FPGA - Prometeo.blog

Level: Basic. Demonstrate how two resistors in series divide the input voltage into predictable proportions.

Objective and use case

In this practical case, you will build a fundamental circuit that uses two resistors in series to reduce a higher DC source voltage to a specific lower voltage level.

Why it is useful:
- Sensor interfacing: Adapts high-voltage sensors (e.g., 12 V automotive sensors) to low-voltage microcontrollers (e.g., 3.3 V or 5 V logic).
- Biasing: Provides stable reference voltages for transistor bases or operational amplifier inputs.
- Level shifting: Simple method to step down signal levels between different circuit stages.
Expected outcome:
- Input Voltage (Vin): Measured at the full 9 V supply.
- Output Voltage (Vout): Measured at the junction between the resistors; expecting exactly 4.5 V (50% of input).
- Current: A small, safe current flows continuously from the source to ground through the series path.
- Ratio verification: The output voltage follows the formula Vout = Vin × (R2 / (R1 + R2)).
Target audience and level: Students starting with Ohm’s Law and Series Circuits (Level: Basic).

Materials

V1: 9 V DC voltage source (battery or power supply).
R1: 10 kΩ resistor, function: high-side element (drops half the voltage).
R2: 10 kΩ resistor, function: low-side element (measurement resistor).
M1: Digital Multimeter (Voltmeter mode), function: measurement tool.

Wiring guide

Construct the circuit using the following node connections. Ensure the power supply is turned off while assembling components.

V1: Connect the positive terminal to node VCC and the negative terminal to node 0 (GND).
R1: Connect between node VCC and node VOUT.
R2: Connect between node VOUT and node 0 (GND).

Conceptual block diagram

Schematic

[ INPUT ]                    [ PROCESSING ]                    [ OUTPUT ]

 [ 9V Source (V1) ] --(VCC)--> [ R1: High-Side 10k ] --(VOUT)--> [ Multimeter (M1) ]
                                          |
                                          v
                                 [ R2: Low-Side 10k ]
                                          |
                                          v
                                    [ Node 0 (GND) ]

Schematic (ASCII)

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

Perform the following steps to validate the circuit behavior.

Set up the Multimeter: Switch your multimeter to DC Voltage mode (20 V range or auto-range).
Measure Input (Vin): Place the red probe on node VCC and the black probe on node 0. Verify the reading is approximately 9 V.
Measure Output (Vout): Place the red probe on node VOUT (the junction between R1 and R2) and the black probe on node 0.
Validate Result: The reading should be approximately 4.5 V.
- Calculation: Vout = 9V × (10kΩ / (10kΩ + 10kΩ)) = 9V × 0.5 = 4.5V.

SPICE netlist and simulation

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

* Title: Simple voltage divider

* --- Power Supply ---
* V1: 9 V DC voltage source
* Connected positive to VCC, negative to 0 (GND)
V1 VCC 0 DC 9

* --- Components ---
* R1: 10 kOhm resistor (High-side)
* Connected between VCC and VOUT
R1 VCC VOUT 10k

* R2: 10 kOhm resistor (Low-side)
* Connected between VOUT and 0 (GND)
R2 VOUT 0 10k

* M1: Digital Multimeter (Voltmeter mode)
* Function: Measurement tool across R2 (VOUT to GND)
* Modeled as a high-impedance resistor (10 MegOhm) to represent input impedance
R_M1 VOUT 0 10Meg
* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Title: Simple voltage divider

* --- Power Supply ---
* V1: 9 V DC voltage source
* Connected positive to VCC, negative to 0 (GND)
V1 VCC 0 DC 9

* --- Components ---
* R1: 10 kOhm resistor (High-side)
* Connected between VCC and VOUT
R1 VCC VOUT 10k

* R2: 10 kOhm resistor (Low-side)
* Connected between VOUT and 0 (GND)
R2 VOUT 0 10k

* M1: Digital Multimeter (Voltmeter mode)
* Function: Measurement tool across R2 (VOUT to GND)
* Modeled as a high-impedance resistor (10 MegOhm) to represent input impedance
R_M1 VOUT 0 10Meg

* --- Simulation and Output ---
* Operating point analysis for DC steady state
.op

* Transient analysis (required for .print tran)
* Simulating for 5ms to show steady DC levels
.tran 100u 5ms

* Print directives
.print tran V(VCC) V(VOUT)

.end

Simulation Results (Transient Analysis)

Show raw data table (59 rows)

Index   time            v(vcc)          v(vout)
0	0.000000e+00	9.000000e+00	4.497751e+00
1	5.000000e-07	9.000000e+00	4.497751e+00
2	1.000000e-06	9.000000e+00	4.497751e+00
3	2.000000e-06	9.000000e+00	4.497751e+00
4	4.000000e-06	9.000000e+00	4.497751e+00
5	8.000000e-06	9.000000e+00	4.497751e+00
6	1.600000e-05	9.000000e+00	4.497751e+00
7	3.200000e-05	9.000000e+00	4.497751e+00
8	6.400000e-05	9.000000e+00	4.497751e+00
9	1.280000e-04	9.000000e+00	4.497751e+00
10	2.280000e-04	9.000000e+00	4.497751e+00
11	3.280000e-04	9.000000e+00	4.497751e+00
12	4.280000e-04	9.000000e+00	4.497751e+00
13	5.280000e-04	9.000000e+00	4.497751e+00
14	6.280000e-04	9.000000e+00	4.497751e+00
15	7.280000e-04	9.000000e+00	4.497751e+00
16	8.280000e-04	9.000000e+00	4.497751e+00
17	9.280000e-04	9.000000e+00	4.497751e+00
18	1.028000e-03	9.000000e+00	4.497751e+00
19	1.128000e-03	9.000000e+00	4.497751e+00
20	1.228000e-03	9.000000e+00	4.497751e+00
21	1.328000e-03	9.000000e+00	4.497751e+00
22	1.428000e-03	9.000000e+00	4.497751e+00
23	1.528000e-03	9.000000e+00	4.497751e+00
... (35 more rows) ...

Common mistakes and how to avoid them

Connecting a heavy load: Connecting a motor or low-resistance load to VOUT will cause the voltage to drop significantly below 4.5 V (loading effect). Solution: Only connect high-impedance loads (like microcontroller inputs) or use a buffer.
Using incorrect resistor ratios: Using random resistor values will result in a random output voltage. Solution: Always calculate the required ratio using the voltage divider formula before building.
Overheating resistors: Using very low resistance values (e.g., 10 Ω) connects the supply almost directly to ground, causing high current. Solution: Use values in the kΩ range for signal reference voltages to minimize power waste.

Troubleshooting

Symptom: VOUT reads 0 V.
- Cause: R1 is open (broken) or R2 is shorted to ground.
- Fix: Check continuity of R1 and ensure R2 legs are not touching.
Symptom: VOUT equals VCC (9 V).
- Cause: R2 is open (broken) or R1 is shorted.
- Fix: Ensure R2 is correctly inserted into the breadboard rails.
Symptom: VOUT is slightly off (e.g., 4.6 V instead of 4.5 V).
- Cause: Resistor tolerance (standard resistors vary by ±5%).
- Fix: This is normal behavior. Use 1% precision resistors if exact values are critical.

