Practical case: Dual Sensor Alarm System

Dual Sensor Alarm System prototype (Maker Style)

Level: Basic — Implement a logic circuit that triggers an alarm if either of two sensors detects an intrusion.

Objective and use case

In this practical case, you will build a digital logic circuit using a 74HC32 (OR gate) integrated circuit. The circuit monitors two switches representing door sensors; if either switch is activated (logic HIGH), the output LED (alarm) turns on.

Why it is useful:
* Home Security: Simulates a system where opening either the front door or the back door triggers the siren.
* Automotive Safety: Functions like the dashboard «door open» light, which illuminates if any passenger door is not fully closed.
* Industrial Controls: Acts as a simplified fault monitor where multiple error signals can trigger a single warning light.

Expected outcome:
* Standby State: When both switches are open (0 V input), the LED remains OFF.
* Active State 1: When Switch A is closed (5 V input), the LED turns ON.
* Active State 2: When Switch B is closed (5 V input), the LED turns ON.
* Dual Active State: When both switches are closed, the LED remains ON.
* Target Audience: Electronics students and hobbyists learning basic digital logic gates.

Materials

  • V1: 5 V DC power supply or battery pack
  • U1: 74HC32 Quad 2-input OR gate IC
  • S1: SPST toggle switch or push-button, function: Front Door Sensor (Input A)
  • S2: SPST toggle switch or push-button, function: Back Door Sensor (Input B)
  • R1: 10 kΩ resistor, function: pull-down for Input A
  • R2: 10 kΩ resistor, function: pull-down for Input B
  • R3: 330 Ω resistor, function: LED current limiting
  • D1: Red LED, function: Alarm indicator
  • Breadboard and hook-up wires

Pin-out of the IC used

Selected Chip: 74HC32 (Quad 2-input OR gate)

Pin Name Logic function Connection in this case
1 1A Input A (Gate 1) Connected to S1 and R1
2 1B Input B (Gate 1) Connected to S2 and R2
3 1Y Output (Gate 1) Connected to R3 (LED driver)
7 GND Ground Connected to 0 (Negative rail)
14 VCC Positive Supply Connected to 5 V rail

Wiring guide

Construct the circuit on the breadboard following these connections. The node names (e.g., IN_A, VCC) indicate electrical junctions.

  • Power Supply:
    • V1: Positive terminal to node VCC.
    • V1: Negative terminal to node 0 (GND).
  • IC Power:
    • U1 (Pin 14): Connect to VCC.
    • U1 (Pin 7): Connect to 0.
  • Sensor A (Front Door):
    • S1: Connect between VCC and node IN_A.
    • R1: Connect between node IN_A and 0 (Functions as a pull-down resistor to ensure logic 0 when switch is open).
    • U1 (Pin 1): Connect to node IN_A.
  • Sensor B (Back Door):
    • S2: Connect between VCC and node IN_B.
    • R2: Connect between node IN_B and 0 (Functions as a pull-down resistor).
    • U1 (Pin 2): Connect to node IN_B.
  • Output Stage:
    • U1 (Pin 3): Connect to node SIG_OUT.
    • R3: Connect between node SIG_OUT and node LED_ANODE.
    • D1: Anode to node LED_ANODE, Cathode to 0.

Conceptual block diagram

Conceptual block diagram — 74HC32 OR gate

Schematic

[ INPUT SENSORS ]                        [ LOGIC PROCESSING ]                  [ OUTPUT ALARM ]

[ VCC ] --> [ S1: Front Door ] --+--(IN_A)--> [ Pin 1 ] --+
                                 |                        |
                           [ R1: 10k ]                    |
                                 |                        v
                               [ GND ]             +-------------+
                                                   |  U1: 74HC32 |
                                                   |  (OR Gate)  | --(Pin 3)--> [ R3: 330 ] --> [ D1: LED ] --> GND
                                                   +-------------+
                               [ GND ]                    ^
                                 |                        |
                           [ R2: 10k ]                    |
                                 |                        |
[ VCC ] --> [ S2: Back Door  ] --+--(IN_B)--> [ Pin 2 ] --+
Schematic (ASCII)

Truth table

The 74HC32 behaves according to the standard OR logic:

Sensor A (S1) Sensor B (S2) Pin 1 (Volts) Pin 2 (Volts) Output Pin 3 (Volts) LED State
Open Open 0 V 0 V 0 V (LOW) OFF
Open Closed 0 V 5 V 5 V (HIGH) ON
Closed Open 5 V 0 V 5 V (HIGH) ON
Closed Closed 5 V 5 V 5 V (HIGH) ON

Measurements and tests

  1. Supply Check: Before inserting the IC, power up the rails and measure the voltage between VCC and 0. It should read approximately 5 V.
  2. Input Verification:
    • Keep U1 inserted. Measure voltage at Pin 1 relative to GND. It should be 0 V.
    • Press S1. The voltage at Pin 1 should jump to ~5 V.
    • Repeat for S2 and Pin 2.
  3. Logic Logic Test:
    • Ensure both switches are open. Measure Pin 3 (Output); it should be close to 0 V.
    • Close S1 only. Measure Pin 3; it should be close to 5 V. The LED should light up.
    • Close S2 only. Measure Pin 3; it should be close to 5 V. The LED should light up.

