Introduction

1.1 Background and Motivation

More than a century ago, the visionary inventor Nikola Tesla first proposed the idea of transmitting electricity through radio waves without physical connections between the source and receiver [1]. Since then, research in wireless energy transfer has produced ground breaking results across numerous fields, including consumer electronics, automotive systems, industrial control, and medical applications [2]. Wireless power transfer (WPT)

can be defined as the process by which electrical energy is supplied from a power source to a load without the use of electrical cables [3]. WPT has been increasingly employed in situations where battery depletion and replacement are major issues, or where wired charging is impractical or hazardous [4]. The demand for wireless power sources is growing in applications such as consumer electronics (smartphones, laptops, and smartwatches), medical implants (pacemakers and neuro stimulators where transdermal wires pose infection risks), electric vehicles (wireless charging systems for autonomous and electric vehicles), industrial applications (sensors embedded in fiber composites), and autonomous underwater vehicles (charging systems for underwater operations) [5-7].

1.2 Problem Statement

Traditional wired power delivery systems face several limitations. First, cables restrict the movement of devices during charging, reducing user convenience. Second, exposed wires and connectors pose electrical and tripping hazards. Third, batteries are expensive, have a limited lifespan, and require periodic replacement. Fourth, visible cables create clutter and reduce aesthetic appeal. Finally, in medical applications, percutaneous wires in implants can cause infections and require surgical replacement [8-9].

1.3 Objectives

The primary objectives of this work are: (1) to design a low-cost wireless power transfer system using inductive coupling; (2) to demonstrate the principle of electromagnetic induction for contactless power transmission; (3) to analyze the effect of distance on power transfer efficiency; (4) to compare different WPT technologies, including inductive coupling, magnetic resonance, and radio frequency; and (5) to develop a working prototype capable of wirelessly powering an LED [10].

1.4 Paper Organization

The remainder of this paper is organized as follows: Section II describes the system architecture and components. Section III presents the hardware implementation. Section IV discusses the working principle and types of WPT. Section V presents results and discussion. Section VI concludes the paper.

2. System architecture and components

2.1 Overall System Design

The proposed wireless power transfer system employs a simple yet effective architecture comprising a transmitter circuit and a receiver circuit. Fig. 1 presents the block diagram. On the transmitter side, a 9V DC battery supplies power to an oscillator circuit built around a 2N2222 transistor, which drives the transmitter coil to generate an alternating magnetic field. On the receiver side, the receiver coil captures this changing magnetic field, inducing an AC voltage that powers the LED load [11]. Transmitter Side: DC Power Supply (9V Battery) → Oscillator Circuit (2N2222 Transistor) → Transmitter Coil → Alternating Magnetic Field

Receiver Side: Receiver Coil → Induced AC Voltage → LED Load

Fig. 1. Block Diagram of Wireless Power Transfer System

2.2 Component Specifications

A. Transmitter Coil (Insulated Copper Wire) – The transmitter coil is made of insulated copper wire wound into a cylindrical shape. When alternating current flows through this coil, it generates a changing magnetic field around it. The coil specifications include a wire gauge of 24–30 AWG, 10–20 turns, and a coil diameter of 3–5 cm [12].

B. Receiver Coil – The receiver coil is similarly constructed from insulated copper wire, typically with 10–20 turns. It is placed close to the transmitter coil to capture the changing magnetic field through electromagnetic induction [13].

C. 2N2222 Transistor – The 2N2222 is an NPN bipolar junction transistor widely used for switching and amplification applications. In this circuit, it acts as a high-frequency switch, converting DC from the battery into AC in the transmitter coil. Key specifications include a collector-emitter voltage (VCEO) of 40V, collector current (IC) of 600 mA, power dissipation of 500 mW, transition frequency (fT) of 250 MHz, and DC current gain (hFE) of 100–300. The transistor is essential because DC current alone cannot create electromagnetic induction; an alternating current is required to generate a changing magnetic field [14-15].

D. Power Supply (9V Battery) – A 9V DC battery serves as the power source. The battery supplies DC voltage, which is converted into high-frequency AC using the transistor oscillator circuit. The alternating current flows through the transmitter coil, creating an alternating magnetic field. The receiver coil captures this field, inducing an AC voltage that can power low-power devices such as LEDs [16].

E. Resistor – A resistor (typically 1 kΩ to 10 kΩ) is connected between the battery positive terminal and the transistor base to limit base current and protect the transistor.

F. LED (Load) – A light emitting diode (LED) serves as the load on the receiver side. The induced AC voltage in the receiver coil directly powers the LED, demonstrating successful wireless power transfer.

2.3 Cost Analysis

Table I presents the detailed cost breakdown of the prototype components. The total cost is approximately ₹175, making the system highly affordable for educational and prototyping purposes [17].

Table 1. Cost and specifications

Note: LED and battery connector are additional if not already available.