Possible improvements and extensions

Variable Divider: Replace R1 and R2 with a single 10 kΩ potentiometer (wiper to output) to create a variable voltage source from 0 V to 9 V.
Buffered Output: Connect the VOUT node to an Operational Amplifier configured as a voltage follower to drive loads like LEDs without dropping the voltage.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Demonstrate how a resistor protects a sensitive component (LED) by limiting current flow according to Ohm’s Law.

Objective and use case

In this case, you will build a fundamental series circuit connecting a DC voltage source, a current-limiting resistor, and a Light Emitting Diode (LED).

Why it is useful:
* Component Protection: Prevents the LED from drawing excessive current and burning out instantly.
* Ohm’s Law Application: Visually demonstrates the relationship between Voltage, Current, and Resistance ($I = V/R$).
* Status Indication: Forms the basis for power indicators on almost every electronic device.
* Diagnostic Tooling: Simple LED circuits are often used to debug logic levels in complex systems.

Expected outcome:
* The LED lights up steadily without overheating.
* The current flowing through the circuit remains within the safe range (typically 10–20 mA).
* The voltage drop across the resistor corresponds to the supply voltage minus the LED forward voltage.

Target audience and level: Beginners and students starting with basic component analysis.

Materials

V1: 5 V DC supply
R1: 220 Ω resistor, function: current limiting
D1: Red LED, function: light emission
M1: Multimeter, function: current measurement (A)
M2: Multimeter, function: voltage measurement (V)

Wiring guide

This circuit uses a series topology. We define the nodes as VCC (5V Source), 0 (Ground), and NODE_A (Intermediate connection).

V1 (DC Source): Positive terminal connects to node VCC. Negative terminal connects to node 0.
R1 (Resistor): Connects between node VCC and node NODE_A.
D1 (LED): Anode connects to node NODE_A. Cathode connects to node 0.

Conceptual block diagram

Schematic

[ SOURCE ]                  [ CURRENT CONTROL ]              [ OUTPUT / LOAD ]

    [ V1: 5V DC ] --(VCC)--> [ R1: 220 Ohm ] --(Node A)--> [ D1: Red LED ] --(0)--> [ GND ]

Schematic (ASCII)

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

To validate Ohm’s Law and component safety:

Calculate Expected Current:
- Assume LED Forward Voltage ($V_f$) $\approx$ 2.0 V.
- Voltage across R1: $V_{R1} = V_{source} – V_f = 5V – 2V = 3V$.
- Expected Current: $I = V_{R1} / R1 = 3V / 220\Omega \approx 13.6 mA$.
Voltage Measurement: Set multimeter M2 to DC Volts. Measure across R1 (leads on VCC and NODE_A). The reading should be approximately 3 V.
Current Measurement: Break the circuit at node VCC or 0 and insert multimeter M1 in series (Amperemeter mode). The reading should be close to 13–14 mA.
Visual Check: The LED should emit a steady, bright red light.

SPICE netlist and simulation

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

* Practical case: Current limiting in an LED

* --- Power Supply ---
* V1: 5V DC Supply connected between VCC and 0 (GND)
V1 VCC 0 DC 5

* --- Components ---
* R1: 220 Ohm Resistor
* Function: Current limiting
* Connected between VCC and NODE_A
R1 VCC NODE_A 220

* D1: Red LED
* Function: Light emission
* Anode connected to NODE_A, Cathode connected to 0 (GND)
D1 NODE_A 0 DLED

* --- Models ---
* Model for D1 (Red LED)
* Parameters: IS (Saturation Current), N (Emission Coefficient), RS (Series Resistance)
* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Current limiting in an LED

* --- Power Supply ---
* V1: 5V DC Supply connected between VCC and 0 (GND)
V1 VCC 0 DC 5

* --- Components ---
* R1: 220 Ohm Resistor
* Function: Current limiting
* Connected between VCC and NODE_A
R1 VCC NODE_A 220

* D1: Red LED
* Function: Light emission
* Anode connected to NODE_A, Cathode connected to 0 (GND)
D1 NODE_A 0 DLED

* --- Models ---
* Model for D1 (Red LED)
* Parameters: IS (Saturation Current), N (Emission Coefficient), RS (Series Resistance)
* Tuned for approximately 1.8V - 2.0V forward voltage drop
.model DLED D (IS=1e-14 N=2.5 RS=5 BV=5 IBV=10u)

* --- Analysis Directives ---
* Calculate DC operating point
.op

* Transient analysis (Required for .print output generation)
* Step: 100us, Stop: 10ms
.tran 100u 10m

* --- Output / Measurements ---
* Simulating M2 (Multimeter - Voltage): Probing NODE_A (Voltage across LED)
* Simulating M1 (Multimeter - Current): Probing I(V1) (Total circuit current)
* Note: I(V1) will be negative as current flows out of the voltage source.
.print tran V(VCC) V(NODE_A) I(V1)

.end

Simulation Results (Transient Analysis)

Show raw data table (108 rows)

Index   time            v(vcc)          v(node_a)       v1#branch
0	0.000000e+00	5.000000e+00	1.880179e+00	-1.41810e-02
1	1.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
2	2.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
3	4.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
4	8.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
5	1.600000e-05	5.000000e+00	1.880178e+00	-1.41810e-02
6	3.200000e-05	5.000000e+00	1.880178e+00	-1.41810e-02
7	6.400000e-05	5.000000e+00	1.880178e+00	-1.41810e-02
8	1.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
9	2.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
10	3.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
11	4.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
12	5.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
13	6.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
14	7.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
15	8.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
16	9.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
17	1.028000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
18	1.128000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
19	1.228000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
20	1.328000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
21	1.428000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
22	1.528000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
23	1.628000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
... (84 more rows) ...

Common mistakes and how to avoid them

Reversed LED Polarity: Connecting the LED cathode to positive. Solution: Ensure the longer leg (Anode) faces the positive voltage side (towards R1).
Omitting the Resistor: Connecting the LED directly to 5V. Solution: Always verify the resistor is in series before applying power to prevent destroying the LED.
Measuring Current in Parallel: Attempting to measure current by probing across the LED like a voltmeter. Solution: Always break the circuit path and place the meter in series for current measurements.

Troubleshooting

Symptom: LED does not light up.
- Cause: LED connected backwards or broken circuit.
- Fix: Check orientation (Anode/Cathode) and ensure all breadboard connections are tight.
Symptom: LED flashes once and dies.
- Cause: No current limiting resistor used (LED burned out).
- Fix: Replace the LED and ensure R1 (220 Ω) is correctly installed.
Symptom: LED is very dim.
- Cause: Resistance value is too high (e.g., using 10 kΩ instead of 220 Ω).
- Fix: Verify the resistor color bands or measure R1 with a multimeter.
Symptom: Multimeter reads 0 A.
- Cause: Blown fuse in the multimeter or improper mode selection.
- Fix: Check probe connections (Com/mA) and ensure the meter dial is set to DC Current.

Possible improvements and extensions

Variable Brightness: Replace R1 with a 1 kΩ potentiometer in series with a 100 Ω safety resistor to manually adjust the brightness.
Multiple Colors: Swap the Red LED for Blue or Green and measure the change in current (different colors have different forward voltages, affecting the calculation).

