Practical case: Water tank level control

Water tank level control prototype (Maker Style)

Level: Basic. Implement a logic safety stop for a water pump using a NAND gate.

Objective and use case

In this session, you will build a digital safety circuit using a 74HC00 NAND gate. The circuit monitors two liquid level sensors and automatically cuts power to a «pump» (simulated by an LED) only when both sensors indicate the tank is dangerously full.

  • Industrial tank filling: Prevents chemical spills by ensuring redundant sensors must agree before triggering an emergency shutdown.
  • Sump pump systems: Prevents motor burnout or overflow by managing logic states between high-water and critical-overflow marks.
  • Home automation: Simple logic for reservoir management without needing a microcontroller.

Expected outcome:
* Normal Operation: The LED (pump) remains ON (Logic High, ~5 V) when the tank is empty or partially full.
* Emergency Stop: The LED turns OFF (Logic Low, ~0 V) immediately when both switch inputs are Logic High (simulating water detection).
* Validation: A specific Truth Table will be verified where only the input condition 1, 1 results in an output of 0.

Target audience: Basic level electronics students and hobbyists.

Materials

  • V1: 5 V DC power supply, function: Main circuit power.
  • U1: 74HC00, function: Quad 2-Input NAND Gate IC.
  • S1: SPST Toggle Switch, function: High Level Sensor simulator.
  • S2: SPST Toggle Switch, function: Safety Level Sensor simulator.
  • R1: 10 kΩ resistor, function: Pull-down for S1.
  • R2: 10 kΩ resistor, function: Pull-down for S2.
  • R3: 330 Ω resistor, function: Current limiting for the Pump Status LED.
  • D1: Green LED, function: Pump status indicator (ON = Running, OFF = Stopped).

Pin-out of the IC used

Chip: 74HC00 (Quad 2-Input NAND Gate)

Pin Name Logic function Connection in this case
1 1 A Input A Connected to Sensor S1
2 1B Input B Connected to Sensor S2
3 1Y Output Y Connected to LED (Pump)
7 GND Ground Connected to 0 V
14 VCC Power Connected to 5 V

Wiring guide

Construct the circuit following these node connections. Ensure the power supply is off while building.

  • Power Rail: Connect V1 positive terminal to node VCC and negative terminal to node 0 (GND).
  • IC Power: Connect U1 pin 14 to VCC and pin 7 to 0.
  • Sensor 1 (Input A):
    • Connect S1 between VCC and node SENSOR_HI.
    • Connect R1 between SENSOR_HI and 0 (Pull-down).
    • Connect U1 pin 1 to node SENSOR_HI.
  • Sensor 2 (Input B):
    • Connect S2 between VCC and node SENSOR_SAFE.
    • Connect R2 between SENSOR_SAFE and 0 (Pull-down).
    • Connect U1 pin 2 to node SENSOR_SAFE.
  • Pump Control (Output):
    • Connect U1 pin 3 to node PUMP_CTRL.
    • Connect D1 (Anode) to node PUMP_CTRL.
    • Connect D1 (Cathode) to node LED_NODE.
    • Connect R3 between LED_NODE and 0.

Conceptual block diagram

Conceptual block diagram — 74HC00 NAND gate
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Title: Practical case: Water tank level control

      INPUTS (Sensors)                       PROCESSING (U1: 74HC00)                  OUTPUT (Pump Indicator)
   ======================                  ===========================              ===========================

   [ VCC ]
      |
   [ S1: High Sensor ]
      |
      +--(Node: SENSOR_HI)---------------> [ U1: Pin 1 (Input A) ]
      |                                             |
   [ R1: 10k Pull-Down ]                            |
      |                                             v
   [ GND ]                                     [ NAND Gate ] --(Node: PUMP_CTRL)--> [ D1: Green LED ]
                                                    ^                                       |
                                                    |                               (Node: LED_NODE)
   [ VCC ]                                          |                                       |
      |                                             |                                  [ R3: 330R ]
   [ S2: Safe Sensor ]                              |                                       |
      |                                             |                                    [ GND ]
      +--(Node: SENSOR_SAFE)-------------> [ U1: Pin 2 (Input B) ]
      |
   [ R2: 10k Pull-Down ]
      |
   [ GND ]

   (Note: U1 Power Connections -> Pin 14: VCC, Pin 7: GND)
Electrical Schematic

Truth table

The 74HC00 acts as a safety interlock. The pump runs (Output 1) by default and only stops (Output 0) when the specific danger condition (1, 1) is met.

