and V_TEMP
R_RT1 VCC V_TEMP 10k
* R1: 10 kΩ resistor (Voltage divider bottom)
* Connected between V_TEMP and 0 (GND)
R1 V_TEMP 0 10k

* Pressure Sensor (RP1 Potentiometer)
* RP1: 10 kΩ linear potentiometer
* Modeled as two resistors (Top/Bot) representing the wiper position.
* Outer legs to VCC and 0, wiper to V_PRESS.
R_RP1_TOP VCC V_PRESS 5k
R_RP1_BOT V_PRESS 0 5k

* --- Dynamic Stimuli (Simulation) ---
* These voltage sources drive the sensor nodes to simulate physical changes
* over time, verifying the logic thresholds (sweeping 1V to 4V).
* They effectively override the static resistor dividers for transient analysis.
V_TEMP_STIM V_TEMP 0 PULSE(1 4 0.5m 100u 100u 1m 3m)
V_PRESS_STIM V_PRESS 0 PULSE(1 4 1m 100u 100u 1.5m 4m)

* --- Reference Thresholds ---
* RP2: 10 kΩ potentiometer (Temperature Reference)
* Configured as divider, wiper to V_REF_T. Set to ~2.5V.
R_RP2_TOP VCC V_REF_T 5k
R_RP2_BOT V_REF_T 0 5k

* RP3: 10 kΩ potentiometer (Pressure Reference)
* Configured as divider, wiper to V_REF_P. Set to ~2.5V.
R_RP3_TOP VCC V_REF_P 5k
R_RP3_BOT V_REF_P 0 5k

* --- Comparator Stage (U2: LM393) ---
* U2: Dual Differential Comparator
* Connections based on Wiring Guide:
*   Comp A (Temp): In+ (3)=V_TEMP, In- (2)=V_REF_T, Out (1)=LOGIC_T
*   Comp B (Press): In+ (5)=V_PRESS, In- (6)=V_REF_P, Out (7)=LOGIC_P
*   Power: VCC (8), GND (4)
XU2 LOGIC_T V_REF_T V_TEMP 0 V_PRESS V_REF_P LOGIC_P VCC LM393

* Pull-up resistors (Required for Open Collector Outputs)
* R2: 4.7 kΩ pull-up for Comparator A
R2 VCC LOGIC_T 4.7k
* R3: 4.7 kΩ pull-up for Comparator B
R3 VCC LOGIC_P 4.7k

* --- Logic Stage (U1: 74HC08) ---
* U1: Quad 2-Input AND Gate
* Connections:
*   Gate 1: Input 1A (1)=LOGIC_T, Input 1B (2)=LOGIC_P, Output 1Y (3)=ALERT
*   Power: VCC (14), GND (7)
*   Unused inputs (4,5,9,10,12,13) connected to 0 (GND) to prevent floating.
XU1 LOGIC_T LOGIC_P ALERT 0 0 0 0 0 0 0 0 0 0 VCC 74HC08

* --- Indicator ---
* R4: 330 Ω resistor (LED current limiting)
R4 ALERT LED_A 330
* D1: Red LED (Cathode to GND)
D1 LED_A 0 DLED

* --- Models and Subcircuits ---

* LED Model
.model DLED D(IS=1e-14 N=1.7 RS=10)

* LM393 Subcircuit (Behavioral Open Collector)
.subckt LM393 1 2 3 4 5 6 7 8
* Pinout: 1=OutA, 2=InA-, 3=InA+, 4=GND, 5=InB+, 6=InB-, 7=OutB, 8=VCC
* Logic: If In+ > In-, Output is High-Z (Pull-up High).
*        If In+ < In-, Output is Low (GND).
* Implementation uses Voltage Controlled Switch to GND.
* Control V = In(-) - In(+). If V > 0 (In- > In+), Switch Closed (Low).
B_A_CTRL 10 0 V = V(2) - V(3)
S_A 1 4 10 0 SW_OC
B_B_CTRL 20 0 V = V(6) - V(5)
S_B 7 4 20 0 SW_OC
.model SW_OC SW(Vt=0 Vh=1m Ron=10 Roff=100Meg)
.ends LM393

* 74HC08 Subcircuit (Behavioral AND Gate)
.subckt 74HC08 1 2 3 4 5 6 7 8 9 10 11 12 13 14
* Pinout: 1=1A, 2=1B, 3=1Y, 7=GND, 14=VCC ...
* Gate 1 Logic: Output High (VCC) if V(1)>2.5 and V(2)>2.5
B_Y1 3 7 V = V(14) * (1 / (1 + exp(-50*(V(1)-2.5)))) * (1 / (1 + exp(-50*(V(2)-2.5))))
.ends 74HC08