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Medium. Design a logic circuit that alerts the user when a water sensor stops detecting liquid using a NOT gate.

Objective and use case

In this case, you will build a monitoring circuit using a 74HC04 inverter that illuminates a red LED when a tank’s liquid level drops below a critical point.

Prevents pump damage: Stops water pumps from running dry in hydroponic systems.
Household safety: Alerts when rooftop reserve tanks are empty.
Industrial maintenance: Visual flag for coolant refill requirements.

Expected outcome:
* Water Present: The sensor is open (Logic 1 input) $\rightarrow$ LED remains OFF.
* Tank Empty: The sensor closes (Logic 0 input) $\rightarrow$ LED turns ON.
* Logic Level: $V_{in} \approx 0\text{ V}$ activates the alert; $V_{in} \approx 5\text{ V}$ indicates normal status.

Target audience: Electronics students and hobbyists familiar with basic digital logic.

Materials

V1: 5 V DC power supply, function: main circuit power
U1: 74HC04 Hex Inverter IC, function: logic inversion
S1: Float switch (SPST, configured to Close when Empty), function: liquid level sensor
R1: 10 kΩ resistor, function: pull-up for sensor signal
R2: 330 Ω resistor, function: LED current limiting
D1: Red LED, function: visual empty alert
C1: 100 nF ceramic capacitor, function: power supply decoupling

Pin-out of the IC used

Selected Chip: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
1	1A	Input	Connected to Sensor Node (SENSE_IN)
2	1Y	Output	Connected to LED circuit (ALERT_OUT)
7	GND	Ground	Connected to GND (0 V)
14	VCC	Power	Connected to 5 V Supply

Wiring guide

V1 connects between node VCC and node GND.
C1 connects between node VCC and node GND (placed physically close to U1).
R1 connects between node VCC and node SENSE_IN.
S1 connects between node SENSE_IN and node GND.
U1 pin 1 connects to node SENSE_IN.
U1 pin 2 connects to node ALERT_OUT.
U1 pin 14 connects to VCC; pin 7 connects to GND.
R2 connects between node ALERT_OUT and node LED_ANODE.
D1 connects between node LED_ANODE (Anode) and node GND (Cathode).

Conceptual block diagram

Schematic

[ INPUT / SENSOR ]                 [ LOGIC PROCESSING ]                 [ OUTPUT / ALERT ]

[ VCC 5V ] --> [ R1: 10k ] --+
               (Pull-Up)     |
                             |
                             V
                        (SENSE_IN) ---->+------------------+
                        (Pin 1)         |    U1: 74HC04    |
                             ^          |   Hex Inverter   |--(ALERT_OUT)--> [ R2: 330R ] --> [ D1: Red LED ] --> GND
                             |          |   (Pin 1 -> 2)   |  (Pin 2)        (Limiting)       (Anode/Cathode)
[ GND 0V ] --> [ S1: Float ]-+          +------------------+
               (Switch)                           ^
                                                  |
                                            [ C1: 100nF ]
                                            (Decoupling)
                                            (VCC / GND)

Schematic (ASCII)

Truth table

Water State	Sensor Switch (S1)	Input Voltage (Pin 1)	Logic Input	Output Voltage (Pin 2)	LED State
Full	OPEN	5 V (via Pull-up)	1	0 V	OFF
Empty	CLOSED	0 V (connected to GND)	0	5 V	ON

Measurements and tests

Supply Check: Measure voltage between VCC and GND. Ensure it is stable at 5 V.
Full Tank Simulation: Manually lift the float (ensure S1 is OPEN). Measure voltage at SENSE_IN. It should be $\approx 5\text{ V}$. Verify LED is OFF.
Empty Tank Simulation: Drop the float (ensure S1 is CLOSED). Measure voltage at SENSE_IN. It should be $\approx 0\text{ V}$.
Logic Output: While S1 is closed (Empty), measure voltage at ALERT_OUT. It should be $\approx 5\text{ V}$.
Current Draw: Measure the current through D1 ($I_{led}$) when ON. It should be approximately 10–12 mA depending on the specific LED voltage drop.

SPICE netlist and simulation

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

* Practical case: Empty Tank Level Indicator

* ==============================================================================
* BILL OF MATERIALS & COMPONENTS
* ==============================================================================

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

* --- Decoupling ---
* C1: 100 nF ceramic capacitor (Power supply decoupling)
C1 VCC 0 100n

* --- Sensor Input Section ---
* R1: 10 kΩ resistor (Pull-up for sensor signal)
R1 VCC SENSE_IN 10k

* S1: Float switch (SPST)
* Wiring: Connects between node SENSE_IN and node GND.
* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Empty Tank Level Indicator

* ==============================================================================
* BILL OF MATERIALS & COMPONENTS
* ==============================================================================

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

* --- Decoupling ---
* C1: 100 nF ceramic capacitor (Power supply decoupling)
C1 VCC 0 100n

* --- Sensor Input Section ---
* R1: 10 kΩ resistor (Pull-up for sensor signal)
R1 VCC SENSE_IN 10k

* S1: Float switch (SPST)
* Wiring: Connects between node SENSE_IN and node GND.
* Simulation: Modeled as a Voltage Controlled Switch (SW).
* Logic: 
*   - Tank Full (Float Up) -> Switch Open -> SENSE_IN pulled to VCC.
*   - Tank Empty (Float Down) -> Switch Closed -> SENSE_IN pulled to GND.
* Control Source V_FLOAT_ACT simulates the float movement.
*   - 0V = Float Up (Full)
*   - 5V = Float Down (Empty)
S1 SENSE_IN 0 FLOAT_CTRL 0 SW_FLOAT
.model SW_FLOAT SW(Vt=2.5 Ron=0.1 Roff=10Meg)

* Stimulus: Float starts Up (Full), drops to Down (Empty) at 50us, returns at 200us.
V_FLOAT_ACT FLOAT_CTRL 0 PULSE(0 5 50u 1u 1u 150u 400u)

* --- Logic Processing ---
* U1: 74HC04 Hex Inverter
* Wiring Guide: Pin 1 (In) -> SENSE_IN, Pin 2 (Out) -> ALERT_OUT
* Power: Pin 14 -> VCC, Pin 7 -> GND
* Implemented as a subcircuit to strictly map pins.
XU1 SENSE_IN ALERT_OUT 0 VCC 74HC04_GATE

* Subcircuit definition for one gate of 74HC04
.subckt 74HC04_GATE IN OUT GND VCC
    * Behavioral voltage source for robust logic inversion
    * Uses sigmoid function for convergence: Vout = VCC if Vin < 2.5V
    B1 OUT GND V = V(VCC) * (1 / (1 + exp(50 * (V(IN) - 2.5))))
.ends

* --- Output Alert ---
* R2: 330 Ω resistor (LED current limiting)
R2 ALERT_OUT LED_ANODE 330

* D1: Red LED (Visual empty alert)
* Wiring: Anode -> LED_ANODE, Cathode -> GND
D1 LED_ANODE 0 LED_RED
.model LED_RED D(IS=1e-14 N=2 RS=5 BV=5 IBV=10u CJO=40p)

* ==============================================================================
* ANALYSIS COMMANDS
* ==============================================================================

