Kids' Basics: Wireless Power

Daniel Koch

Issue 48, July 2021

A fun and versatile concept demonstrator to light up your day.

Skill Level: Beginner

The concept of wireless power has fascinated scientists, inventors, and the public since the first real developments in understanding electricity. Even before electricity’s nature and potential were defined, people recognised that lightning had lots of energy, and proposed to capture and harness it. Then, there are famous attempts like Nikola Tesla’s wireless power theories.

Many of these fell by the wayside. Some were built on flawed understandings or untested (and later to be proven incorrect) theories. Others were rendered invalid by aspects of science that just hadn’t been discovered yet. In recent years, however, wireless power transmission has made something of a comeback.

The uses for it are divided into two broad categories: transmitting signals across barriers that need to be kept sealed, and charging without plugs and sockets. In this last case, some people have used wireless phone, tablet, or laptop chargers where the device just sits on a pad and charges. What fewer people realise is that this technology keeps some people alive or functioning healthily, or both: Wireless charging is important in some medical implants like pacemakers.

Our circuit won’t be anywhere near that level, and you certainly won’t be charging a laptop or pacemaker with it. However, what you will be able to do is have a bit of fun learning the principles behind the technology, and get creative at the same time. We’re going to build a desktop decoration that can be a lamp, a display base, or a novelty artwork. That will make more sense once you see the finished product.


This month, you’ll need to find a round object between 4cm and 8cm diameter, or thereabouts. This will be used to form coils of wire and will not stay part of the build, however it may get slightly scratched by the wire so don’t use something valuable or spotlessly shiny. We used the lid of a spray paint can, and we’ve used aerosol cans themselves in the past but they usually have a lip at the base. Just make sure there is no lip at the end to stop you sliding a coil off. This object is called a ‘mandrel’ when used to form loops like this, but we’ll call it a ‘former’ so you don’t have to remember that rare term.

We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on. You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.

As always with Kids' Basics, we're building on a solderless breadboard. We avoid soldering to make Kids' Basics accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step.

TOOLS & MATERIALS (See Text for details):
Masking Tape
Small File or Sandpaper
Cardboard Box
LED Light Strings, see text

TOOLS & MATERIALS (See Text for details):

Electronics Parts Required:JaycarAltronicsCore Electronics
1 x Solderless BreadboardPB8820P1002CE05102
1 x Socket-to-Socket Jumper WireWC6026P1023CE05098
1 x LED of choiceZD0185Z0869CE05103
1 x BD139 NPN TransistorZT2189Z1068CE07455
1 x Self-Adhesive HeatsinkHH8581H0600FIT0367
1 Roll of 0.5mm Enamelled Copper WireWW4016W0405CE06932
1 x 1kΩ ResistorRR0572R7558CE05092
1 x 2AA Switched Battery PackPH9280S5042POLOLU-1160
2 x AA BatteriesSB2424S4955BCE04629

Electronics Parts Required:

The Electronics Build:

Step 1:

Hold the end of the enamelled copper wire (ECW) with one finger, taken straight from the roll uncut, and wind ten turns around the former.

Step 2:

Take some extra ECW and form a loop, twist a couple of turns near the finger holding the coil, and take the end back to be held under that finger.

Step 3:

Wind on another ten turns of ECW. You should now have twenty turns all up, with a loop sticking out in the middle. This is called a ‘centre tap’.

Step 4:

Gently slide the coil off the end of the former, being careful not to let any slip away and unwind. Tape next to where the wires exit the coil. You might need a second pair of hands for this.

Step 5:

Using the same technique, wind another coil with twenty turns, but this time there is no loop in the middle. Take it off the former and tape as before.

Step 6:

Cut the loop on the centre tap, and use a file or sandpaper to take the enamel coating off the ends of all six wires. Be careful to go all the way around each wire, to a length of at least 1cm.

Step 7:

Plug a BD139 transistor into the breadboard with the writing on the front facing you. Add a 1kΩ resistor (brown-black-black-brown--brown) between the right-hand leg of the transistor and a spot to the right. Stick the adhesive heatsink to the back of the transistor.

