A Simple Wireless Charger For Smartphones


efy triedSmartphone batteries need to be charged regularly, usually more than once a day. But their charging cables tend to wear out after a while due to overuse, which can be annoying—especially if they fail when they’re needed the most. A wireless charger can be a convenient alternative; you just set it up and forget it. But how does it work, exactly?

fig.  1 Winding the coils
fig 1 Winding the coils

While wireless charging may seem like a recent invention, its origins date back more than a hundred years. The credit for this goes to the famous Serbian-American inventor, Nikola Tesla.

Working principle

In the late 1800s, Nikola Tesla successfully transmitted electricity through air. He used a process called resonant-inductive coupling for this, which works by creating a magnetic field between a transmitter (to send electricity) and a receiver (to receive electricity) to operate the light bulb in his laboratory in New York City.

fig.  2: The transmitter and receiver coils
fig 2 The transmitter and receiver coils

A few years later, he patented the Tesla coil—a tower with a coil on top that fired bolts of electricity. Tesla had a bigger vision of a wireless power grid, but his dreams never came true. Now the same basic principle of inductive charging can be used for charging a smartphone wirelessly.

Bill of Materials
0.3 – 0.4mm dia enamelled copper wire5 metersFor transmitter and receiver coils
Resistor 1k1For the current limit
Transistor BC5471For transfer
LEDs1For half-wave rectifiers
USB cable1For mobile charging
9V battery with connector1For the transmitter power supply


You can use a plastic bottle with a diameter of about 8cm, like the author, for winding a coil on it. Or you can use PVC pipe or any other cylindrical object to run the coil. Wind 15 turns of enamelled copper wire, finish, and wrap another 15 turns, like a center-tapped transformer. Similarly, wind 15 turns of enamelled copper wire to make the receiving coil.

The author used a BC547 NPN transistor for the oscillator in his circuit, but a TTC5200 transistor can be used for better results.

Connect a 1k resistor to the base of the transistor to limit the current. Connect the center-tapped coil to the positive terminal of the battery whose negative terminal should be connected to the emitter of the transistor. The other two ends of the transmitter coil should be connected to the transistor’s collector and base terminals, as shown in Fig. 3, through a 1k current-limiting resistor.

fig.  3 The circuit diagram
fig 3 The circuit diagram

The receiving ends of the coil are connected to the USB cable through the LED, which is used here for the indicator and as a half-wave rectifier.

fig.  4: Prototype of the author
fig 4 Prototype of the author

Principle of operation

The transmitter coil connected to the battery through a transistor makes the oscillating circuit. The oscillating current inside the transmitting coil causes it to emit a 1MHz magnetic field. This oscillating magnetic field induces an electric current in the receiver coil.

The induced current is an alternating current, so it must be converted to DC for mobile charging. The LED used in the circuit rectifies the current to DC besides using it as an indicator.

Sakthivignesh R. is the Engineer – SCADA system


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