* Operating Point Analysis
.op

* Transient Analysis
* Run for 500us to capture the float switch activation cycle
.tran 1u 500u

* Output Printing
* Monitor Sensor Input, Inverter Output, and LED Voltage
.print tran V(SENSE_IN) V(ALERT_OUT) V(LED_ANODE) V(FLOAT_CTRL)

.end

Simulation Results (Transient Analysis)

Show raw data table (1190 rows)

Index   time            v(sense_in)     v(alert_out)    v(led_anode)
0	0.000000e+00	4.995005e+00	3.316079e-54	-1.70080e-28
1	1.000000e-08	4.995005e+00	3.316079e-54	-9.73961e-29
2	2.000000e-08	4.995005e+00	3.316079e-54	-1.41516e-29
3	4.000000e-08	4.995005e+00	3.316079e-54	8.723601e-29
4	8.000000e-08	4.995005e+00	3.316079e-54	1.163518e-28
5	1.600000e-07	4.995005e+00	3.316079e-54	4.380930e-29
6	3.200000e-07	4.995005e+00	3.316079e-54	-1.45299e-29
7	6.400000e-07	4.995005e+00	3.316079e-54	-1.01395e-29
8	1.280000e-06	4.995005e+00	3.316079e-54	-5.46095e-32
9	2.280000e-06	4.995005e+00	3.316079e-54	4.098577e-31
10	3.280000e-06	4.995005e+00	3.316079e-54	2.282032e-32
11	4.280000e-06	4.995005e+00	3.316079e-54	-9.50625e-33
12	5.280000e-06	4.995005e+00	3.316079e-54	-1.09186e-33
13	6.280000e-06	4.995005e+00	3.316079e-54	1.911218e-34
14	7.280000e-06	4.995005e+00	3.316079e-54	3.847480e-35
15	8.280000e-06	4.995005e+00	3.316079e-54	-2.97995e-36
16	9.280000e-06	4.995005e+00	3.316079e-54	-1.15977e-36
17	1.028000e-05	4.995005e+00	3.316079e-54	1.723722e-38
18	1.128000e-05	4.995005e+00	3.316079e-54	3.117034e-38
19	1.228000e-05	4.995005e+00	3.316079e-54	1.177223e-39
20	1.328000e-05	4.995005e+00	3.316079e-54	-7.52109e-40
21	1.428000e-05	4.995005e+00	3.316079e-54	-6.99870e-41
22	1.528000e-05	4.995005e+00	3.316079e-54	1.597704e-41
23	1.628000e-05	4.995005e+00	3.316079e-54	2.660714e-42
... (1166 more rows) ...

Common mistakes and how to avoid them

Leaving inputs floating: Even though we only use one gate (Pin 1/2), unused inputs on CMOS chips (pins 3, 5, 9, 11, 13) should be tied to GND or VCC to prevent oscillation and excess power consumption.
Incorrect Pull-up wiring: Connecting the resistor in series with the input instead of as a pull-up to VCC. Ensure R1 goes strictly to 5V.
Sensor Logic inversion: Using a sensor that is Open when Empty without changing the circuit logic. This would cause the light to be ON when the tank is full. Ensure the mechanical action matches the truth table.

Troubleshooting

LED is always ON: Check if S1 is stuck in the Closed position or if pin 1 is shorted to ground.
LED never turns ON: Check if the float switch is actually closing the circuit to ground. Measure resistance across S1 terminals while moving the float.
Chip gets hot: Check for short circuits at the output or if VCC/GND are reversed (Pins 14 and 7).
LED flickers: The liquid might be turbulent. Add a capacitor (e.g., 10 µF) in parallel with S1 to create a hardware debounce delay.

Possible improvements and extensions

Audio Alert: Add a 5V active buzzer in parallel with the LED/Resistor combo to provide an audible alarm when the tank is empty.
Hysteresis: Replace the 74HC04 with a 74HC14 (Schmitt Trigger Inverter). This prevents the LED from jittering if the water level is right at the switching threshold.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Build a safety circuit that cuts a ‘Ready’ signal instantly when a stop button is pressed.

Objective and use case

In this tutorial, you will build a digital logic circuit that inverts an input signal. Specifically, a «System Ready» indicator (Green LED) will remain active by default and will immediately turn off when an emergency pushbutton is pressed.

Why it is useful:
- Industrial safety: Simulates an emergency kill switch where the active state shuts down machinery.
- Security systems: Sensors (like door contacts) often break a circuit to trigger an alarm or change a status.
- Fail-safe logic: Ensures a system defaults to «safe» (off) when an active intervention occurs.
Expected outcome:
- Idle State: When the button is NOT pressed (Logic 0), the Green LED is ON (Logic 1).
- Active State: When the button IS pressed (Logic 1), the Green LED turns OFF (Logic 0).
- Signal Voltage: Input transitions between 0 V and 5 V; Output inverts logically.
Target audience and level: Students and hobbyists learning basic digital inversion.

Materials

V1: 5 V DC supply, function: main power source.
U1: 74HC04 Hex Inverter IC, function: logic inversion (NOT gate).
S1: Pushbutton (Normally Open), function: emergency signal trigger.
R1: 10 kΩ resistor, function: pull-down resistor for input stability.
R2: 330 Ω resistor, function: current limiting for the LED.
D1: Green LED, function: ‘System Ready’ indicator.

Pin-out of the IC used

Selected Chip: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
14	VCC	Power Supply (+5V)	Connect to 5 V rail
7	GND	Ground (0V)	Connect to 0 V rail
1	1A	Input 1	Connect to Pushbutton and Pull-down resistor
2	1Y	Output 1	Connect to LED resistor (R2)

Wiring guide

Construct the circuit following these node connections (Nodes: VCC, 0, V_IN, V_OUT):

Power Supply:
- V1 connects between VCC (positive) and 0 (negative/GND).
- U1 Pin 14 connects to VCC.
- U1 Pin 7 connects to 0.
Input Stage (Button Logic):
- S1 connects between VCC and V_IN.
- R1 connects between V_IN and 0 (This pulls the input to 0 V when the button is open).
- U1 Pin 1 (Input 1A) connects to V_IN.
Output Stage (Indicator):
- U1 Pin 2 (Output 1Y) connects to V_OUT.
- R2 connects between V_OUT and node LED_ANODE.
- D1 Anode connects to LED_ANODE.
- D1 Cathode connects to 0.

Conceptual block diagram

Schematic

[ INPUT STAGE ]                          [ LOGIC STAGE ]                       [ OUTPUT STAGE ]

    [ V1: 5V Supply ] --(Power VCC)--------> [ U1 Power: Pin 14 ]

    [ S1: Pushbutton ] --(Press = 5V)--+
    (Emergency Trig)                   |
                                       v
                                  [ Node V_IN ] --(Pin 1)--> [   U1: 74HC04   ] --(Pin 2)--> [ R2: 330 Ohm ] --> [ D1: Green LED ] --> [ GND ]
                                       ^                     [ Hex Inverter IC]              (Current Limit)     (System Ready)
                                       |                     [   (NOT Gate)   ]
    [ R1: 10k Resistor ] --(Open = 0V)-+                     [  GND: Pin 7    ]
    (Pull-down to GND)                                             |
                                                                   v
                                                                [ GND ]

Schematic (ASCII)

Truth table

The 74HC04 implements the Boolean NOT function ($Y = \overline{A}$).

