Course Title: Electronic Circuits II Laboratory
Course Code: EEE 206
Name of The Project: N-P-N Transistor Tester Using NE555 IC
Date of Submission: January 05, 2025
Submitted To
Salma Nazia Rahman
Assistant Professor
Department of EEE
University of Asia Pacific
Submitted By
A.S. M Mahmudun Hasan Nahin
Reg. No.: 22208007
Shihab Ahmmad Raju
Reg. No.: 22208021
Ahasan Shamim Dipu
Reg. No.: 22208031
Group No.: 03
Section-A-1
Name of The Project:
N-P-N Transistor Tester using NE555 IC
Objectives of The Project:
The objective of this project is to design and develop an N-P-N Transistor Tester
using the NE555 IC, enabling users to check the functionality and performance of
NPN transistors efficiently. The circuit will verify the operating condition of the
transistor by checking its ability of switching between cutoff and saturation states.
The circuit will utilize the NE555 timer IC in a simple and cost-effective
configuration to produce the required pulses for testing, making it suitable for
educational, hobbyist, and small-scale electronic troubleshooting purposes.
Research Question:
Why is the NE555 IC a suitable choice for designing a low-cost NPN transistor
validation tool?
Probable Answer:
1. Cost-Effectiveness
•
•
Affordable Price: The NE555 timer IC is one of the most cost-effective
integrated circuits available in the market, making it available for students,
hobbyists, and small-scale projects.
Minimal External Components: The NE555 requires only a few external
components (resistors, capacitors, and a power source) to operate. This
reduces the overall cost of the circuit compared to other solutions.
2. Versatility of the NE555 IC
•
•
•
Astable Mode Functionality: The NE555 can be easily configured in astable
mode to generate continuous square wave pulses. These pulses are perfect for
driving the base terminal of the NPN transistor during testing.
Precise Timing Control: By adjusting the value of the components, the signal
frequency and duty cycle of the output pulses can be controlled, allowing the
circuit to accommodate various types of NPN transistors.
Dual Functionality: The NE555 IC is versatile and can be adapted for other
modes (monostable or astable) if future improvements or modifications are
required.
3. Reliability and Simplicity
•
•
Stable and Consistent Output: The NE555 IC provides a stable and reliable
output signal, ensuring accurate and repeatable testing of NPN transistors.
Simple Design: The NE555 IC simplifies to design the circuit. It reduces the
complexity compared to microcontroller-based circuits that might require
programming.
4. Educational Value
•
•
Widely Taught Component: The NE555 is a fundamental component in
electronics education, making it an ideal choice for beginners or educational
projects. Students are likely familiar with its operation, which simplifies the
learning process.
Demonstrates Key Principles: The NE555 IC in this project demonstrates the
concepts such as pulse generation, transistor operation, and troubleshooting of
electronic components.
5. Compatibility with Testing Requirements
•
•
Base Signal Generation: The NE555 generates the necessary pulses to switch
the mode of operation of the NPN transistor between active (ON) and cutoff
(OFF) states.
Ease of Output Observation: The tester circuit can incorporate simple output
indicators, such as LEDs or a multimeter, to observe the transistor's behavior
during testing. The NE555 provides clean and consistent pulses, ensuring
accurate results.
6. Low Power Consumption
•
The NE555 IC operates efficiently and requires only a small amount of power
to generate the necessary signals. This makes it suitable for battery-powered or
portable applications, further enhancing its cost-effectiveness.
7. Alternative Options Would Be Less Practical
•
•
Microcontroller Based Design: While microcontrollers can also generate test
signals, they require programming skills, higher costs, and additional
peripherals, making them less practical for a low-cost solution.
Discrete Components: Using discrete components to build a pulse generator
would make the circuit more complex and less reliable than using the NE555
IC.
Equipment List:
The N-P-N Transistor Tester circuit is built around the NE555 timer IC, which is
configured in its astable mode to generate continuous square wave pulses. These
pulses are used as input signals to test the switching behavior of the NPN transistor.