S1 (High Level) S2 (Safety Level) Voltage at Pin 3 Pump Status (LED) Logic State
0 (Dry) 0 (Dry) High (~5 V) ON Safe
0 (Dry) 1 (Wet) High (~5 V) ON Sensor Error/Safe
1 (Wet) 0 (Dry) High (~5 V) ON Warning Level
1 (Wet) 1 (Wet) Low (~0 V) OFF STOP (Danger)

Measurements and tests

  1. Default State Check: Ensure S1 and S2 are open (OFF). Power on the circuit. Measure the voltage at node PUMP_CTRL relative to GND. It should read approximately 5 V. The Green LED should be lit.
  2. Single Sensor Test: Close S1 only. The LED should remain ON. Open S1 and close S2 only. The LED should remain ON.
  3. Safety Stop Test: Close both S1 and S2 simultaneously.
    • Measure the voltage at node PUMP_CTRL. It should drop to near 0 V (< 0.1 V).
    • Confirm the LED turns OFF immediately.
  4. Recovery: Open either switch; the LED should turn back ON.

SPICE netlist and simulation

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

* Practical case: Water tank level control
.width out=256

* --- Models ---
* Generic Green LED Model
.model DLED D(IS=1e-14 N=2 RS=10 BV=5 IBV=10u CJO=10p)
* Ideal Voltage-Controlled Switch Model
.model SW_IDEAL sw(vt=2.5 vh=0 ron=1 roff=10Meg)

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

* --- Input Sensors (Switches + Pull-downs) ---
* S1: High Level Sensor simulator
* Modeled as a switch connected to VCC, controlled by a pulse source (V_ACT1)
* Timing: Period 100us, covers logic states quickly
V_ACT1 ACT1 0 PULSE(0 5 0 1u 1u 50u 100u)
S1 VCC SENSOR_HI ACT1 0 SW_IDEAL
R1 SENSOR_HI 0 10k
* ... (truncated in public view) ...

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

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* Practical case: Water tank level control
.width out=256

* --- Models ---
* Generic Green LED Model
.model DLED D(IS=1e-14 N=2 RS=10 BV=5 IBV=10u CJO=10p)
* Ideal Voltage-Controlled Switch Model
.model SW_IDEAL sw(vt=2.5 vh=0 ron=1 roff=10Meg)

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

* --- Input Sensors (Switches + Pull-downs) ---
* S1: High Level Sensor simulator
* Modeled as a switch connected to VCC, controlled by a pulse source (V_ACT1)
* Timing: Period 100us, covers logic states quickly
V_ACT1 ACT1 0 PULSE(0 5 0 1u 1u 50u 100u)
S1 VCC SENSOR_HI ACT1 0 SW_IDEAL
R1 SENSOR_HI 0 10k

* S2: Safety Level Sensor simulator
* Modeled as a switch connected to VCC, controlled by a pulse source (V_ACT2)
* Timing: Period 200us, provides different state combinations with S1
V_ACT2 ACT2 0 PULSE(0 5 0 1u 1u 100u 200u)
S2 VCC SENSOR_SAFE ACT2 0 SW_IDEAL
R2 SENSOR_SAFE 0 10k

* --- Logic IC: U1 (74HC00 Quad 2-Input NAND Gate) ---
* Wiring Guide connections:
* Pin 1 (Input A) -> SENSOR_HI
* Pin 2 (Input B) -> SENSOR_SAFE
* Pin 3 (Output)  -> PUMP_CTRL
* Pin 7 (GND)     -> 0
* Pin 14 (VCC)    -> VCC

.subckt 74HC00 1 2 3 7 14
    * Behavioral NAND implementation using continuous sigmoid functions for convergence
    * V(3) = VCC * (1 - (Sigmoid(In1) * Sigmoid(In2)))
    * Threshold is set to VCC/2 (approx 2.5V)
    B_NAND 3 7 V = V(14) * (1 - ( (1/(1+exp(-50*(V(1)-0.5*V(14))))) * (1/(1+exp(-50*(V(2)-0.5*V(14))))) ))
.ends

XU1 SENSOR_HI SENSOR_SAFE PUMP_CTRL 0 VCC 74HC00

* --- Output Stage ---
* D1: Pump Status LED (Green)
* R3: Current limiting resistor
D1 PUMP_CTRL LED_NODE DLED
R3 LED_NODE 0 330

* --- Simulation Directives ---
.op
* Transient analysis for 500us to capture full truth table sequence
.tran 1u 500u