Button State	Input Voltage (V_IN)	Logic Input (A)	Logic Output (Y)	LED State
Released	0 V (Pulled down)	0	1	ON
Pressed	5 V (VCC)	1	0	OFF

Measurements and tests

Follow these steps to validate the emergency deactivation logic:

Idle Check:
- Ensure the power supply is on. Do not touch the button.
- Visual: The Green LED should be lit.
- Measurement: Use a multimeter to measure voltage at V_IN (Pin 1). It should be approx 0 V.
- Measurement: Measure voltage at V_OUT (Pin 2). It should be approx 5 V (Logic High).
Activation Check:
- Press and hold the pushbutton S1.
- Visual: The Green LED must turn OFF immediately.
- Measurement: Voltage at V_IN should rise to 5 V.
- Measurement: Voltage at V_OUT should drop to approx 0 V (Logic Low).

SPICE netlist and simulation

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

* Practical case: Emergency deactivation
* Circuit: Inverter Logic (NOT Gate) with LED Indicator

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

* --- Input Stage (Button Logic) ---
* Components: S1 (Pushbutton), R1 (Pull-down)
* Connectivity: S1 connects VCC to V_IN. R1 connects V_IN to 0.
* Logic: 
*   - Button Released (Default): S1 Open -> V_IN pulled to 0V by R1.
*   - Button Pressed (Emergency): S1 Closed -> V_IN pulled to 5V (VCC).

* Simulation of S1 (Normally Open Pushbutton):
* Modeled as a Voltage-Controlled Switch (S1) driven by SW_CTRL.
* Vt=2.5V ensures switch closes when control signal is 5V.
S1 VCC V_IN SW_CTRL 0 SW_BTN
.model SW_BTN SW(Vt=2.5 Vh=0.1 Ron=1 Roff=10Meg)

* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Emergency deactivation
* Circuit: Inverter Logic (NOT Gate) with LED Indicator

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

* --- Input Stage (Button Logic) ---
* Components: S1 (Pushbutton), R1 (Pull-down)
* Connectivity: S1 connects VCC to V_IN. R1 connects V_IN to 0.
* Logic: 
*   - Button Released (Default): S1 Open -> V_IN pulled to 0V by R1.
*   - Button Pressed (Emergency): S1 Closed -> V_IN pulled to 5V (VCC).

* Simulation of S1 (Normally Open Pushbutton):
* Modeled as a Voltage-Controlled Switch (S1) driven by SW_CTRL.
* Vt=2.5V ensures switch closes when control signal is 5V.
S1 VCC V_IN SW_CTRL 0 SW_BTN
.model SW_BTN SW(Vt=2.5 Vh=0.1 Ron=1 Roff=10Meg)

* Control Signal (User Finger Simulation):
* Generates a pulse: 0V (Released) -> 5V (Pressed) -> 0V (Released).
* Timeline: Idle for 100us, Press for 300us, then Release.
V_BTN_CTRL SW_CTRL 0 PULSE(0 5 100u 1u 1u 300u 1000u)

* R1: 10k Pull-down resistor
R1 V_IN 0 10k

* --- Logic Stage (U1) ---
* Component: 74HC04 Hex Inverter
* Connectivity: Pin 1 (Input) -> V_IN, Pin 2 (Output) -> V_OUT.
* Power: Pin 14 -> VCC, Pin 7 -> 0.
XU1 V_IN V_OUT 0 VCC 74HC04_INV

* Subcircuit for 74HC04 Inverter
* Behavioral model: Output is High when Input is Low.
* Uses a sigmoid function for smooth switching and convergence.
.subckt 74HC04_INV In Out Gnd Vcc
B1 Out Gnd V = V(Vcc,Gnd) / (1 + exp(50 * (V(In,Gnd) - V(Vcc,Gnd)/2)))
.ends

* --- Output Stage (Indicator) ---
* Components: R2 (Resistor), D1 (Green LED)
* Connectivity: V_OUT -> R2 -> LED_ANODE -> D1 -> 0
* Logic: 
*   - V_IN=0 (Ready) -> V_OUT=5 -> LED ON.
*   - V_IN=5 (Emergency) -> V_OUT=0 -> LED OFF.

R2 V_OUT LED_ANODE 330

* D1: Green LED
D1 LED_ANODE 0 LED_GREEN
.model LED_GREEN D(Is=1e-22 Rs=5 N=1.5 Cjo=10p Vj=0.75 M=0.33 BV=5 Ibv=10u)

* --- Simulation Directives ---
* Transient analysis to observe the button press event
.tran 10u 600u

* Output data for analysis
.print tran V(V_IN) V(V_OUT) V(LED_ANODE) V(SW_CTRL)

* Calculate DC operating point
.op

.end

Simulation Results (Transient Analysis)

Show raw data table (260 rows)

Index   time            v(v_in)         v(v_out)        v(led_anode)
0	0.000000e+00	4.995005e-03	5.000000e+00	1.833072e+00
1	6.000000e-08	4.995005e-03	5.000000e+00	1.833072e+00
2	1.200000e-07	4.995005e-03	5.000000e+00	1.833072e+00
3	2.400000e-07	4.995005e-03	5.000000e+00	1.833072e+00
4	4.800000e-07	4.995005e-03	5.000000e+00	1.833072e+00
5	9.600000e-07	4.995005e-03	5.000000e+00	1.833072e+00
6	1.920000e-06	4.995005e-03	5.000000e+00	1.833072e+00
7	3.840000e-06	4.995005e-03	5.000000e+00	1.833072e+00
8	7.680000e-06	4.995005e-03	5.000000e+00	1.833072e+00
9	1.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
10	2.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
11	3.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
12	4.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
13	5.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
14	6.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
15	7.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
16	8.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
17	9.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
18	1.000000e-04	4.995005e-03	5.000000e+00	1.833072e+00
19	1.001000e-04	4.995005e-03	5.000000e+00	1.833072e+00
20	1.002750e-04	4.995005e-03	5.000000e+00	1.833072e+00
21	1.003234e-04	4.995005e-03	5.000000e+00	1.833072e+00
22	1.004082e-04	4.995005e-03	5.000000e+00	1.833072e+00
23	1.004317e-04	4.995005e-03	5.000000e+00	1.833072e+00
... (236 more rows) ...

Common mistakes and how to avoid them

Floating Input: Omitting R1 (pull-down resistor) causes the input to float when the button is released.
- Solution: Always ensure the input pin is connected to GND via a resistor (e.g., 10 kΩ) when the switch is open.
LED Reversed: The LED does not light up even when the output is High.
- Solution: Check D1 polarity. The longer leg (Anode) must face the resistor/IC output; the shorter leg (Cathode) goes to Ground.
Short Circuiting Power: Connecting the button directly between VCC and GND without the gate input in between or wiring the button in parallel with the supply.
- Solution: Follow the node list carefully. The button connects VCC to the Input Pin, not directly to Ground.