The key components and working of the circuit are as follows:
Key Components:
Hardware:
1. NE555 Timer IC: Acts as the pulse generator, producing a stable square wave
signal with adjustable frequency and duty cycle.
2. Resistors and Capacitors: Define the timing parameters (frequency and duty
cycle) of the NE555 output signal. The frequency of the signal can be adjusted
using a variable resistor.
3. NPN Transistor (Under Test): The device being tested for proper
functionality.
4. LED: Displays the results of the test. If the LED blinks in sync with the
NE555 pulses, the transistor is functional.
5. Power Supply: Provides the necessary voltage for the circuit to operate,
typically 9V, depending on the transistor specifications.
6. Veroboard: A prototyping board used to construct the circuit.
7. Terminal Blocks: It is used to connect the battery.
8. Dip Switch(SPST): It is used to turn on and off the circuit.
9. IC Rails: IC rails are used for easy user interference. It makes the transistor
easy to connect and disconnect for the circuit.
Software: Proteus Professionals v8.17
Introduction:
The N-P-N Transistor Tester using NE555 IC is a simple yet effective circuit
designed to test the functionality of NPN transistors. Transistors are fundamental
components in electronics, widely used as switches or amplifiers. However,
identifying whether a transistor is functioning correctly can sometimes be a challenge,
especially for hobbyists or those without access to expensive test equipment. This
project aims to address this issue by providing a low-cost, user-friendly solution for
testing NPN transistors.
The circuit uses the NE555 IC, a highly versatile and widely available timer IC, to
generate square wave pulses. These pulses are applied to the base of the NPN
transistor under test. If the transistor is functioning properly, it will switch on and off
in response to the pulses, allowing current to flow through its collector-emitter path.
A visual indicator, such as an LED, is used to visually represent the transistor's
operation, making it easy to determine its status.
This project is ideal for educational purposes, hobbyists, and small-scale
troubleshooting tasks. It demonstrates fundamental principles of transistor operation,
pulse generation, and electronic circuit design, making it valuable for beginners and
students in electronics.
Description:
555 Timer IC:
The 555 monolithic timing circuit is a highly stable controller capable of producing
accurate time delays, or oscillation. In the time delay mode of operation, the time is
precisely controlled by one external resistor and capacitor. For a stable operation as an
oscillator, the free running frequency and the duty cycle are both accurately controlled
with two external resistors and one capacitor. The circuit may be triggered and reset
on falling waveforms, and the output structure can source or sink up to 200mA [1].
Pin Diagram:
Fig-1 [1]
Pin Configurations:
Table-01: Pin Configuration of NE555 IC [2]
PIN
1
NAME OF THE PIN
GROUND / GND
FUNCTION
Ground reference voltage, low level voltage (0V).
2
TRIGGER / TRIG
Triggers the internal flip-flop with a negative pulse that causes the
output switching from low to high when Vtrig drops below 1/3 VCC.
3
OUTPUT / OUT
Drives any TTL circuit and can source or sink up to 200mA of current
with an output voltage of approximately VCC-1.5V.
4
RESET
Resets the internal flip-flop and controls the state of the output on pin
3. Typically connected to a logic "1" level when not in use.
5
CONTROL / CTRL
Controls the timing of the 555 timer by varying the width of the output
signal independently of the RC timing network. Connected to ground
through a 10nF capacitor when not in use.
6
THRESHOLD / THR
Resets the flip-flop by causing the output to switch from high to low
when the voltage applied to it exceeds 2/3Vcc. Connects directly to the
RC timing circuit.
7
DISCHARGE / DIS
Discharges the timing capacitor to ground through an internal.
Connects to ground when output is low
8
Vcc / V+ SUPPLY
Supplies power, typically between 4.5V and 15V for general purpose
TTL 555 timers.