SPICE netlist and simulation

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

* Practical case: Dual Sensor Alarm System
* Corrected SPICE Netlist based on BOM and Wiring Guide

* ==============================================================================
* POWER SUPPLY
* ==============================================================================
* V1: 5V DC Supply
* Wiring: Positive to VCC, Negative to 0 (GND)
V1 VCC 0 DC 5

* ==============================================================================
* INPUT SENSORS
* ==============================================================================
* Sensor A: Front Door (S1, R1)
* Wiring: S1 connects VCC to IN_A. R1 connects IN_A to 0 (Pull-down).
* Simulation: S1 is modeled as a voltage-controlled switch driven by a control pulse
* to simulate a button press sequence.
V_CTRL_A CTRL_A 0 PULSE(0 5 10u 1u 1u 100u 200u)
S1 VCC IN_A CTRL_A 0 SW_GEN
R1 IN_A 0 10k

* Sensor B: Back Door (S2, R2)
* Wiring: S2 connects VCC to IN_B. R2 connects IN_B to 0 (Pull-down).
* Simulation: S2 control pulse is offset to test all truth table combinations.
V_CTRL_B CTRL_B 0 PULSE(0 5 10u 1u 1u 200u 400u)
S2 VCC IN_B CTRL_B 0 SW_GEN
R2 IN_B 0 10k

* ==============================================================================
* LOGIC IC: U1 (74HC32)
* ... (truncated in public view) ...

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* Practical case: Dual Sensor Alarm System
* Corrected SPICE Netlist based on BOM and Wiring Guide

* ==============================================================================
* POWER SUPPLY
* ==============================================================================
* V1: 5V DC Supply
* Wiring: Positive to VCC, Negative to 0 (GND)
V1 VCC 0 DC 5

* ==============================================================================
* INPUT SENSORS
* ==============================================================================
* Sensor A: Front Door (S1, R1)
* Wiring: S1 connects VCC to IN_A. R1 connects IN_A to 0 (Pull-down).
* Simulation: S1 is modeled as a voltage-controlled switch driven by a control pulse
* to simulate a button press sequence.
V_CTRL_A CTRL_A 0 PULSE(0 5 10u 1u 1u 100u 200u)
S1 VCC IN_A CTRL_A 0 SW_GEN
R1 IN_A 0 10k

* Sensor B: Back Door (S2, R2)
* Wiring: S2 connects VCC to IN_B. R2 connects IN_B to 0 (Pull-down).
* Simulation: S2 control pulse is offset to test all truth table combinations.
V_CTRL_B CTRL_B 0 PULSE(0 5 10u 1u 1u 200u 400u)
S2 VCC IN_B CTRL_B 0 SW_GEN
R2 IN_B 0 10k

* ==============================================================================
* LOGIC IC: U1 (74HC32)
* ==============================================================================
* Wiring: Pin 1=IN_A, Pin 2=IN_B, Pin 3=SIG_OUT, Pin 7=0, Pin 14=VCC
* Uses a subcircuit to model the OR gate logic
XU1 IN_A IN_B SIG_OUT 0 VCC 74HC32

* ==============================================================================
* OUTPUT STAGE
* ==============================================================================
* Wiring: SIG_OUT -> R3 -> LED_ANODE -> D1 -> 0
R3 SIG_OUT LED_ANODE 330
D1 LED_ANODE 0 LED_RED

* ==============================================================================
* MODELS & SUBCIRCUITS
* ==============================================================================

* Model for Switch (Idealized Push-Button)
.model SW_GEN SW(Vt=2.5 Ron=0.1 Roff=10Meg)

* Model for Red LED
.model LED_RED D(IS=1u N=3 RS=5)

