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Practical case: Conditional automatic irrigation system

Conditional automatic irrigation system prototype (Maker Style)

Level: Basic – Build a logic circuit to activate a pump only when soil is dry and water is available.

Objective and use case

In this practical case, you will build a safety interlock circuit using a 74HC08 AND gate. The circuit simulates a smart irrigation controller that decides whether to turn on a water pump based on two environmental conditions.

Why this is useful:
* Equipment Protection: Prevents pumps from running «dry» (without water input), which often causes mechanical failure.
* Resource Conservation: Ensures water is only dispensed when the soil actually needs moisture.
* Industrial Logic: Demonstrates the fundamental «safety interlock» concept used in heavy machinery (e.g., machine runs ONLY if guard is closed AND operator presses button).
* Digital Logic Basics: Provides a clear physical representation of the Boolean AND function ($Y = A \cdot B$).

Expected outcome:
* The Output LED (Pump) turns ON only when Switch A (Soil Sensor) is HIGH AND Switch B (Tank Sensor) is HIGH.
* If the Tank is Empty (Switch B = LOW), the pump remains OFF even if the soil is dry.
* Logic 0: Voltage $\approx$ 0 V. Logic 1: Voltage $\approx$ 5 V.

Target audience: Electronic students and hobbyists at a basic level.

Materials

  • U1: 74HC08, function: Quad 2-Input AND Gate IC.
  • S1: SPST Switch, function: Soil Moisture Sensor simulation (Closed = Dry/Logic 1).
  • S2: SPST Switch, function: Water Tank Level simulation (Closed = Water Present/Logic 1).
  • R1: 10 kΩ resistor, function: pull-down for Input A.
  • R2: 10 kΩ resistor, function: pull-down for Input B.
  • R3: 330 Ω resistor, function: LED current limiting.
  • D1: Green LED, function: Water Pump active indicator.
  • V1: 5 V DC supply, function: Main power source.

Pin-out of the IC: 74HC08

The 74HC08 contains four independent AND gates. We will use only one of them for this experiment.

PinNameLogic FunctionConnection in this case
11AInput AConnected to S1 (Soil Status)
21BInput BConnected to S2 (Tank Status)
31YOutput YConnected to LED (Pump indicator)
7GNDGroundConnected to Power Supply Ground (0V)
14VCCPower SupplyConnected to +5V Source

Wiring guide

Follow these connections carefully. The node names correspond to the function of the wire in the circuit.

  • V1 connects between node VCC and node 0 (GND).
  • U1 (Pin 14) connects to node VCC.
  • U1 (Pin 7) connects to node 0 (GND).
  • S1 connects between node VCC and node SOIL_Status.
  • R1 connects between node SOIL_Status and node 0 (GND) (Keeps input Low when switch is open).
  • S2 connects between node VCC and node TANK_Status.
  • R2 connects between node TANK_Status and node 0 (GND) (Keeps input Low when switch is open).
  • U1 (Pin 1, Input A) connects to node SOIL_Status.
  • U1 (Pin 2, Input B) connects to node TANK_Status.
  • U1 (Pin 3, Output Y) connects to node PUMP_Cmd.
  • R3 connects between node PUMP_Cmd and node LED_Anode.
  • D1 connects between node LED_Anode (Anode) and node 0 (GND) (Cathode).

Conceptual block diagram

Conceptual block diagram — 74HC08 AND gate

Schematic

[ INPUTS ]                                  [ LOGIC ]                             [ OUTPUT ]

[ S1: Soil Sensor ]
[ (Switch to VCC) ] --(Node: SOIL_Status)-->+---------------------+
[ (R1: 10k to GND)]                         |      U1: 74HC08     |
                                            |      (AND Gate)     |
                                            |                     |--(Node: PUMP_Cmd)--> [ R3: 330 Ohm ] --> [ D1: Green LED ] --> GND
                                            |   Pin 1 (Input A)   |                      (Current Lim.)      (Pump Active)
                                            |                     |
                                            |   Pin 2 (Input B)   |
[ S2: Tank Level  ] --(Node: TANK_Status)-->|                     |
[ (Switch to VCC) ]                         +---------------------+
[ (R2: 10k to GND)]
Schematic (ASCII)

