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Mini Projects #021 – by Tim Blythman SILICON CHIP Wireless Flashing LEDs Wireless power transmission has been researched for over 100 years ago and is currently used for charging things like toothbrushes, smartphones and even electric cars. Here we show you how to build wireless LEDs that can be programmed to flash in sequence. I t’s difficult to transmit a lot of power wirelessly, so we decided to see what was possible on a smaller scale. Most of these technologies depend on the transformer principle: an alternating current in a coil will induce a current in a second nearby coil. Since the inverse square law applies, the closer the coils, the much more effective the energy transmission. We first saw the concept of wireless LEDs on YouTube (Big Clive – youtu. be/UQ3K0suY1Dc). In the video, he demonstrates a kit purchased online. It consists of a small circuit board attached to a coil about 8cm across and some small LED modules. When the circuit is powered, it drives the coil at around 200kHz and LED modules that are nearby light up. He goes on to explain the circuit and show some waveforms. We wanted to see if this was something we could replicate, or possibly improve on. By adding some smarts in the form of an Arduino Uno, we’ve made it possible to control the LEDs better and make them flash in a pattern. You might have seen the Circuit Notebook entry on Wireless power transfer from our December 2023 issue (siliconchip.au/Article/16048). The principle here is similar, although our circuitry is simpler and much more compact. However, it does require a microcontroller. Circuit details Fig.1 shows the fairly simple circuit. It depends on using the values shown, particularly the capacitors and inductors, for proper operation. In the Transmitter on the right, the Uno generates a PWM waveform at its D3 pin, which drives the coil/capacitor pair via a transistor. The 100μF capacitor provides a steady power supply for this part of the circuit. When pin D3 goes high and the transistor switches on, the current builds in the inductor until D3 goes low, and Parts List – Wireless LEDs (JMP021) Transmitter (one required) 1 Arduino Uno compatible main board [Jaycar XC4410] 1 TIP31 3A NPN transistor, TO-220 [Jaycar ZT2285] 1 100μF electrolytic capacitor [Jaycar RE6130] 1 100nF MKT capacitor [Jaycar RM7125] 1 1kW resistor [Jaycar RR0572] 1 2m length of 0.5mm solid-core insulated wire (enamelled or with plastic insulation) 1 2-pin header [cut from Jaycar HM3211] 1 length of electrical tape to secure coil Receiver (per unit, multiple can be used) 1 100μH unshielded SMD inductor [Jaycar LF1400, pack of 10] 1 6.8–10nF ceramic capacitor [Jaycar RC5346, RC5347 or RC5348; see text] 1 high-brightness 5mm LED [Jaycar ZD0290, ZD0291, ZD0292, ZD0293 or ZD0295] 48 Silicon Chip Australia's electronics magazine A screenshot from the YouTube video by Big Clive (https://youtu. be/UQ3K0suY1Dc). It shows the inspiration for this project. siliconchip.com.au Fig.1: a parallel tuned LC network is driven by a PWM signal from a microcontroller. Energy is radiated from the coil that’s part of that LC network; the Receiver picks up the energy and uses it to power the LED. the transistor switches off. The current continues to flow through the inductor to charge up the capacitor. The energy in the capacitor is released on the next cycle. By using a capacitor here instead of a diode to catch the inductive spike, the energy in the inductor is saved instead of being dissipated. The well-known formula for the resonant frequency of an LC (inductor-capacitor) circuit is: f = 1÷(2π√LC) For the components we have chosen and the coil’s dimensions, this works out to around 250kHz. As we will see later, the circuit will operate mostly below that frequency, from about 160kHz to 200kHz. The components in the Receiver on the left have a resonant frequency of 193kHz for a 6.8nF capacitor, down to 159kHz for a 10nF capacitor. An 8.2nF capacitor would be resonant at 176kHz. The presence of a so-called non-linear device (the LED) will change this somewhat. It is not a sharp resonance (like a radio tuner), so the circuit will also respond to close frequencies as well. In this case, resonance means that the circuit will tend to reinforce signals that occur at a specific frequency. An analogy would be pushing a Similarly, our coil has a diameter of 8cm, with five windings. We used a bottle as a former, then taped up the wires to help it keep its shape. Twisting the two wires together also helps to keep the coil from unravelling. siliconchip.com.au Australia's electronics magazine playground swing; when the pushes occur at the correct frequency, it will swing higher than if they are not. The operation of the two parts of the circuit depends on the two inductors being ‘coupled’; their magnetic fields must interact. This will also change the behaviour of the two circuits. They can be considered to form an air-cored transformer. In practice, the frequency of the Transmitter coil is determined by the PWM frequency. As it is operating near resonance, the waveform is amplified somewhat. The Receiver will also resonate