555 Timer as Astable Multivibrator:
(a)
Fig-2:(a) The 555 Timer connected as a astable multivibrator,(b) Waveforms of the Circuit [3]
Transistor:
A transistor is a fundamental semiconductor device used to control the flow of
electrical current in electronic circuits.Transistors revolutionized electronics by
replacing bulky vacuum tubes, enabling the creation of compact and efficient devices
such as computers, radios, and smartphones.
Figure 3 shows a simplified structure for the BJT, the BJT consists of three
semiconductor regions: the emitter region (n type), the base region ( p type), and the
collector region (n type). Such a transistor is called an npn transistor. A terminal is
connected to each of the three semiconductor regions of the transistor, with the
terminals labeled emitter (E), base (B), and collector (C). The transistor consists of
two pn junctions, the emitter–base junction (EBJ) and the collector–base junction
(CBJ). Depending on the bias condition (forward or reverse) of each of these
junctions, different modes of operation of the BJT are obtained, as shown in Table-2.
The active mode is the one used if the transistor is to operate as an amplifier.
Switching applications (e.g., logic circuits) utilize both the cutoff mode and the
saturation mode. As the name implies, in the cutoff mode no current flows because
both junctions are reverse biased. As we will see shortly, charge carriers of both
polarities—that is, electrons and holes—participate in the current-conduction process
in a bipolar transistor, which is the reason for the name bipolar [4].
Fig-3: A Simplified Structure of the npn Transistor [4]
Table-2-Mode of Operation of a Transistor
Mode of Operation
Active
Saturation
Cut-off
Active Reverse
E-B junction
Reverse
Forward
Forward
Reverse
C-B junction
Reverse
Reverse
Forward
Forward
Switching Operation of a Transistor:
Using a transistor as a switch is the simplest application of the device. A transistor can
be extensively used for switching operations, either for opening or closing a circuit.
On the other hand, the basic concept behind the operation of a transistor as a switch
relies on its mode of operation. Generally, the low-voltage DC is turned on or off by
transistors in this mode.
Both PNP and NPN transistors can be utilized as switches. A basic terminal transistor
can be handled differently from a signal amplifier by biasing both NPN and PNP
bipolar transistors with an “ON / OFF” static switch. One of the main uses of the
transistor to transform a DC signal “ON” or “OFF” is solid-state switches.
Some devices, including LEDs, only require several milliamperes of DC voltages at
the logical level and can be directly controlled via the logical gate output. High-power
devices such as generators, solenoids or lamps usually need more power to use
transistor switches than the usual logic gate.
Transistor Switch’s Working Regions or Operating Modes
The saturation zone and cut-off area are also known as the transistor switch’s working
regions. This implies that, by switching between its “top-off” (saturation) and
“absolute OFF,” the transistor is used as a switch to basically overwrite its Q-Point
and the voltage divider circuit that is needed for amplification.
Cut-off Region
The “cut-off” area is at the bottom of the curves, the blue, shaded area and the yellow
zone on the left is the transistor “saturation” region.
The transistor’s operating specifications include the base current (IB), the collector
current (IC) and the emitter-collector voltage (VCE).
Characteristics of Cut-off Region
The transistor is used as an “open switch”
• The bases and input are grounded (0v)
• The base emission voltage is VBE > 0.7 V
• The basic emitter is reversed
• The full-OFF (cut-off area) transistor (“Collector Flow = 0”) • VOUT =
VCC = “1′′”
• No collector current flows (IC = 0)
Instead, we can describe the “cut-off region” or “OFF mode,” both in reverse bias,
with Vb < 0.7 V and IC = 0, when using a bipolar transistor as a switch.
•
Saturation Region
In this mode or region, the highest base current is applied, leading to the overall
collector current, causing the average collector-emitter voltage to fall and the leakage
surface as small as possible and the maximum current that flows across this transistor.
That is why the “Fully ON” transistor is triggered.
Alternatively, we can define the “saturation field” or “ON step,” all junctions forward,
VW > 0.7 V and IK = complete when using a bipolar transistor as a switch.
