Practical case: Floating Inputs and Noise Sensitivity

Level: Basic. Demonstrate the instability of high-impedance inputs using a weak pull-down resistor.

Objective and use case

In this practical case, you will build a NOT gate circuit where the input is pulled to ground by an extremely high resistance (10 MΩ) instead of a standard value. This configuration simulates a «floating» or high-impedance (High-Z) wire that is technically connected but susceptible to interference.

• Understanding Input Impedance: Demonstrates why CMOS inputs (like the 74HC series) must never be left unconnected.
• Noise Susceptibility: Shows how weak connections allow environmental electromagnetic noise (EMI) to trigger false logic states.
• Leakage Simulation: Models scenarios where dirt, humidity, or PCB flux creates unintended high-resistance paths.
• Target Audience: Basic electronics students and hobbyists learning digital logic best practices.

Expected Outcome:
* When the switch is pressed, the output behaves normally (LED OFF).
* When the switch is released, the voltage at the input returns to 0 V very slowly or becomes unstable due to the weak discharge path.
* In a real-world assembly, the LED may flicker or glow dimly due to ambient mains hum (50/60 Hz) coupling into the high-impedance node.

Materials

• U1: 74HC04 Hex Inverter IC, function: Logic inversion.
• V1: 5 V DC supply, function: Main power source.
• S1: Momentary pushbutton (Normally Open), function: Input signal trigger.
• R_WEAK: 10 MΩ resistor, function: Parasitic/Weak pull-down (simulates near-floating condition).
• R_LED: 330 Ω resistor, function: Current limiting for output.
• D1: Red LED, function: Logic High indicator.
• C_PARASITIC: 10 pF capacitor, function: Represents trace/breadboard capacitance (optional but recommended for accurate simulation physics).

Pin-out of the IC used

Chip Selected: 74HC04 Hex Inverter

Note: Pins 3, 4, 5, 6, 8, 9, 10, 11, 12, 13 are unused and should theoretically be tied to GND in a final product, but are left open here for simplicity.

Wiring guide

• Power Supply:

  • V1 connects between node VCC and node 0 (GND).
  • U1 pin 14 connects to VCC.
  • U1 pin 7 connects to 0.

• Input Stage (High Impedance):

  • S1 connects between VCC and node IN_WEAK.
  • R_WEAK connects between IN_WEAK and 0.
  • C_PARASITIC connects between IN_WEAK and 0 (Simulates stray capacitance).
  • U1 pin 1 (Input 1 A) connects to IN_WEAK.

• Output Stage:

  • U1 pin 2 (Output 1Y) connects to node OUT.
  • R_LED connects between OUT and node LED_ANODE.
  • D1 connects between LED_ANODE (Anode) and 0 (Cathode).

Conceptual block diagram

Schematic

Title: Practical case: Floating Inputs and Noise Sensitivity

      INPUT STAGE (High Impedance)                        PROCESSING UNIT                        OUTPUT STAGE
   (Converging signals at Input Node)                    (Logic Inversion)                      (Visual Load)

[ VCC ] --> [ S1: Pushbutton ] --(Strong High)------\
                                                     \
                                                      \
[ GND ] --> [ R_WEAK: 10M ] ---(Weak Pull-Down)-------+--> [ U1: 74HC04 ] --(Low/High)--> [ R_LED: 330 ] --> [ D1: LED ] --> GND
                                                      /      (Pin 1 -> 2)
                                                     /
[ GND ] --> [ C_PARASITIC ] ---(Noise/Delay)--------/

Truth table

Note: With a standard 10 kΩ pull-down, the «Low» state is solid 0 V. With 10 MΩ, the pin is easily influenced by external electric fields.

Measurements and tests

1. Baseline Check: Connect the power supply. With the switch S1 open (released), measure voltage at node IN_WEAK.
   • Expectation: It should eventually settle near 0 V, but if you touch the wire insulation with your finger, the voltage might fluctuate.
2. Active State: Press S1.
   • Expectation: IN_WEAK jumps to 5 V immediately. The LED D1 turns OFF.
3. Release and Decay: Release S1 and observe D1.
   • Expectation: Unlike a standard circuit, the transition might not be instant. In a simulation with parasitic capacitance, the voltage at IN_WEAK will decay slowly (t = R × C). With 10 MΩ, even small capacitance creates a noticeable delay or «soft» transition.

SPICE netlist and simulation

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

* Practical case: Floating Inputs and Noise Sensitivity
.width out=256

* --- Power Supply ---
* V1 connects between node VCC and node 0 (GND)
V1 VCC 0 DC 5

* --- Control Signal for Pushbutton ---
* Simulates the user pressing the button.
* Logic High closes the switch, Logic Low opens it.
* Pulse: Delay 50us, Width 100us, Period 1ms (Single press simulation)
V_SW_CTRL SW_CTRL 0 PULSE(0 5 50u 1u 1u 100u 1m)

* --- Input Stage (High Impedance) ---
* S1: Momentary pushbutton connecting VCC to IN_WEAK.
* Modeled as a voltage-controlled switch (ideal switch) driven by V_SW_CTRL.
S1 VCC IN_WEAK SW_CTRL 0 SW_IDEAL

* R_WEAK: 10 M resistor (Parasitic/Weak pull-down).
* Connects between IN_WEAK and 0.
* ... (truncated in public view) ...