* Subcircuit for 74HC32 (Quad 2-Input OR Gate)
* Implements OR logic: Y = A OR B
* Mathematical implementation using De Morgan's Law for continuous signals:
* Y = 1 - ( (1-A) * (1-B) )  (normalized 0-1 logic)
.subckt 74HC32 A B Y GND_PIN VCC_PIN
    * Sigmoid function to normalize inputs: 1/(1+exp(-20*(V(in)-2.5)))
    * Logic formula: V(Y) = V(VCC) * (1 - ( (1-Sig(A)) * (1-Sig(B)) ))
    B_OR Y GND_PIN V = V(VCC_PIN) * (1 - ( (1 - 1/(1+exp(-20*(V(A)-2.5)))) * (1 - 1/(1+exp(-20*(V(B)-2.5)))) ))
.ends

* ==============================================================================
* ANALYSIS
* ==============================================================================
* Transient analysis to verify truth table (00, 10, 01, 11)
.tran 1u 500u

* Monitor Input and Output Voltages
.print tran V(IN_A) V(IN_B) V(SIG_OUT) V(LED_ANODE)

* Compute DC Operating Point
.op

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (1202 rows) 
Index   time            v(in_a)         v(in_b)         v(sig_out)
0	0.000000e+00	4.995005e-03	4.995005e-03	5.000000e+00
1	1.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00
2	2.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00
3	4.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00
4	8.000000e-08	4.995005e-03	4.995005e-03	5.000000e+00
5	1.600000e-07	4.995005e-03	4.995005e-03	5.000000e+00
6	3.200000e-07	4.995005e-03	4.995005e-03	5.000000e+00
7	6.400000e-07	4.995005e-03	4.995005e-03	5.000000e+00
8	1.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
9	2.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
10	3.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
11	4.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
12	5.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
13	6.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
14	7.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
15	8.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
16	9.280000e-06	4.995005e-03	4.995005e-03	5.000000e+00
17	1.000000e-05	4.995005e-03	4.995005e-03	5.000000e+00
18	1.010000e-05	4.995005e-03	4.995005e-03	5.000000e+00
19	1.026000e-05	4.995005e-03	4.995005e-03	5.000000e+00
20	1.030750e-05	4.995005e-03	4.995005e-03	5.000000e+00
21	1.039062e-05	4.995005e-03	4.995005e-03	5.000000e+00
22	1.041363e-05	4.995005e-03	4.995005e-03	5.000000e+00
23	1.045390e-05	4.995005e-03	4.995005e-03	5.000000e+00
... (1178 more rows) ...

Common mistakes and how to avoid them

  1. Floating Inputs: Forgetting R1 or R2 (pull-down resistors).
    • Solution: Logic gates behave unpredictably if inputs are not connected to a definite voltage. Always use pull-down resistors (to ground) or pull-up resistors (to VCC) for mechanical switches.
  2. Missing LED Resistor: Connecting the LED directly to the IC output.
    • Solution: Always include R3 (330 Ω) to limit current. Without it, you may damage the LED or the 74HC32 output stage.
  3. Incorrect IC Orientation: Inserting the 74HC32 backwards.
    • Solution: Locate the notch or dot on the IC package. The notch indicates the end with Pin 1 and Pin 14.

Troubleshooting

  • LED is always ON:
    • Check if R1 or R2 is disconnected (floating inputs often drift HIGH).
    • Verify S1 or S2 are not wired as «normally closed» by mistake.
    • Check for short circuits between VCC and Pin 1/Pin 2.
  • LED never turns ON:
    • Check if the IC is powered (Pin 14 at 5V, Pin 7 at GND).
    • Verify LED polarity (Anode must face the resistor/IC, Cathode to GND).
  • LED is very dim:
    • The value of R3 might be too high (e.g., using 10 kΩ instead of 330 Ω).
    • Power supply voltage might be too low (< 3 V).

Possible improvements and extensions

  1. Latched Alarm: Add a flip-flop or create a latch circuit so the alarm stays ON even after the intruder closes the door (S1/S2 open again), requiring a manual reset.
  2. Audible Alert: Connect an active buzzer in parallel with the LED (driven by a transistor if the current requirement exceeds 20mA) to add sound to the visual alarm.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: Which integrated circuit is specified in the text to build the logic circuit?




Question 2: What specific logic function does the 74HC32 IC perform in this circuit?




Question 3: According to the expected outcome, what is the state of the LED when both switches are open (0 V input)?




Question 4: What is the function of the 10 kΩ resistors (R1 and R2) typically found in this type of circuit setup?




Question 5: If Switch A is closed (5 V input) and Switch B is open, what happens to the LED?




Question 6: Which of the following is listed as a real-world use case for this circuit?




Question 7: What is the primary purpose of the 330 Ω resistor (R3) connected to the output?




Question 8: What is the expected outcome if both Switch A and Switch B are closed simultaneously?




Question 9: Based on the input levels mentioned (5 V input), what is the appropriate power supply voltage for this circuit?




Question 10: Who is explicitly mentioned as the target audience for this practical case?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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