around its characteristic frequency, allowing it to develop enough voltage to light the LED. We wrote an Arduino sketch that allows the frequency to be manually adjusted. Broadly speaking, the closer the Receiver is to resonance, the brighter the LED will light. You might also see a neat trick in our design. By having Receivers with different resonant frequencies, we can change the Transmitter frequency and have different Receivers light up at different times. By cycling through different frequencies, the different LEDs will flash in a sequence. Construction We wound a five-turn coil on a bottle; you can use a similar cylindrical object, making sure to leave 10cm or so of wire to connect at each end. You could use enamelled wire, but we had some solid-core cable (from an old network cable). You might prefer this as it is easier to strip than enamelled wire. Use tape to secure the windings neatly and twist the ends to keep them Each Receiver consists of an inductor, capacitor and LED soldered in parallel. Using different capacitor values allows the Receiver to be tuned to react to different frequencies. The Receivers are small and cheap enough that you can easily build a dozen using different coloured LEDs. February 2025  49 Just a handful of parts are needed to experiment with wireless power. We built our Transmitter circuit on a small pair of header pins, and it too only needs a handful of components. Fig.2: this is how we laid out our prototype; only three connections are needed to the Uno. Check with the circuit diagram as you go to ensure you wire things up correctly. together, as you can see in our photos. Place the two-pin header in the 5V-GND position on the Uno and solder the 100μF capacitor across it, making sure that the negative stripe on the capacitor goes to ground. Follow with the transistor; its pin 3 emitter should go to ground, too. Note from the photos how it is mounted upside-down. Solder the 1kW resistor to the base pin (pin 1) of the transistor and push the other lead into the socket for pin D3. The coil and 100nF capacitor are both connected between the collector (pin 2) of the transistor and 5V. Look closely at our photos to check that this is all correct. Receiver Each Receiver simply has the LED, an inductor and a capacitor wired in parallel. We soldered the capacitor to the inductor first after trimming its leads down to a few millimetres. The LEDs leads are similarly trimmed to sit over the capacitor and also soldered in place. Check out our photos; you can see that the solder pads on the inductor are quite large. You should be able to straighten the leads so that the Receiver sits flat on the top of the inductor. This will help with testing. Place the Receivers inside the Transmitter coil. 50 Silicon Chip Software The software uses direct register writes to achieve a higher PWM frequency than plain Arduino code would allow. Thus, it only works on the Uno or ATmega328-based boards, such as the Nano. The code in setup and the setF() function could be used in your own sketches. You can download the Arduino sketches from siliconchip.au/Shop/6/583 Load the sketch file WIRELESS_ LED_FREQUENCY_TEST onto the Uno and open the Serial Monitor at 115,200 baud. You should see a report that the Uno is delivering a 222kHz waveform. Change the frequency by entering a value in kHz (eg, 220<Enter>). The Uno will find the nearest achievable match (between 100kHz and 250kHz) and display it. We found that the Receivers with 6.8nF capacitors were brightest at around 195kHz, 8.2nF worked best at 185kHz and 10nF at 165kHz, although the 10nF Receivers were much less bright than the others. This is due to the Transmitter coil being further from its 250kHz resonance. The red and yellow LEDs also tended to be less sensitive to frequency; their lower forward voltage allows operation over a wider range. You might find that the transistor is getting warm at this stage. The Australia's electronics magazine WIRELESS_LED_FLASHING sketch cycles through three different frequencies in turn and flashes the Receivers briefly, so the transistor is not working all the time. You can tweak the frequencies and delays with the arrays and defines near the start of the sketch. Using frequencies between the peaks noted above will allow multiple LEDs to light at the same time. The Receivers work best in the plane of the coil (and parallel to it), but will still work if a thin piece of paper or plastic is between. Thus, you can hide the Transmitter if you want to. Of course, the Receivers are very simple and well-suited to experimentation. It appears that around 200kHz is the sweet spot and you could try different value capacitors. You can also try different inductors, but we found that this style and value worked best. You can also use the Receivers to test other wireless power devices. A mobile phone charging pad causes the Receivers to flash briefly; many of these use a coded protocol to avoid running continuously and wasting power. Wireless power transmission is still only practical over short distances, but this project shows how to easily experiment with the concept and create an entertaining display. SC siliconchip.com.au