Let us consider a base-biased transistor in a CE configuration. When we extend the
voltage rule of Kirchhoff to the circuit’s input and output side, we can write,
VBB = IBRB + VBE … (1)
VCE = VCC – ICRC … (2)
VBB is the input voltage (Vi), and VCE is the output voltage DC (Vo). That’s why
we get,
Vi = IBRB + VBE
Vo = VCC – ICRC
First, let’s look at the shift in Vo as Vi rises from zero. A Silicon junction transistor
remains in a cutoff state as long as Vi is less than 0.6 V. Also, IC= 0. Vo= VCC. Thus,
the transistor switches into an active state when Vi goes past 0.6 V. IC >0, and Vo is
also decreasing (because ICRC is increasing). Originally, with rising Vi, IC increases
almost linearly.
Vo also decreases linearly until its value drops below 1 V. Post this, the change
becomes non-linear, and the transistor moves into the state of saturation. Vo continues
to decrease on increasing Vi but never becomes zero. Here’s a Vo vs Vi plot (also
referred to as the transition features of a reference transistor).
When Vi is low and the transistor is unable to forward bias, Vo is high(= VCC).
If Vi is sufficiently high to saturate the transistor, Vo is very low (~0).
It is also switched off when a transistor is not conducting. On the other side, it is
turned on when it is in a state of depletion. Bringing these components together,
imagine a resistor that determines the low and high values below and above those
points of voltage.
Such levels suit the transistor’s cutoff and saturation. We might say in such a situation
that a small input turns off the transistor, and a high input switches on it. These
circuits are designed to prevent the transistor from staying in an active state. This is
how a transistor can act as a switch [5].
Working Principle of the Suggested Design:
1. The NE555 timer IC is configured in astable mode, which generates a
continuous square wave as an output. The frequency and duty cycle of the
pulses can be adjusted by changing the pot value connected to the IC.
2. The square wave output from the NE555 is connected to the base terminal of
the NPN transistor. This serves as the control signal for switching the
transistor on and off.
3. When the base of the transistor receives a HIGH signal (logic 1) from the
NE555 output, the transistor enters saturation mode, allowing current to flow
from the collector to the emitter.
4. Conversely, when the base receives a LOW signal (logic 0), the transistor
turns off, and no current flows through its collector-emitter junction.
5. An LED is connected in the collector terminal of the transistor. If the transistor
functions correctly, the LED will blink in sync with the square wave pulses,
visually confirming the transistor's capability of changing mode of operation.
Circuit Diagram:
PCB Layout:
3D Visualization:
Fig: Front View
Fig: Bottom View
Cost Analysis:
Product
NE555 Timer
IC Base 8 Pin
Battery 9V
Blue Screw Terminal Connector 2 Pin
9V Battery Connector
IC Rail
100uF 16V Capacitor
100 ohm resistor
1K ohm resistor
2.2k ohm resistor
2N222 Transistor
Dip Switch- 1 position
Variable Resistor Pot 10k
Veroboard
Others
Quantity
1
1
1
1
1
1
1
1
2
1
1
1
1
1
Price(BDT)
15.06
5.03
70.05
13.00
10.08
33.15
2.99
1.98
2.50
0.75
5.00
16.86
26.39
35.00
80.00
Total=
Subtotal(BDT)
15.06
5.03
70.05
3.00
10.08
33.15
2.99
1.98
5.00
0.75
5.00
16.86
26.39
35.00
80.00
310.34(BDT)
Comparison of Existing Methods to Proposed Design:
a. Commercial Transistor Testers
•
Advantages of Commercial Testers:
1. High precision and multifunctional (e.g., can test both NPN and PNP
transistors).
2. Can test additional parameters like gain (hFE), leakage current, etc.
•
Limitations Compared to Proposed Design:
1. Expensive and not cost-effective for hobbyists or students.
2. May be overly complex for simple testing needs.
b. Multimeters
•
Advantages of Multimeters:
1. Widely available and easy to use for basic transistor testing (e.g.,
checking continuity and junction polarity).