* --- Simulation Directives ---
.tran 10u 5ms
.print tran V(V_TEMP) V(V_PRESS) V(LOGIC_T) V(LOGIC_P) V(ALERT)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (1124 rows)
Index   time            v(v_temp)       v(v_press)      v(logic_t)
0	0.000000e+00	1.000000e+00	1.000000e+00	1.061571e-02
1	1.000000e-07	1.000000e+00	1.000000e+00	1.061571e-02
2	2.000000e-07	1.000000e+00	1.000000e+00	1.061571e-02
3	4.000000e-07	1.000000e+00	1.000000e+00	1.061571e-02
4	8.000000e-07	1.000000e+00	1.000000e+00	1.061571e-02
5	1.600000e-06	1.000000e+00	1.000000e+00	1.061571e-02
6	3.200000e-06	1.000000e+00	1.000000e+00	1.061571e-02
7	6.400000e-06	1.000000e+00	1.000000e+00	1.061571e-02
8	1.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
9	2.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
10	3.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
11	4.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
12	5.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
13	6.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
14	7.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
15	8.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
16	9.280000e-05	1.000000e+00	1.000000e+00	1.061571e-02
17	1.028000e-04	1.000000e+00	1.000000e+00	1.061571e-02
18	1.128000e-04	1.000000e+00	1.000000e+00	1.061571e-02
19	1.228000e-04	1.000000e+00	1.000000e+00	1.061571e-02
20	1.328000e-04	1.000000e+00	1.000000e+00	1.061571e-02
21	1.428000e-04	1.000000e+00	1.000000e+00	1.061571e-02
22	1.528000e-04	1.000000e+00	1.000000e+00	1.061571e-02
23	1.628000e-04	1.000000e+00	1.000000e+00	1.061571e-02
... (1100 more rows) ...

Common mistakes and how to avoid them

  1. Missing Pull-up Resistors on Comparators: The LM393 has open-collector outputs. If you omit R2 and R3, the inputs to the 74HC08 will float or remain low, preventing the circuit from working. Solution: Always install pull-ups (4.7kΩ to 10kΩ) from the output pin to VCC.
  2. Incorrect NTC Wiring: Connecting the NTC to ground and the fixed resistor to VCC creates a voltage that drops as temperature rises. Solution: Connect the NTC to VCC and the fixed resistor to Ground to ensure voltage increases with temperature, matching the non-inverting comparator logic.
  3. Floating Inputs on 74HC08: Leaving unused inputs on the logic chip connected to nothing can cause noise and higher power consumption. Solution: Connect unused inputs (e.g., pins 4, 5, 9, 10, 12, 13) to GND.

Troubleshooting

  • LED never turns ON: Check if R2 or R3 are missing. Without them, the AND gate inputs see Logic 0. Verify the orientation of the LED.
  • LED is always ON: Check RP2 and RP3. If the reference voltage is set to 0V, the sensors will always appear «High» relative to the reference.
  • Erratic/Flickering LED: The voltage at the comparator inputs might be hovering exactly at the threshold. This creates noise. Adding a hysteresis feedback resistor can solve this, but ensuring clean power connections usually suffices for basic tests.

Possible improvements and extensions

  1. Add Hysteresis: Connect a high-value resistor (e.g., 100kΩ) between the comparator output and the non-inverting input. This prevents the «chattering» effect when sensor values hover near the threshold.
  2. Audible Alarm: Connect a buzzer with a transistor driver (like a 2N2222) to the output of the 74HC08 alongside the LED for an audible warning in a noisy industrial environment.

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Quick Quiz

Question 1: What is the primary function of the LM393 component in this circuit?




Question 2: Which logic gate is used to process the safety logic ensuring both conditions must be met?




Question 3: Under what condition will the Red LED turn ON (Critical State)?




Question 4: What is the expected voltage level of the 74HC08 output in the 'Critical State'?




Question 5: Which of the following is listed as a specific use case for this circuit?




Question 6: What state is the LED in if only the temperature exceeds the critical limit but pressure does not?




Question 7: What is the primary purpose of this industrial safety circuit?




Question 8: In the context of Hydraulic Systems, what does this circuit help prevent?




Question 9: Who is the target audience for this circuit project?




Question 10: What logic level represents the 'Critical State' at the 74HC08 output?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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