3. Hardware implementation

3.1 Transmitter Circuit Design

The transmitter circuit is designed as a simple oscillator using the 2N2222 transistor. The circuit connections are as follows: the first loop of the transmitter coil is connected to the transistor emitter terminal and the battery negative terminal; the second loop of the transmitter coil is connected to the battery positive terminal and one end of the resistor; the second terminal of the resistor is connected to the transistor base terminal; and the third loop of the transmitter coil is connected to the transistor collector terminal. The transistor acts as a high-frequency switch, turning on and off rapidly to convert DC into AC. This AC current flowing through the transmitter coil creates a changing magnetic field around it [18].

3.2 Receiver Circuit Design

The receiver circuit is simple: the receiver coil is connected directly to the LED terminals. When the receiver coil is placed within the changing magnetic field of the transmitter coil, an alternating voltage is induced across the coil terminals, which illuminates the LED.

Fig. 2. System Circuit Design (Fritzing Environment)

(Figure placeholder: Hardware system design created using the Fritzing environment, illustrating connections of all electronic components.)

3.3 Working Principle

The wireless power transfer operates on the principle of electromagnetic induction, discovered by Michael Faraday in 1831. The process involves three steps. First, the 2N2222 transistor converts the 9V DC battery supply into high-frequency AC. Second, the AC current in the transmitter coil creates a time-varying magnetic field around it. Third, when the receiver coil is placed within this changing magnetic field, an electromotive force (EMF) is induced in the receiver coil according to Faraday's Law. Finally, the induced voltage powers the LED connected to the receiver coil [19]. The relationship between the coils follows Faraday's Law of Electromagnetic Induction:

EMF = -N × (dΦ/dt)

where EMF is the induced electromotive force (voltage), N is the number of turns in the receiver coil, and dΦ/dt is the rate of change of magnetic flux.

3.4 Prototype Development

Fig. 3. Complete Hardware Model – (Figure placeholder: Complete hardware model of the proposed Wireless Power Transfer System, consisting of the transmitter coil (cylindrical shape), receiver coil, 2N2222 transistor, resistor, 9V battery, and LED load.)

4. Types of wireless power transfer technology

Wireless power transmission is broadly divided into three categories based on the transmission mechanism and distance capabilities [20], [21].

4.1 Inductive Coupling

Inductive coupling deals with the transmission of a magnetic field generated by a transmitter coil to an electrical load at the receiver coil. This method employs inductive magnetic coupling at low-frequency (LF) and high-frequency bands. It has a very short range (typically a few centimeters) but is highly efficient (70–80%). This method is based on the use of coils on both transmit and receive ends, essentially functioning as an air-core transformer [22], [23].

Advantages: Inductive coupling offers high power conversion efficiency (70–80%). Low-frequency operation increases efficiency. Magnetic fields are generally harmless to human health compared to electromagnetic waves. The circuit design is simple with low implementation cost.

Disadvantages: The transmission range is very short (typically 2–10 cm). Precise alignment between transmitter and receiver coils is required. Efficiency drops sharply with increasing distance.

4.2 Magnetic Resonance Coupling

Magnetic resonance, also known as inductive resonance coupling, uses the same resonant frequency for both transmitter and receiver coils. When both coils resonate at the same frequency, power transmission becomes significantly stronger. This technique was famously demonstrated by MIT researchers in 2007, who wirelessly lit a 60W bulb from 2 meters away [24], [25].

Advantages: Magnetic resonance coupling provides a longer transmission distance than inductive coupling (up to several meters). It is less sensitive to coil misalignment and can transfer higher power levels (kilowatts for EV charging). It remains efficient even with obstacles between coils.

Disadvantages: This method is more expensive to acquire and implement than inductive coupling. It is extremely difficult to maintain high-quality resonance and requires precise component matching. The circuit design is complex [26].

4.3 Radio Frequency (RF) Power Transfer

Radio frequency (RF) technology transmits electromagnetic signals to send both information and power. An RF power harvesting system receives and converts electromagnetic energy into useful direct current (DC) voltage. The key components are the antenna and rectifier circuit that convert RF power (AC) into DC energy [27], [28].

Advantages: RF power transfer is capable of long-distance transmission (meters to kilometers) and enables simultaneous data communication with power transfer. Radio frequencies are relatively safe for humans at low power levels. Waves can penetrate through walls and obstacles, and no definite path is required for transmission.

Disadvantages: Power conversion efficiency is very low (typically 1–10%). High power loss occurs over distance due to the inverse square law. Regulatory restrictions apply to RF transmission power, and complex antenna and rectifier design is required [29].

4.4 Comparative Analysis

Table II presents a comparative analysis of the three WPT technologies discussed above.                                            Table 2. Comparison of wpt technologies

5. Results and discussion

5.1 Experimental Setup

The prototype was tested under various conditions to evaluate performance. Test cases included coils at 0 cm distance (direct contact), coils at 1 cm separation, coils at 2 cm separation, coils at 3 cm separation, and coils at 5 cm separation.