OVERVIEW:
I am Rositha Lingamallu studying in Hindu College of Engineering & Technology, I present this design and implementation of a world’s simple and low cost fire alarm system using a thermistor, a resistor, a BC547 transistor, and a buzzer on a printed circuit board (PCB). The thermistor, a temperature-sensitive resistor, detects an increase in temperature, indicative of a fire. Under normal conditions, the thermistor maintains a high resistance, keeping the transistor in its off state. As the ambient temperature rises due to a fire, the thermistor's resistance decreases, causing a voltage drop across it and activating the BC547 transistor. When the transistor turns on, it completes the circuit, allowing current to flow through the buzzer, which then emits a sound to alert the presence of a fire. This simple yet effective fire alarm circuit offers a cost-effective solution for early fire detection in residential and small commercial applications. The compact design on a PCB ensures reliability and ease of installation.
STEPS INVOLVED IN PCB DESIGNING:
Design Specification: Requirement Analysis: Define the functional requirements and constraints for the PCB, including size, number of layers, and electrical specifications. Schematic Design: Create a schematic diagram showing the electrical connections between components using PCB design software.
PCB Layout Design: Component Placement: Arrange the components on the PCB layout according to the schematic. Ensure that components are placed for optimal signal flow and ease of routing. Routing: Connect the components with traces, considering electrical constraints such as trace width and spacing. Use routing tools in the PCB design software to automate or manually route the connections. Design Rule Check (DRC): Run DRC to verify that the design adheres to manufacturing and electrical specifications.
Generating Gerber Files: File Generation: Create Gerber files, which are standard file formats used to describe the PCB layers, drill holes, and other manufacturing details.
Bill of Materials (BoM): Prepare a BoM listing all components and their specifications needed for the PCB assembly.
PCB Fabrication: Penalization: Combine multiple PCBs into a single panel for efficient manufacturing. Layer Printing: Print the copper layers on the PCB substrate using photolithography or other methods. Etching: Remove unwanted copper from the PCB to create the traces and pads.
Drilling: Drill holes for component leads, vias, and other features.
Plating: Plate the drilled holes with copper to create electrical connections between layers if the PCB is multilayer.
Solder Mask and Silkscreen: Apply a solder mask to protect the traces and a silkscreen layer for component markings.
PCB Assembly:
Component Placement: Place the components onto the PCB, typically using automated pick-and-place machines. Soldering: Solder the components to the PCB, which can be done through techniques like wave soldering, reflow soldering, or hand soldering.
Testing and Quality Control: Electrical Testing: Perform tests such as in-circuit testing (ICT) or functional testing to ensure the PCB operates as intended.
Inspection: Inspect the PCB visually or using automated optical inspection (AOI) to check for soldering issues or defects.
Final Assembly and Packaging: Enclosure: Place the PCB in an enclosure if required. Final Assembly: Complete any additional assembly tasks, such as attaching connectors, displays, or other peripherals. Packaging: Package the final product for shipment or distribution.
COMPONENTS REQUIRED:
Thermistor: A temperature-sensitive resistor that changes resistance with temperature.
Resistor: (100Ω) Used to control current flow and create voltage dividers.
Transistor: (BC547) A general-purpose NPN transistor used as a switch.
Buzzer: (12MM) An audible alarm indicator.
Power Supply: Provides the necessary voltage to the circuit, typically 9V or 12V.
HDR M Connectors:
JST Connectors
PCB board
CONSTRUCTION: Open EASYEDA software and select the components and connect the circuit diagram in the below manner. Consider a thermistor and 100 ohms resistor connected in series. Consider a BC547 transistor and connect the emitter of the BC547 to the ground (anode terminal) of the power supply then connect the collector of the BC547 to one terminal of the buzzer. Now another terminal of the buzzer is connected to the positive terminal (Cathode terminal) of the power supply. The base of the transistor is connected to the thermistor and resistor
Fig1: Schematic diagram
After completing the construction of the Schematic diagram, we have to save it and convert it into the Layout diagram.
Fig2: Layout diagram
7. WORKING PRINCIPLE:
Thermistor Sensing:
The thermistor is placed in the area to be monitored for temperature changes.At normal temperatures, the thermistor has a specific resistance. As temperature increases, the resistance of the thermistor changes (typically decreases for an NTC thermistor).
Voltage Divider:
The thermistor and a fixed resistor form a voltage divider circuit.The output voltage of this divider depends on the thermistor’s resistance, which changes with temperature.This voltage is connected to the base of the BC547 transistor.
Transistor Switching:
The BC547 transistor is configured as a switch. Its base is connected to the voltage divider output.When the temperature is below a certain threshold, the voltage at the base of the transistor is low, keeping the transistor in the off state (no current flows from collector to emitter).As the temperature rises and the thermistor’s resistance drops, the voltage at the base increases.Once the base voltage reaches a certain level (around 0.7V for BC547), the transistor switches on, allowing current to flow from collector to emitter.
Buzzer Activation:
The buzzer is connected in series with the collector of the BC547 transistor.When the transistor turns on, current flows through the buzzer, causing it to sound an alarm.
Power Supply:
The power supply provides the necessary voltage to the entire circuit.A voltage regulator might be used to ensure a stable voltage supply to sensitive components.
8. SUMMARY:
This simple fire alarm system uses the temperature-sensitive properties of a thermistor to detect high temperatures, and a BC547 transistor to switch on a buzzer, providing an audible alarm. The circuit is powered by a power supply and organized on a PCB for reliability and compactness.
9. OUTCOME:
Fig3: Sample Circuit was Tested.
Developed and Written by
Name: Rositha Lingamallu
College: Hindu College of Engineering & Technology, Guntur
Degree: Diploma ECE
Year: 2024
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