•
Limitations Compared to Proposed Design:
1. Cannot dynamically test the switching behavior of a transistor.
2. Does not provide a visual or continuous indication of performance like
your LED-based tester.
c. Discrete Component-Based Testers
•
Advantages of Discrete Testers:
1. Does not rely on integrated circuits, showcasing the basic principles of
transistor operation.
•
Limitations Compared to Proposed Design:
1. Bulky and complicated circuits with more components required.
2. Less reliable and harder to troubleshoot due to increased connections.
d. Microcontroller Based Testers
•
Advantages of Microcontroller Testers:
1. Highly customizable and capable of testing both NPN and PNP
transistors, as well as advanced parameters like gain.
2. Programmable for various applications.
•
Limitations Compared to Proposed Design:
1. Higher cost and complexity.
2. Requires programming knowledge and additional components like
oscillators and displays.
Result:
(a)
(b)
Fig:(a) NPN Transistor Tester Circuit Simulation Output, (b) Output Waveforms(Yellow-555 Timer Output, BlueCollector Terminal, Red- Base Terminal)
The result of this project is a fully functional and cost-effective n-p-n transistor
analyzer that reliably determines the operational status of NPN transistors.
Areas for Improvement
The proposed tester might be limited compared to more advanced tools, such as:
1. The circuit cannot test FET transistors or other parameters like gain and
leakage current.
2. It is not suitable for high-power consuming transistors.
3. It can test single transistors at a time.
Discussions:
The Social and Economic Impact of the N-P-N Transistor Tester prototype:
The proposed model of the tester is a simple, low-cost tool designed to test NPN
transistors, but its impact extends beyond its technical utility. The project has several
social and economic implications, particularly in the context of education, technology
access, and sustainability. Below is a detailed explanation of its social and economic
impact:
Social Impact
1. Enhanced Accessibility to Technology
•
•
•
Bridging the Knowledge Gap: The tester is accessible to students, hobbyists,
and electronics enthusiasts in low-resource settings, where expensive test
equipment like multimeters or professional transistor testers might not be
affordable.
Promotes DIY Culture: The model Encourages individuals, especially in
developing countries, to repair and reuse electronic devices rather than
discarding them due to lack of testing tools. This fosters a culture of selfreliance and problem-solving.
Hands-On Learning: The design promotes practical, hands-on learning of
electronics concepts, such as transistor operation, circuit design, and timer IC
functionality. This can inspire interest in STEM (Science, Technology,
Engineering, and Mathematics) fields, especially among young learners.
2. Empowering Education
•
•
Ideal for Electronics Education: The tester can be used as an excellent
teaching aid in schools, colleges, and technical institutions. It provides
students with a tangible way to understand the functionality of transistors and
timer ICs.
Low-Cost Lab Equipment: For educational institutions in low-income areas,
this design can be used as a low-cost alternative to expensive laboratory
equipment, making electronics education more inclusive and accessible.
3. Fostering Repair Culture
•
Promotes Electronics Repair: With an affordable transistor tester,
Individuals and small repair shops can diagnose and repair electronic devices
instead of replacing faulty components blindly or discarding entire devices due
to affordability. This reduces e-waste and promotes a repair-and-reuse culture.
4. Encourages Sustainability
•
•
Reduction in Electronic Waste (E-Waste): By identifying faulty transistors
in circuits, the design helps prevent unnecessary disposal of electronic devices.
This contributes to reducing environmental pollution caused by e-waste.
Awareness of Sustainable Practices: As users engage with this project, they
become more aware of the importance of reusing components and reducing
electronic waste.
Economic Impact
1. Cost-Effective Solution
•
Affordable Testing Tool: The tester is highly cost-effective compared to
commercial transistor testers or advanced testing equipment. This makes it
•
economically viable for small-scale electronics businesses, repair shops, and
educational institutions.