Troubleshooting

Symptom: LED is always ON, pressing the button does nothing.
- Cause: The button is not connected to VCC, or the input pin is permanently grounded.
- Fix: Check continuity across S1 when pressed. Ensure S1 connects to Pin 1.
Symptom: LED is always OFF.
- Cause: IC not powered, LED reversed, or input permanently connected to VCC.
- Fix: Measure Pin 14 (VCC) and Pin 7 (GND). Check V_IN voltage; it should be 0 V when the button is released.
Symptom: LED flickers when your hand gets close to the wire.
- Cause: Floating input (Missing R1).
- Fix: Install the 10 kΩ pull-down resistor securely between Pin 1 and Ground.

Possible improvements and extensions

Add a «Stop» Indicator: Add a second inverter (or use another gate on the same chip) to drive a Red LED that turns ON when the system is stopped (Output High when Input High).
Latching Circuit: Replace the simple NOT gate with a Flip-Flop logic circuit so that once the emergency button is pressed, the system stays off even if the button is released, requiring a separate «Reset» button.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic — Use a 74HC04 inverter and an LDR to automatically switch on an LED when ambient light drops.

Objective and use case

You will build an automatic light control circuit that detects darkness using a Light Dependent Resistor (LDR) and activates an LED using a 74HC04 digital inverter.

Why it is useful:
- Automating streetlights to turn on only at night to save energy.
- Activating emergency pathway lighting during power failures or darkness.
- Controlling garden solar lights automatically.
- Adjusting screen brightness on mobile devices based on ambient light.
Expected outcome:
- When the LDR is exposed to bright light, the LED remains OFF.
- When the LDR is covered (darkness), the LED turns ON.
- The voltage at the logic gate input transitions from Logic High (5V) to Logic Low (0V) as it gets darker.
Target audience and level: Students and hobbyists familiar with basic breadboarding.

Materials

V1: 5 V DC supply, function: Main power source.
R1: LDR (GL5528 or similar), function: Light sensor (Variable resistor).
R2: 10 kΩ potentiometer, function: Sensitivity calibration (Pull-down).
U1: 74HC04, function: Hex Inverter (NOT gate).
R3: 330 Ω resistor, function: LED current limiting.
D1: Red LED, function: Visual output indicator.

Pin-out of the IC used

Chip: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
14	VCC	Power (+)	Connect to `VCC` (5V)
7	GND	Ground (-)	Connect to `0` (GND)
1	1A	Input	Connect to sensor node `VSENSE`
2	1Y	Output	Connect to LED node `VOUT`

(Note: Pins 3, 5, 9, 11, 13 are unused inputs and should ideally be connected to GND in permanent circuits to prevent noise, though not strictly required for this quick test.)

Wiring guide

Use the following explicit node connections to build the circuit on your breadboard:

Power Supply:
- V1 positive terminal connects to node VCC.
- V1 negative terminal connects to node 0 (GND).
Sensor Stage (Voltage Divider):
- R1 (LDR) connects between VCC and node VSENSE.
- R2 (Potentiometer) connects between node VSENSE and 0 (GND).
- Note: Adjust R2 so the voltage at VSENSE varies when light changes.
Logic Stage (Inverter):
- U1 Pin 14 connects to VCC.
- U1 Pin 7 connects to 0.
- U1 Pin 1 (Input) connects to node VSENSE.
- U1 Pin 2 (Output) connects to node VOUT.
Output Stage:
- R3 connects between node VOUT and node LED_ANODE.
- D1 connects between node LED_ANODE (Anode/Long leg) and 0 (Cathode/Short leg).

Conceptual block diagram

Schematic

[ INPUT / SENSOR STAGE ]               [ LOGIC STAGE ]                  [ OUTPUT STAGE ]

 [ VCC ] --> [ R1: LDR (Sensor) ] --+
                                    |
                                    v
                               [ VSENSE ] --(Pin 1)--> [ U1: 74HC04 ] --(Pin 2)--> [ R3: 330 Ohm ] --> [ D1: LED ] --> GND
                                    ^                  [  NOT Gate  ]
                                    |
 [ GND ] --> [ R2: Pot (Calib) ] ---+

Schematic (ASCII)

Truth table

The 74HC04 inverts the input signal. We configure the sensors so that «Bright» creates a HIGH input.

Ambient Condition	LDR Resistance	Voltage at `VSENSE` (Input)	Logic Input	Logic Output (`VOUT`)	LED State
Bright	Low	High (> 2.5V)	1	0 (GND)	OFF
Dark	High	Low (< 1.5V)	0	1 (5V)	ON

Measurements and tests

Calibration: Expose the LDR to normal room light. Adjust potentiometer R2 until the LED turns OFF.
Voltage Check (Bright): Measure voltage between VSENSE and GND. It should be close to 5V (Logic 1). The output at VOUT should be near 0V.
Activation: Cover the LDR with your hand to simulate darkness.
Voltage Check (Dark): Measure VSENSE again. It should drop towards 0V (Logic 0). The output VOUT should jump to approx. 5V, turning the LED ON.

SPICE netlist and simulation

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

* Practical case: Automatic darkness sensor

* --- Models ---
* Generic Red LED Model
.model DLED D (IS=1e-22 N=1.5 RS=5 BV=5 IBV=10u CJO=10p)

* --- Subcircuits ---
* 74HC04 Hex Inverter Model (Behavioral)
* Pins: 1=Input, 2=Output, 7=GND, 14=VCC
* Maps to subckt args: In Out GND VCC
.subckt 74HC04 In Out GND VCC
  * Robust Sigmoid Transfer Function for Inverter
  * Threshold is VCC/2. Output swings between GND and VCC.
  * Formula: Vout = VCC * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
  B_INV Out GND V = V(VCC) * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
.ends

* --- Main Circuit Components ---

* 1. Power Supply
* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Automatic darkness sensor

* --- Models ---
* Generic Red LED Model
.model DLED D (IS=1e-22 N=1.5 RS=5 BV=5 IBV=10u CJO=10p)

* --- Subcircuits ---
* 74HC04 Hex Inverter Model (Behavioral)
* Pins: 1=Input, 2=Output, 7=GND, 14=VCC
* Maps to subckt args: In Out GND VCC
.subckt 74HC04 In Out GND VCC
  * Robust Sigmoid Transfer Function for Inverter
  * Threshold is VCC/2. Output swings between GND and VCC.
  * Formula: Vout = VCC * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
  B_INV Out GND V = V(VCC) * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
.ends

* --- Main Circuit Components ---

* 1. Power Supply
* V1: 5V DC supply
V1 VCC 0 DC 5

* 2. Sensor Stage (Voltage Divider)
* R1: LDR (Light Dependent Resistor)
* Implementation: A dummy R1 is placed to satisfy the BOM.
* A parallel behavioral source (B_LDR) implements the dynamic resistance change.
R1 VCC VSENSE 100Meg
B_LDR VCC VSENSE I = V(VCC, VSENSE) / V(RES_CTRL)

* R2: 10k Potentiometer (Sensitivity Calibration)
R2 VSENSE 0 10k

* Dynamic Stimulus for LDR (Simulates Light Conditions)
* Generates a control voltage representing Ohms.
* Pulse sweeps from 1k (Light) to 100k (Dark).
* Logic: Light(1k) -> VSENSE High -> LED OFF. Dark(100k) -> VSENSE Low -> LED ON.
V_LDR_CTRL RES_CTRL 0 PULSE(1k 100k 0 200u 200u 400u 2ms)