Step 8:

Insert the wires from a 2AA battery pack so that the red wire (+) is to the left of the transistor and the black wire (-) goes to the left-hand leg of the transistor.

Step 9:

Install both of the wires from the centre tap to the red battery pack wire (+). One of the wires from the ends of the coil goes to the middle leg of the transistor, while the other goes to the 1kΩ resistor.

Step 10:

Cut a socket-to-socket jumper wire in half and bare the ends. Twist them firmly to the ends of the untapped coil, and tape over the exposed wire. We left ours open so you can see it. Insert an LED into the sockets.

Step 11:

Insert batteries into the holder and turn on the switch. Move the coil with the LED right on top of the centre-tapped coil, and the LED should illuminate. If it does not, try flipping the coil over first, then switch off the power and check all connections.

Step 12:

Use masking tape to secure both the coil wires and battery wires into the breadboard so it can be moved easily.

How Does It Work?

Any conductor with an electric current moving through it has a magnetic field around it. We won’t get into Lenz’s Law here, because it doesn’t matter as much in this case. We’ll cover that and several other rules when we build an electric motor. The short version is: The more current flowing through the conductor, the stronger the magnetic field.

If you take a nail and wind lots of turns of wire around it, you make an electromagnet. The turns of wire produce a magnetic field that is concentrated by the nail and you can therefore pick things up. However, if you took the coil with the LED connected to it that you have just built, and sat it next to the electromagnet, nothing would happen. Why??

Only a change in the relationship between a conductor and a magnetic field will produce an electric current. This can happen if the magnetic field changes strength or direction, or if the conductor moves in the magnetic field, or both. In our case, the coils stay still, so we need a changing magnetic field.

This is most easily achieved by switching on and off the electric current that creates the magnetic field in the first place. As the magnetic field rises then falls with the change in current, it induces a corresponding current in the other coil. This is the basis of ‘induction’.

When current is first applied to the coil from the battery, it is to the centre tap, with current paths to both sides. One end of the coil is connected to the collector of the transistor, which is currently off. This means current can only flow through the other half of the coil to the 1kΩ resistor, through it, and to the base of the transistor.

Current at the base of the transistor can reach ground and complete the circuit via the base-emitter junction. The coil has a DC resistance, but we need the 1kΩ because there is still, well and truly, enough current to damage the base of the transistor.

At this point, the current at the base causes the transistor to go into saturation and conduct. This causes current to flow in the other half of the coil, through the transistor, to ground. The thing is, this current path has a lower resistance than the base-emitter junction even without the 1kΩ resistor, and so much of the current goes this way. So much does, in fact, that there is not enough to keep the base of the transistor saturated, and it starts to turn off. This reduces the current through the coil until the process repeats.

What we have made is a very basic oscillator, the name for a circuit that switches on and off. This circuit self-oscillates at a frequency that is set by the dynamics of the components involved, including the coil.

Coils like to oppose change in magnetic fields. They will oppose rising fields but also try to hang onto fields that are falling. This slows down the process a bit. It is also where most of the radiating is happening. The magnetic field produced by the current flowing through the coils not only radiates outward, but across the other turns of the coil, so the interaction is actually very complex and builds on itself. This is why there is such a thing as too few and too many turns of the coil.

The image here is taken with a test instrument called an ‘oscilloscope’, which is able to graph electrical signals. The screenshot here shows a voltage taken from across the transistor half of the coil in yellow, and across the resistor/base half in blue.

As soon as another coil is brought close enough to this one, which has the rising and falling magnetic field around it, that coil interacts with the moving magnetic field. It has a current induced in it which is opposite the one that created the field.

Our LED only works with current flowing through it one way, but the current reverses so many times a second that the human eye cannot tell that the LED is only lit for half the time. The closer the two coils are together, the more of the magnetic field that is involved and therefore the stronger the current in the secondary coil.