Minimal Investment: The design uses readily available, inexpensive
components (such as NE555 IC, resistors, capacitors, LED), keeping the
overall cost low. This ensures that even those with limited financial resources
can build or acquire it.
2. Support for Small Businesses and Repair Shops
•
•
Boosting Local Electronics Repair: Affordable tools like this tester empower
small repair shops to offer efficient and reliable services. This creates
opportunities for entrepreneurship, particularly in developing regions where
access to advanced testing tools is limited.
Reducing Repair Costs: By providing an affordable way to diagnose faulty
NPN transistors, the tester reduces repair costs for consumers and businesses,
making electronics repair more economical.
3. Promoting Local Manufacturing
•
•
Encourages Local Production: This design can be manufactured locally
using easily sourced components. This can boost local manufacturing
industries, providing income and employment opportunities.
Revenue Generation: Entrepreneurs can assemble and sell these testers as
low-cost, portable diagnostic tools, especially in regions where commercial
testers are expensive or unavailable.
4. Reduced Dependency on Imported Tools
•
By providing an alternative to expensive, imported testing equipment, this
design reduces dependency on foreign products. This is especially beneficial
for developing countries striving to build self-reliant industries.
5. Long-Term Savings
•
•
For Individuals: Instead of purchasing new transistors or devices due to
uncertainty about a component's functionality, users can identify and replace
only the faulty components, saving money.
For Businesses: Electronics companies and repair services can save on testing
equipment costs while improving efficiency in diagnosing faulty transistors.
Broader Impacts
1. Democratization of Technology
•
This design lowers the barriers to entry for working with and understanding
electronics, ensuring that even those with limited financial resources or
technical expertise can participate in the field of electronics.
2. Empowerment of Underprivileged Communities
•
By providing affordable and accessible tools like this tester can empower
underprivileged communities to engage in electronics repair, education, and
innovation. This can improve livelihoods and foster technological
development at the grassroots level.
3. Contribution to the Circular Economy
•
The tester supports the principles of a circular economy by enabling the repair
and reuse of electronic components, thereby reducing waste and conserving
resources.
Conclusion:
The developed prototype of the transistor analyzing tool using 555 timer exemplifies
how innovative thinking and basic electronic components can be combined to address
practical challenges. The project is a testament to the power of simplicity and the
importance of accessibility in technology. By offering an affordable, easy-to-use, and
effective solution for testing transistors, the design has the potential to make a
meaningful impact in education, repair, and sustainability.
In conclusion, the project not only achieves its intended purpose but also opens doors
for further exploration and innovation in the field of electronics. It is an excellent
example of how small, low-cost designs can create significant social, economic, and
environmental benefits.
References
[1] P. S. L. Products, "ALL DATASHEET.COM," 31 08 1994. [Online]. Available:
https://www.alldatasheet.com/manufacture/view.jsp?Searchword=NE555%20SIGNETICS.
[Accessed 04 01 2025].
[2] K. Thakur, "EIM TECHMOLOGY," 06 2024. [Online]. Available:
https://www.eimtechnology.com/blogs/articles/pin-configuration-of-555timer?srsltid=AfmBOoorAh5c3INjSIhhYfmkU2_4cwCMLssuSd39PtKeUZJzlMVEbNHG. [Accessed 04
01 2025].
[3] K. C. S. Adel S. Sedra, "18.7.3 An Astable Multivibrator Using the 555 IC,7th ed.," in Microelectronic
Circuits, New York Oxford, OXFORD UNIVERSITY PRESS, 2015, pp. 1420-1422.
[4] K. C. S. Adel S. Sedra, "6.1.1 Simplified Structure and Modes of Operation," in Microelectronic
Circuit, New York, OXFORD UNIVERSITY PRESS, 2015, pp. 306-307.
[5] "BYJU'S," [Online]. Available: https://byjus.com/jee/transistor-as-a-switch/. [Accessed 05 01
2025].