Truth table

This table defines the logic states.
0 = Switch Open / 0V / Wet Soil / Empty Tank / Pump OFF
1 = Switch Closed / 5V / Dry Soil / Full Tank / Pump ON

Soil Status (A)Tank Status (B)Output Pump (Y)Real-world State
000Soil wet, Tank empty -> Standby
010Soil wet, Tank full -> Standby
100Soil dry, Tank empty -> Safety Cutoff (Protect Pump)
111Soil dry, Tank full -> Irrigation Active

Measurements and tests

Validate your circuit using a multimeter set to DC Voltage (20V range).

  1. Standby Check: Ensure both S1 and S2 are Open (OFF). Measure voltage at Pin 3 of U1.
    • Expected: ~0 V. D1 is OFF.
  2. Dry Run Protection Test: Close S1 (Soil is Dry) but leave S2 Open (Tank Empty).
    • Expected: Pin 1 reads 5 V, Pin 2 reads 0 V. Output Pin 3 must remain at 0 V. D1 is OFF.
  3. No Demand Test: Open S1 (Soil Wet) and Close S2 (Tank Full).
    • Expected: Pin 1 reads 0 V, Pin 2 reads 5 V. Output Pin 3 must remain at 0 V. D1 is OFF.
  4. Active Irrigation Test: Close both S1 and S2.
    • Expected: Pin 1 reads 5 V, Pin 2 reads 5 V. Output Pin 3 should read ~5 V (Logic High). D1 lights up Green.

SPICE netlist and simulation

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

* Title: Practical case: Conditional automatic irrigation system

* -----------------------------------------------------------------------------
* POWER SUPPLY
* -----------------------------------------------------------------------------
* V1: 5V DC supply, function: Main power source.
V1 VCC 0 DC 5

* -----------------------------------------------------------------------------
* STIMULI GENERATION (Simulating User Interaction)
* -----------------------------------------------------------------------------
* These voltage sources drive the control pins of the ideal switches (S1, S2)
* to simulate the physical sensors changing state over time.
* They are not part of the BOM but are necessary for dynamic simulation.

* Control signal for S1 (Soil Sensor): Period 200us
* Logic: 0 -> 1 -> 0 -> 1
V_CTRL_S1 N_CTRL_S1 0 PULSE(0 5 10u 1u 1u 100u 200u)

* Control signal for S2 (Tank Sensor): Period 400us
* Logic: 0 -> 0 -> 1 -> 1
V_CTRL_S2 N_CTRL_S2 0 PULSE(0 5 10u 1u 1u 200u 400u)

* -----------------------------------------------------------------------------
* INPUT STAGE (Sensors and Pull-downs)
* -----------------------------------------------------------------------------
* S1: SPST Switch, function: Soil Moisture Sensor simulation.
* Wiring: Connects between node VCC and node SOIL_Status.
* Logic: Closed (Controlled by V_CTRL_S1 High) = Dry/Logic 1.
S1 VCC SOIL_Status N_CTRL_S1 0 SW_IDEAL
* ... (truncated in public view) ...

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

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* Title: Practical case: Conditional automatic irrigation system

* -----------------------------------------------------------------------------
* POWER SUPPLY
* -----------------------------------------------------------------------------
* V1: 5V DC supply, function: Main power source.
V1 VCC 0 DC 5

* -----------------------------------------------------------------------------
* STIMULI GENERATION (Simulating User Interaction)
* -----------------------------------------------------------------------------
* These voltage sources drive the control pins of the ideal switches (S1, S2)
* to simulate the physical sensors changing state over time.
* They are not part of the BOM but are necessary for dynamic simulation.

