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Practical case: Open door alarm

Open door alarm prototype (Maker Style)

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)

PinNameLogic functionConnection in this case
11AInputConnected to SW1 and R1
21YOutputConnected to LED resistor R2
7GNDGroundConnected to 0V (Power Supply Ground)
14VCCPowerConnected 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

Conceptual block diagram — 74HC04 NOT gate

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 StatusSwitch (SW1)Input Voltage (Pin 1)Logic InputLogic Output (Pin 2)LED Status
ClosedClosed5 V (High)10OFF
OpenOpen0 V (Low)01ON

Measurements and tests

  1. Supply Check: Before inserting the IC, verify V1 provides exactly 5 V.
  2. 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.
  3. 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.

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* 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)

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

  1. 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.
  2. 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.
  3. 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

  1. Audible Alarm: Connect an NPN transistor and a buzzer to the output to generate sound alongside the light when the door opens.
  2. 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.

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Quick Quiz

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




Question 2: Which logic gate is used to build this monitoring circuit?




Question 3: In the expected outcome, what is the state of the LED when the door (switch) is closed?




Question 4: What real-world application uses the principle described in this circuit?




Question 5: What is the function of the 10 kΩ resistor (R1) in this circuit?




Question 6: Which specific IC chip is listed in the materials for the inverter function?




Question 7: According to standard pinouts for the 74HC04 chip, where is Ground (GND) typically connected?




Question 8: What voltage level represents a Logic High input in this specific circuit?




Question 9: What is the function of the 330 Ω resistor (R2)?




Question 10: When the switch is open (Door Open), what is the logic state at the input?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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