* 3. Logic Stage
* U1: 74HC04 Hex Inverter
* Connections: Pin 1 (In)=VSENSE, Pin 2 (Out)=VOUT, Pin 7=0, Pin 14=VCC
XU1 VSENSE VOUT 0 VCC 74HC04

* 4. Output Stage
* R3: LED Current Limiting Resistor (330 Ohm)
R3 VOUT LED_ANODE 330

* D1: Red LED
D1 LED_ANODE 0 DLED

* --- Analysis Directives ---
* Transient analysis to capture the Light/Dark transition
.tran 10u 2ms

* Print specific node voltages for validation
.print tran V(VSENSE) V(VOUT) V(LED_ANODE)

* Compute DC operating point
.op

.end

Simulation Results (Transient Analysis)

Show raw data table (224 rows)

Index   time            v(vsense)       v(vout)         v(led_anode)
0	0.000000e+00	4.545459e+00	1.916016e-44	6.555013e-37
1	1.000000e-07	4.525005e+00	3.875543e-44	2.124754e-38
2	2.000000e-07	4.504821e+00	1.070470e-43	-1.98700e-38
3	4.000000e-07	4.464726e+00	4.391831e-43	-3.30922e-39
4	8.000000e-07	4.386087e+00	5.351931e-42	4.963938e-40
5	1.600000e-06	4.240174e+00	7.789996e-38	7.726704e-38
6	3.200000e-06	3.973321e+00	1.292803e-32	1.287493e-32
7	6.400000e-06	3.529123e+00	-6.61237e-21	-6.59876e-21
8	1.280000e-05	2.884261e+00	2.263832e-08	2.262430e-08
9	1.905731e-05	2.447108e+00	4.668386e+00	1.823995e+00
10	2.344117e-05	2.212214e+00	4.999997e+00	1.833723e+00
11	2.751655e-05	2.030989e+00	5.000000e+00	1.833029e+00
12	3.266976e-05	1.840361e+00	5.000000e+00	1.833116e+00
13	4.266976e-05	1.556825e+00	5.000000e+00	1.833028e+00
14	5.266976e-05	1.349010e+00	5.000000e+00	1.833116e+00
15	6.266976e-05	1.190157e+00	5.000000e+00	1.833028e+00
16	7.266976e-05	1.064784e+00	5.000000e+00	1.833116e+00
17	8.266976e-05	9.633175e-01	5.000000e+00	1.833028e+00
18	9.266976e-05	8.795141e-01	5.000000e+00	1.833116e+00
19	1.026698e-04	8.091310e-01	5.000000e+00	1.833028e+00
20	1.126698e-04	7.491835e-01	5.000000e+00	1.833116e+00
21	1.226698e-04	6.975110e-01	5.000000e+00	1.833028e+00
22	1.326698e-04	6.525106e-01	5.000000e+00	1.833116e+00
23	1.426698e-04	6.129684e-01	5.000000e+00	1.833028e+00
... (200 more rows) ...

Common mistakes and how to avoid them

Swapping LDR and Potentiometer: If you swap R1 and R2, the logic inverts: the light will turn ON when it is bright and OFF when it is dark. Ensure the LDR is connected to VCC and the Potentiometer to GND.
LED inserted backwards: If D1 does not light up when VOUT is high, check the polarity. The longer leg (anode) must face the resistor R3.
Sensitivity too low: If the LED never turns off, R2 might be set to too high a resistance, keeping voltage at VSENSE always high. Turn the knob to lower the resistance.

Troubleshooting

LED is always ON:
- Cause: Potentiometer resistance is too high or LDR is broken (open circuit).
- Fix: Decrease R2 value by turning the knob. Check LDR connections.
LED is always OFF:
- Cause: Potentiometer resistance is too low (shorting input to ground) or U1 is not powered.
- Fix: Verify Pin 14 has 5V. Increase R2 resistance slightly.
LED flickers:
- Cause: The light level is right at the switching threshold of the 74HC04.
- Fix: Adjust R2 slightly to move away from the threshold or shade the LDR more decisively.

Possible improvements and extensions

Add Hysteresis: Replace the 74HC04 with a 74HC14 (Schmidt Trigger Inverter). This prevents flickering when the light transitions slowly (dusk/dawn).
High Power Load: Connect the output pin to a transistor (like a 2N2222) and a relay module to switch a 110V/220V desk lamp instead of a small LED.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Objective: Build a logic circuit using a NOT gate that activates an LED when a switch contact is opened.

Objective and use use case

You will build a digital monitoring circuit that illuminates an LED indicator whenever a switch (representing a door sensor) breaks contact. This demonstrates the fundamental operation of the NOT gate (Inverter) in security logic.

Why it is useful:
- Home Security: Basic principle behind magnetic reed switches used on windows and doors.
- Appliance Safety: Ensures devices like microwaves or washing machines do not run if the door is open.
- Industrial Interlocks: Visual warning systems for machine guards.
Expected outcome:
- Door Closed (Switch Closed): Input logic High (5V), Output logic Low (0V), LED OFF.
- Door Open (Switch Open): Input logic Low (0V), Output logic High (5V), LED ON.
- Target audience and level: Introductory Electronics Students (Basic).

Materials

V1: 5 V DC supply, function: Main power source
U1: 74HC04, function: Hex Inverter (NOT gate logic)
SW1: SPST Switch, function: Simulates door sensor (Closed = Door Closed)
R1: 10 kΩ resistor, function: Pull-down for U1 input
R2: 330 Ω resistor, function: LED current limiting
D1: Red LED, function: Visual alarm indicator

Pin-out of the IC used

Chip: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
1	1A	Input	Connected to SW1 and R1
2	1Y	Output	Connected to LED resistor R2
7	GND	Ground	Connected to 0V (Power Supply Ground)
14	VCC	Power	Connected to 5V (Power Supply Positive)

Wiring guide

VCC connects to V1 positive terminal, U1 pin 14, and one side of SW1.
0 (GND) connects to V1 negative terminal, U1 pin 7, R1, and cathode of D1.
DOOR_STATUS (Node A) connects to the other side of SW1, the other side of R1, and U1 pin 1.
ALARM_OUT (Node Y) connects to U1 pin 2 and one side of R2.
LED_ANODE connects to the other side of R2 and the anode of D1.

Conceptual block diagram

Schematic

[ INPUT / SENSOR ]                 [ LOGIC PROCESSING ]               [ OUTPUT / ALARM ]

    [ VCC (5V Source) ]
             |
             v
    [ SW1 (Door Switch) ]
             |
             v
          (Node A) -------------------->+------------------+
             |                          |    U1: 74HC04    |
             v                          |    (NOT Gate)    | --(Pin 2)--> [ R2: 330Ω ] --> [ D1: LED ] --> GND
    [ R1 (10k Pull-down) ]              |  Input: Pin 1    |
             |                          +------------------+
             v
            GND

Schematic (ASCII)

Truth table

Door Status	Switch (SW1)	Input Voltage (Pin 1)	Logic Input	Logic Output (Pin 2)	LED Status
Closed	Closed	5 V (High)	1	0	OFF
Open	Open	0 V (Low)	0	1	ON

Measurements and tests

Supply Check: Before inserting the IC, verify V1 provides exactly 5 V.
State 1 (Secure): Close SW1. Measure voltage at Pin 1 (Input). It should be ~5 V. Measure Pin 2 (Output). It should be ~0 V. Verify LED is OFF.
State 2 (Alarm): Open SW1. Measure voltage at Pin 1 (Input). It should drop to 0 V (pulled down by R1). Measure Pin 2 (Output). It should rise to ~5 V. Verify LED is ON.