* Control signal for S1 (Soil Sensor): Period 200us
* Logic: 0 -> 1 -> 0 -> 1
V_CTRL_S1 N_CTRL_S1 0 PULSE(0 5 10u 1u 1u 100u 200u)

* Control signal for S2 (Tank Sensor): Period 400us
* Logic: 0 -> 0 -> 1 -> 1
V_CTRL_S2 N_CTRL_S2 0 PULSE(0 5 10u 1u 1u 200u 400u)

* -----------------------------------------------------------------------------
* INPUT STAGE (Sensors and Pull-downs)
* -----------------------------------------------------------------------------
* S1: SPST Switch, function: Soil Moisture Sensor simulation.
* Wiring: Connects between node VCC and node SOIL_Status.
* Logic: Closed (Controlled by V_CTRL_S1 High) = Dry/Logic 1.
S1 VCC SOIL_Status N_CTRL_S1 0 SW_IDEAL

* R1: 10 kΩ resistor, function: pull-down for Input A.
* Wiring: Connects between node SOIL_Status and node 0 (GND).
R1 SOIL_Status 0 10k

* S2: SPST Switch, function: Water Tank Level simulation.
* Wiring: Connects between node VCC and node TANK_Status.
* Logic: Closed (Controlled by V_CTRL_S2 High) = Water Present/Logic 1.
S2 VCC TANK_Status N_CTRL_S2 0 SW_IDEAL

* R2: 10 kΩ resistor, function: pull-down for Input B.
* Wiring: Connects between node TANK_Status and node 0 (GND).
R2 TANK_Status 0 10k

* -----------------------------------------------------------------------------
* LOGIC STAGE (74HC08 Quad 2-Input AND Gate)
* -----------------------------------------------------------------------------
* U1: 74HC08
* Wiring Guide:
* - Pin 1 (Input A) -> SOIL_Status
* - Pin 2 (Input B) -> TANK_Status
* - Pin 3 (Output Y) -> PUMP_Cmd
* - Pin 7 -> GND (0)
* - Pin 14 -> VCC
* Implemented as a subcircuit to strictly expose pins as nodes.
XU1 SOIL_Status TANK_Status PUMP_Cmd 0 VCC 74HC08_Behavioral

* -----------------------------------------------------------------------------
* OUTPUT STAGE (Indicator)
* -----------------------------------------------------------------------------
* R3: 330 Ω resistor, function: LED current limiting.
* Wiring: Connects between node PUMP_Cmd and node LED_Anode.
R3 PUMP_Cmd LED_Anode 330

* D1: Green LED, function: Water Pump active indicator.
* Wiring: Connects between node LED_Anode (Anode) and node 0 (GND).
D1 LED_Anode 0 LED_Green

* -----------------------------------------------------------------------------
* MODELS & SUBCIRCUITS
* -----------------------------------------------------------------------------
* Switch Model: Low On-Resistance, High Off-Resistance, Threshold 2.5V
.model SW_IDEAL SW(Vt=2.5 Ron=0.1 Roff=100Meg)

* LED Model: Generic Green LED approximation
.model LED_Green D(IS=1e-22 N=1.5 RS=5 BV=5 IBV=10u)

* 74HC08 Subcircuit (Behavioral Implementation)
* Pinout: 1=A, 2=B, 3=Y, 7=GND, 14=VCC
.subckt 74HC08_Behavioral 1 2 3 7 14
* Logic Y = A AND B
* Implementation: Continuous sigmoid function for robust convergence.
* Output voltage swings to V(14) (VCC) when both inputs > 2.5V.
B_AND 3 7 V = V(14,7) * (1 / (1 + exp(-40 * (V(1,7) - 2.5)))) * (1 / (1 + exp(-40 * (V(2,7) - 2.5))))
.ends

* -----------------------------------------------------------------------------
* ANALYSIS COMMANDS
* -----------------------------------------------------------------------------
* Transient analysis: 500us duration to capture all logic states (00, 10, 01, 11)
.tran 1u 500u

* Print critical nodes for verification
.print tran V(SOIL_Status) V(TANK_Status) V(PUMP_Cmd) V(LED_Anode)