* --- Output Printing ---
* Required to generate simulation log
.print tran V(SENSOR_HI) V(SENSOR_SAFE) V(PUMP_CTRL) V(LED_NODE)

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation successfully cycles through all 4 logic states. When both inputs are High (~5V), the output PUMP_CTRL goes Low (~0V) and the LED voltage drops to ~0V (OFF). In all other states (00, 01, 10), the output is High (~5V) and the LED node is ~3.46V (ON).
Show raw data table (810 rows) 
Index   time            v(sensor_hi)    v(sensor_safe)  v(pump_ctrl)    v(led_node)
0	0.000000e+00	4.995005e-03	4.995005e-03	5.000000e+00	3.463208e+00
1	1.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
2	2.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
3	4.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
4	8.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
5	1.600000e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
6	3.200000e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
7	3.562500e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
8	4.196875e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
9	4.372461e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
10	4.679736e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
11	4.795524e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
12	4.902290e-07	4.995005e-03	4.995005e-03	5.000000e+00	3.463209e+00
13	5.023412e-07	4.999500e+00	4.999500e+00	4.417025e-69	-7.81556e-01
14	5.138120e-07	4.999500e+00	4.999500e+00	4.417025e-69	1.002344e-01
15	5.170059e-07	4.999500e+00	4.999500e+00	4.417025e-69	3.466376e-02
16	5.182905e-07	4.999500e+00	4.999500e+00	4.417025e-69	2.349502e-02
17	5.201200e-07	4.999500e+00	4.999500e+00	4.417025e-69	1.345184e-02
18	5.222326e-07	4.999500e+00	4.999500e+00	4.417025e-69	7.036188e-03
19	5.244685e-07	4.999500e+00	4.999500e+00	4.417025e-69	3.539225e-03
20	5.268938e-07	4.999500e+00	4.999500e+00	4.417025e-69	1.673565e-03
21	5.291278e-07	4.999500e+00	4.999500e+00	4.417025e-69	8.446489e-04
22	5.313933e-07	4.999500e+00	4.999500e+00	4.417025e-69	4.221950e-04
23	5.337647e-07	4.999500e+00	4.999500e+00	4.417025e-69	2.037947e-04
... (786 more rows) ...

Common mistakes and how to avoid them

  1. Floating Inputs: Forgetting R1 or R2 results in erratic switching because the CMOS inputs pick up electrical noise when switches are open. Fix: Always ensure inputs are pulled to Ground via resistors when the switch is open.
  2. LED Polarity: Connecting the LED backwards prevents it from lighting even when logic is High. Fix: Ensure the longer leg (Anode) faces the IC output pin.
  3. Shorting Output to Ground: Connecting Pin 3 directly to Ground to «test» it will damage the IC when it tries to output High. Fix: Always measure voltage with a multimeter in parallel, never force a node to ground with a wire.

Troubleshooting

  • Symptom: LED is always ON, even when both switches are closed.
    • Cause: Resistors R1/R2 might be connected to VCC instead of GND, or the IC is bypassed.
    • Fix: Check that R1 and R2 connect to the negative rail (0) and switches connect to VCC.
  • Symptom: LED never turns ON.
    • Cause: LED reversed or R3 is too high value/open.
    • Fix: Check D1 orientation and continuity of R3.
  • Symptom: Circuit behaves erratically when touching wires.
    • Cause: Floating inputs (Missing pull-down resistors).
    • Fix: Verify R1 and R2 are securely connected to node 0.

Possible improvements and extensions

  1. Visual and Audible Alarm: Connect an additional active buzzer (via a transistor driver) to the output, but invert the signal first so the buzzer sounds when the pump stops.
  2. Motor Drive: Replace the LED with an NPN transistor (like 2N2222) and a relay to control a real DC water pump motor, adding a flyback diode for protection.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary objective of the circuit described in the text?




Question 2: Which specific logic gate IC is used to build this safety circuit?




Question 3: In the 'Normal Operation' state, what is the status of the LED (pump)?




Question 4: What condition triggers the 'Emergency Stop' where the LED turns OFF?




Question 5: What component is used to simulate the 'Pump' in this circuit?




Question 6: According to the expected outcome, what is the only input condition that results in an output of 0?




Question 7: Which of the following is NOT listed as a use case for this circuit?




Question 8: What is the target audience for this specific electronics session?




Question 9: Why is this circuit useful for industrial tank filling?




Question 10: What logic voltage level represents the LED being ON in 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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