The voltage induced by the rising and collapsing magnetic field is actually far higher than that which created it. The circuit uses 2xAA batteries for power, but the signals coming from the oscilloscope showed much higher voltages than that. The height of each of those grid squares on the oscilloscope represents 10V. The efficiency of the transfer is quite low, however. The screenshot with a single yellow line is the oscilloscope across the LED. This time, the grid lines are 2V apart. This is still quite high, but we cannot use a resistor because the current is so low, the LED would barely light.


Outside of this demonstration, the principle is applied with a bit more complexity, such as voltage regulating circuits, or oscillators built to a specification rather than self-oscillating. You have likely seen wireless device charging, particularly at some fast food outlets and eateries where any device with a wireless charger built-in can be charged without having to worry about different, broken, or missing plugs.

These systems can also be used where signals have to be transmitted across barriers. For example, if a signal were to produce a current in a coil rather than an oscillator, the receiving coil would experience a signal that is smaller than, and opposite, the one that created it. Responding circuits would take this into account, so induction coils can be used to couple sensors across waterproof barriers without having to make a hole in it, or something that needs to remain gas-tight.

The efficiency of these systems is woeful, which is why cables are still used to bring meaningful amounts of electricity anywhere. Only a small amount of the total power from the start reaches the end, even if voltages increase: They do so at the expense of current. This means there has to be a good reason to choose such a system. Medical implant charging is a great example of a situation where the benefit outweighs the losses. As a device charger, they’re far less effective than plugging in a lead, but in public situations, leads are too problematic. In this case, the wireless charger wins.

RFID (Radio Frequency Identification) tags s, and more recent evolutions like NFC (Near-Field Communications), are also based on wireless power, although the signal itself is sent as a radio signal. The big wire loop around the outer edge of the tags or cards is the coil that has power induced in it from a magnetic field generated by the reader, and this power is just enough to run the microcircuitry in the card.

The Craft Build:

While there are few practical applications for this circuit as it is, we’re going to have a bit of fun anyway. In searching for practical ways to use this technology in the real world we came across the aforementioned gas-tight barrier situations. That made us think of a balloon, so without further ado, here is a wirelessly-powered balloon lamp!

It is based on the small sets of string lights available from dollar shops and hardware stores, and quite often party shops too. The best ones to use have a coin-cell battery and are meant to be placed in the neck of a bottle, with the string inside.

Step 1:

Cut the LED string from the battery pack/bottle stopper assembly. File the wires the same way you did for the ECW earlier.

Step 2:

Un-tape and un-twist the jumper wires from the single-winding LED coil, twist on the wires from the LED string, then carefully tape these new joins. Make sure no filed or bare wire, sharp points, or adhesive are exposed.

Step 3:

Slide the LED string into a balloon, and then gently work the coil in as well. You may need to stretch the balloon neck while someone else works the coil in.

Step 4:

Inflate the balloon and tie it off, then carefully shake the assembly until the coil sits near the base of the balloon.

Step 5:

Take the circuit made in the electronics build and tape the centre-tapped coil to the lid of a cardboard box. You can decorate a plain one or use a pre-decorated gift box. Place the circuit inside the box.

Step 6:

Turn on the power, close the lid, and place the balloon on top of the lid so that the coil in the balloon is over the top of the coil in the box. The LEDs should light up.


You can experiment with different coil sizes and combinations. You can try as few as five turns per side on the centre-tapped coil but we recommend staying above ten. You can also experiment with different coil diameters and see what difference that makes.

The most obvious choice is to explore different lighting options. What about a coil in the balloon that has a breathing LED attached instead of a string? You could also use white balloons and multi-colour LED strings if you can find them.

We chose the white lights so we can utilise the colour of the balloon, so we made a bunch of coil/LED sets and put them in different coloured balloons. These we can change out whenever we want a different coloured light.

Of course, the challenge with balloons is that they deflate. You might like to come up with a different lamp option, like a tissue paper lamp or sculpture. As usual, the main limits will be imagination, and what materials you can source.