* Calculate DC operating point
.op

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (1202 rows)
Index   time            v(soil_status)  v(tank_status)  v(pump_cmd)
0	0.000000e+00	4.999500e-04	4.999500e-04	7.201843e-87
1	1.000000e-08	4.999500e-04	4.999500e-04	7.201843e-87
2	2.000000e-08	4.999500e-04	4.999500e-04	7.201843e-87
3	4.000000e-08	4.999500e-04	4.999500e-04	7.201843e-87
4	8.000000e-08	4.999500e-04	4.999500e-04	7.201843e-87
5	1.600000e-07	4.999500e-04	4.999500e-04	7.201843e-87
6	3.200000e-07	4.999500e-04	4.999500e-04	7.201843e-87
7	6.400000e-07	4.999500e-04	4.999500e-04	7.201843e-87
8	1.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
9	2.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
10	3.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
11	4.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
12	5.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
13	6.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
14	7.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
15	8.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
16	9.280000e-06	4.999500e-04	4.999500e-04	7.201843e-87
17	1.000000e-05	4.999500e-04	4.999500e-04	7.201843e-87
18	1.010000e-05	4.999500e-04	4.999500e-04	7.201843e-87
19	1.026000e-05	4.999500e-04	4.999500e-04	7.201843e-87
20	1.030750e-05	4.999500e-04	4.999500e-04	7.201843e-87
21	1.039062e-05	4.999500e-04	4.999500e-04	7.201843e-87
22	1.041363e-05	4.999500e-04	4.999500e-04	7.201843e-87
23	1.045390e-05	4.999500e-04	4.999500e-04	7.201843e-87
... (1178 more rows) ...

Common mistakes and how to avoid them

  • Floating Inputs: Forgetting R1 or R2 causes the inputs to «float,» making the LED flicker or turn on randomly when switches are open. Solution: Always verify pull-down resistors are connected to Ground.
  • LED Orientation: Placing the LED backwards prevents it from lighting up even when logic is correct. Solution: Ensure the longer leg (Anode) faces the resistor and the IC.
  • Confusing Chips: Using a 74HC32 (OR gate) instead of 74HC08 (AND gate). Solution: Read the text printed on the top of the IC before insertion. If it behaves like «Pump on if EITHER condition is met,» you have the wrong chip.

Troubleshooting

  • Symptom: LED is always ON, regardless of switches.
    • Cause: Inputs might be shorted directly to VCC, or the IC is damaged.
    • Fix: Check wiring at Pin 1 and 2. Ensure R1 and R2 go to Ground, not VCC.
  • Symptom: LED is very dim when active.
    • Cause: R3 value is too high.
    • Fix: Replace R3 with a value between 220 Ω and 470 Ω.
  • Symptom: Circuit works inversely (LED off when switches are closed).
    • Cause: You might be using a NAND gate (like 74HC00) or connected the LED to VCC instead of Ground (sourcing vs sinking).
    • Fix: Verify part number is 74HC08 and LED Cathode is at Ground.

Possible improvements and extensions

  1. High Power Interface: Replace the LED with an NPN transistor (e.g., 2N2222) and a relay to control a real 12V water pump.
  2. Manual Override: Add a third switch connected to an OR gate after the AND gate output, allowing a user to force the pump ON regardless of sensors.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary function of the 74HC08 IC used in this circuit?




Question 2: What real-world device does the circuit simulate?




Question 3: Under what specific condition will the Output LED (Pump) turn ON?




Question 4: What is the main purpose of the 'safety interlock' concept demonstrated here?




Question 5: What does Switch A (Soil Sensor) represent in the logic of this project?




Question 6: What is the function of the 10 kΩ resistors (R1 and R2) connected to the switches?




Question 7: If the Water Tank is Empty (Switch B = LOW), what is the state of the pump?




Question 8: What voltage level corresponds to Logic 1 in this circuit?




Question 9: What does the Output LED visually indicate when it is lit?




Question 10: Which Boolean algebraic expression represents the logic 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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