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)

Pin Name Logic function Connection in this case
1 1A Input A Connected to Temperature Comparator Output
2 1B Input B Connected to Pressure Comparator Output
3 1Y Output Connected to LED (via R4)
7 GND Ground Connected to 0V supply rail
14 VCC Power Supply Connected 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: Temperature Sensor: Pressure Input 1A (Temp Alert) Input 1B (Press Alert) Output 1Y (System Alarm) LED State
Low (< Ref) Low (< Ref) 0 0 0 OFF
Low (< Ref) High (> Ref) 0 1 0 OFF
High (> Ref) Low (< Ref) 1 0 0 OFF
High (> Ref) High (> Ref) 1 1 1 ON

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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