SPICE netlist and simulation

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

* Practical case: Open door alarm
*
* BILL OF MATERIALS:
* V1: 5V DC Supply
* U1: 74HC04 Hex Inverter (Behavioral Model)
* SW1: SPST Switch (Modeled as Voltage-Controlled Switch)
* R1: 10k Pull-down Resistor
* R2: 330 Ohm Current Limiting Resistor
* D1: Red LED
*
* WIRING CONNECTIONS:
* VCC: V1(+), U1(14), SW1(1)
* GND: V1(-), U1(7), R1(2), D1(Cathode)
* DOOR_STATUS: SW1(2), R1(1), U1(1)
* ALARM_OUT: U1(2), R2(1)
* LED_ANODE: R2(2), D1(Anode)

* --- Main Power Supply ---
V1 VCC 0 DC 5

* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Open door alarm
*
* BILL OF MATERIALS:
* V1: 5V DC Supply
* U1: 74HC04 Hex Inverter (Behavioral Model)
* SW1: SPST Switch (Modeled as Voltage-Controlled Switch)
* R1: 10k Pull-down Resistor
* R2: 330 Ohm Current Limiting Resistor
* D1: Red LED
*
* WIRING CONNECTIONS:
* VCC: V1(+), U1(14), SW1(1)
* GND: V1(-), U1(7), R1(2), D1(Cathode)
* DOOR_STATUS: SW1(2), R1(1), U1(1)
* ALARM_OUT: U1(2), R2(1)
* LED_ANODE: R2(2), D1(Anode)

* --- Main Power Supply ---
V1 VCC 0 DC 5

* --- User Interaction (Door Sensor) ---
* Model SW1 as a voltage-controlled switch S1 driven by a pulse source.
* Logic: Control High = Switch Closed (Door Closed). Control Low = Switch Open (Door Open).
* Pulse: Starts 0V (Open/Alarm ON), goes to 5V (Closed/Alarm OFF) at 1ms, stays for 2ms.
V_SW_CTRL SW_CTRL 0 PULSE(0 5 1m 10u 10u 2m 5m)

* S1 connects VCC to DOOR_STATUS when SW_CTRL is High.
S1 VCC DOOR_STATUS SW_CTRL 0 SW_DOOR
.model SW_DOOR SW(Vt=2.5 Ron=0.1 Roff=100Meg)

* --- Pull-down Resistor ---
R1 DOOR_STATUS 0 10k

* --- 74HC04 Hex Inverter (U1) ---
* Implements NOT gate logic: ALARM_OUT = NOT(DOOR_STATUS)
* Pin mapping: 1=In, 2=Out, 7=GND, 14=VCC
XU1 DOOR_STATUS ALARM_OUT 0 VCC 74HC04_GATE

* --- Output Stage ---
R2 ALARM_OUT LED_ANODE 330
D1 LED_ANODE 0 D_RED

* --- Models and Subcircuits ---

* LED Model
.model D_RED D(IS=1e-22 RS=6 N=1.5 CJO=50p BV=5 IBV=10u)

* 74HC04 Single Gate Behavioral Model
* Pins: In Out GND VCC
.subckt 74HC04_GATE 1 2 7 14
* Continuous sigmoid function for robust NOT logic
* Vout goes Low when Vin > 2.5V, High when Vin < 2.5V
B_INV 2 7 V = V(14,7) * (1 / (1 + exp(50 * (V(1,7) - 2.5))))
.ends

* --- Simulation Directives ---
.tran 10u 5ms
.op

* --- Output Printing ---
.print tran V(DOOR_STATUS) V(ALARM_OUT) V(LED_ANODE) V(SW_CTRL)

.end

Simulation Results (Transient Analysis)

Show raw data table (1126 rows)

Index   time            v(door_status)  v(alarm_out)    v(led_anode)
0	0.000000e+00	4.999500e-04	5.000000e+00	1.842385e+00
1	1.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
2	2.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
3	4.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
4	8.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
5	1.600000e-06	4.999500e-04	5.000000e+00	1.842385e+00
6	3.200000e-06	4.999500e-04	5.000000e+00	1.842385e+00
7	6.400000e-06	4.999500e-04	5.000000e+00	1.842385e+00
8	1.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
9	2.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
10	3.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
11	4.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
12	5.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
13	6.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
14	7.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
15	8.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
16	9.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
17	1.028000e-04	4.999500e-04	5.000000e+00	1.842385e+00
18	1.128000e-04	4.999500e-04	5.000000e+00	1.842385e+00
19	1.228000e-04	4.999500e-04	5.000000e+00	1.842385e+00
20	1.328000e-04	4.999500e-04	5.000000e+00	1.842385e+00
21	1.428000e-04	4.999500e-04	5.000000e+00	1.842385e+00
22	1.528000e-04	4.999500e-04	5.000000e+00	1.842385e+00
23	1.628000e-04	4.999500e-04	5.000000e+00	1.842385e+00
... (1102 more rows) ...

Common mistakes and how to avoid them

Floating Input: Forgetting the pull-down resistor (R1). Without R1, when the switch opens, the input pin floats and the LED may flicker or remain in an unpredictable state. Always tie CMOS inputs to a defined logic level.
No LED Resistor: Connecting the LED directly to the 74HC04 output without R2. This can burn out the LED or damage the IC output stage due to excessive current.
Wrong Polarity: Inserting the LED backwards (anode to ground). The LED will never light up. Ensure the longer leg (anode) faces the resistor coming from the IC.

Troubleshooting

LED always ON: Check if SW1 is actually closing. If using a push-button, ensure it is connected to VCC. Verify R1 is connected to Ground.
LED always OFF: Check if the 74HC04 has power (Pin 14) and Ground (Pin 7). Check LED polarity. Ensure SW1 is actually disconnecting VCC when «Open».
LED is dim: The value of R2 might be too high (e.g., 10kΩ instead of 330Ω) or the 5V supply is sagging.
IC gets hot: Immediate disconnect power. Check for short circuits between Output (Pin 2) and Ground, or if the chip is inserted backwards.

Possible improvements and extensions

Audible Alarm: Connect an NPN transistor and a buzzer to the output to generate sound alongside the light when the door opens.
Latch Circuit: Add a feedback loop or a Flip-Flop so that once the alarm triggers, it stays ON even if the door is closed again, requiring a manual reset button.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Practical case: Simple voltage divider

Objective and use case

Materials

Wiring guide

Conceptual block diagram

Schematic

Electrical diagram

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Current limiting in an LED

Objective and use case

Materials

Wiring guide

Conceptual block diagram

Schematic

Electrical diagram

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Empty Tank Level Indicator

Objective and use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Emergency deactivation

Objective and use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Automatic darkness sensor

Objective and use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Open door alarm

Objective and use use case