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* Practical case: Floating Inputs and Noise Sensitivity
.width out=256

* --- Power Supply ---
* V1 connects between node VCC and node 0 (GND)
V1 VCC 0 DC 5

* --- Control Signal for Pushbutton ---
* Simulates the user pressing the button.
* Logic High closes the switch, Logic Low opens it.
* Pulse: Delay 50us, Width 100us, Period 1ms (Single press simulation)
V_SW_CTRL SW_CTRL 0 PULSE(0 5 50u 1u 1u 100u 1m)

* --- Input Stage (High Impedance) ---
* S1: Momentary pushbutton connecting VCC to IN_WEAK.
* Modeled as a voltage-controlled switch (ideal switch) driven by V_SW_CTRL.
S1 VCC IN_WEAK SW_CTRL 0 SW_IDEAL

* R_WEAK: 10 M resistor (Parasitic/Weak pull-down).
* Connects between IN_WEAK and 0.
R_WEAK IN_WEAK 0 10Meg

* C_PARASITIC: 10 pF capacitor (Trace/Breadboard capacitance).
* Connects between IN_WEAK and 0.
C_PARASITIC IN_WEAK 0 10p

* --- Logic Stage (U1: 74HC04 Hex Inverter) ---
* U1 Pin 1 (Input 1A) connects to IN_WEAK
* U1 Pin 2 (Output 1Y) connects to OUT
* U1 Pin 7 connects to 0 (GND)
* U1 Pin 14 connects to VCC
* Implemented as a behavioral subcircuit to ensure robust convergence.
XU1 IN_WEAK OUT 0 VCC 74HC04_GATE

* --- Output Stage ---
* R_LED connects between OUT and LED_ANODE
R_LED OUT LED_ANODE 330

* D1: Red LED connects between LED_ANODE (Anode) and 0 (Cathode)
D1 LED_ANODE 0 D_RED

* --- Component Models ---

* Switch Model:
* Roff=1G ensures the switch is effectively open circuit compared to R_WEAK (10M).
* Ron=1 ensures good connection to VCC when closed.
.model SW_IDEAL SW(Vt=2.5 Ron=1 Roff=1G)

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

* 74HC04 Inverter Subcircuit:
* Uses a continuous sigmoid function for logic inversion.
* Vout = VCC / (1 + exp(k * (Vin - Vthreshold)))
.subckt 74HC04_GATE A Y GND VCC
B1 Y GND V={V(VCC) / (1 + exp(40 * (V(A) - V(VCC)/2)))}
.ends

* --- Simulation Directives ---
.op
* Transient analysis: 1us step, 500us stop time to capture discharge curve
.tran 1u 500u

* --- Output Print Directives ---
* Required for batch execution data logging
* Order: Inputs, Outputs, then others
.print tran V(IN_WEAK) V(OUT) V(SW_CTRL) V(LED_ANODE)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation correctly demonstrates the inverter logic. Initially (0-50us), the switch is open, IN_WEAK is near 0V (pulled down by 10M), and OUT is High (5V). When the switch closes (approx 50us-150us), IN_WEAK rises to 5V, and OUT drops to ~0V. After the switch opens (150us+), IN_WEAK slowly discharges through the 10M resistor and 10pF capacitor (RC time constant ~100us), causing OUT to eventually switch back High around 280us.

Show raw data table (554 rows)

Index   time            v(in_weak)      v(out)          v(sw_ctrl)      v(led_anode)
0	0.000000e+00	4.950495e-02	5.000000e+00	0.000000e+00	1.536792e+00
1	1.000000e-08	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
2	2.000000e-08	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
3	4.000000e-08	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
4	8.000000e-08	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
5	1.600000e-07	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
6	3.200000e-07	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
7	6.400000e-07	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
8	1.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
9	2.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
10	3.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
11	4.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
12	5.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
13	6.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
14	7.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
15	8.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
16	9.280000e-06	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
17	1.028000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
18	1.128000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
19	1.228000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
20	1.328000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
21	1.428000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
22	1.528000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
23	1.628000e-05	4.950495e-02	5.000000e+00	0.000000e+00	1.536791e+00
... (530 more rows) ...

Common mistakes and how to avoid them

1. Using a standard resistor: Do not use 10 kΩ or 100 kΩ. You must use 10 MΩ (Megohms) to successfully simulate the «near-floating» fragility.
2. Ignoring breadboard capacitance: On a real breadboard, the rows have stray capacitance. This combined with 10 MΩ creates a low-pass filter, making the button feel «sluggish.»
3. Touching the metal pins: If you touch the input pin of the 74HC04 with your finger while using a 10 MΩ pull-down, your body acts as an antenna, injecting 50/60 Hz noise, causing the LED to flicker visibly.

Troubleshooting

• LED is permanently OFF: The 10 MΩ resistor might be open-circuit, or the input is picking up static charge keeping it Logic HIGH. Ensure the resistor is actually connected to Ground.
• LED flickers constantly: This is actually a successful demonstration! The high impedance input is picking up ambient noise.
• Simulation error «Node is floating»: Ensure R_WEAK is strictly connected to node 0. Even though it is high resistance, it provides the DC path to ground required by SPICE solvers.

Possible improvements and extensions

1. Compare Pull-downs: Add a second switch with a standard 10 kΩ pull-down to compare the signal integrity and discharge speed against the 10 MΩ version.
2. Touch Sensor: Replace S1 with a bare copper wire. Touch it to inject noise and turn the LED on/off, effectively creating a crude touch sensor using the high input impedance.

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Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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