# Wireless L E D

100,850

163

44

This is a way  to light up an LED without connecting any wires to it.

(The music is from Rick Astley's "Never Gonna Give You Up")

A simple transistor oscillator circuit generates radio frequency energy, and this energy is coupled to the LED's leads which are bent into a circle and soldered to form the receiving inductor.

The circuit is as simple as it can be made, and it makes for a nice demonstration - magic trick, even.

## Step 1: The Circuit

The circuit diagram is shown. A high frequency transistor is connected to an inductor, and biased to its active region. Due to feedback inside it, it oscillates, converting a portion of energy from the battery into radio frequency energy.

The inductor is a wire formed into a circular loop. The LED's leads are formed into a matching loop and soldered to form the receiving loop. When the two loops are placed close together (but not touching) the LED lights up.

I shall explain the circuit diagram, taking each component (or its representation) from left to right.

On the extreme left is the symbol for the battery. The longer dash represents the positive tab, the "pip" if you are using AA cells. I used four AAA cells in a holder to test this circuit. The voltage is not critical, and I tested it using the output from my mobile phone charger too.

That squiggly line is a resistor. The 33K resistor will have three orange bands on it, and then a silver or gold band.

The next symbol is a capacitor. I used a 0.1 microfarad capacitor, although any capacitor in the range 100 picofarad and upwards will work.

Then comes the transistor - I first tested the circuit using a BF494 transistor - this is a high frequency low power transistor used in the front end of medium wave radios. It overheated and died when I tried to increase the brightness of the LED by increasing the voltage to 12 V - so, keep your battery voltage down to 6V if using the BF494 or equivalent transistor.

On the collector of the transistor is the induction loop - this is just a single circular loop of wire, to match the loop of wire connected to the LED.

On the emitter of the transistor is an inductor - the value of this, too, is not critical, and any value from a few tens of microhenries upwards would work. The one I used (found in my box of junk) had a value of 330 microhenries - marked by two orange bands and a brown band.

This completes the oscillator. The resistor supplies current to the base of the transistor so that it will turn on and pass current. The base is grounded for rf by the capacitor, while still allowing the base to get current.

The emitter is allowed to float at rf by the inductor, while the current returns to the battery through it.

On the receiving side, the leads of the LED are bent over to form a circle. The rf induced in it is rectified by the LED and it simultaneously emits light.

## Step 2: Components

Here is a list of components:

1. 6 volt battery - four AA or AAA cells in a battery box.
2. One red Light Emitting Diode (red LED)
3. One transistor, type BF494 or equivalent
4. One capacitor, 0.1 microfarad (disk ceramic)
5. One resistor, 33 Kilohm (33K 1/4 watt) - orange, orange, orange, (gold/silver)
6. One inductor 330 microhenries - orange, orange, brown, (gold/silver)

Wire, solder, etc. as needed.

The resistor and the inductor both look alike - this can get you confused. The inductor is likely to be larger, and will be attracted strongly by a magnet because it is a fine coil of wire on a ferrite core.

The resistor can be identified if you have a meter - it will measure the value marked on it, while the inductor will show as a very low resistor, despite being marked exactly like a resistor is of a higher value.

## Step 3: Construction: LED

The LED can be prepared by bending its leads around any convenient round object and soldering its ends together. If you have extracted it from somewhere and its leads are too short bend another piece of wire into a circle and solder it to the ends to make an assembly as shown here.

## Step 4: Construction: Oscillator

Constructing the oscillator circuit is very simple: All the components are just soldered together to form a self-supporting assembly.

The transistor I mentioned in the parts list (BF494) died, and the one in the pictures is another one (one of many that I tried) that worked, that I found in my junk box.

First, the capacitor and the resistor are soldered together in parallel. Keep the leads short. They may go together any way - they are not polarised, ie, have a "positive" or "negative" terminal. When both their leads have been joined together by two blobs of solder they may be handled as a unit.

Next, identify the base lead of the transistor and solder one end of the resistor-capacitor parallel network to it. Some types of transistors have the base lead in the middle, so look up the data sheet to find out. Or you can try to identify the base by using the multimeter (if you have one, and you know how to use it).

Next, take a length of stiff wire, form it into a circle about the size of the loop on the LED and solder it between the other side of the resistor-capacitor combination and the collector of the transistor.

The inductor gets connected to the emitter, and the supply wires get connected as shown.

The only tricky bit here is identifying the three terminals of the transistor, once you have done that, the rest is a walk over.

## Step 5: Operation

Connect the oscillator to the battery. The emitter of the transistor goes to the negative pole of the battery, and one end of the loop that connects to the resistor-capacitor combo goes to the positive pole.

The transistor might get slightly warm - this is normal.

The LED will light when it is placed over the loop. The maximum brightness will be observed when the two loops are most close together. The range of this simple circuit is only a few millimeters at most, but even then it might puzzle a few people.

Hide the circuit under a sheet of paper and your LED will mysteriously light up when it is placed in just the right spot! It's magic!

More importantly, building and making this simple circuit is fun, and the experience will give you the confidence and the motivation to attempt more complex and more rewarding things.

Have Fun.

## Step 6: Conclusion

This circuit intentionally generates high frequency energy. This might potentially interfere with other sensitive devices. Use it at your own risk. Connecting a large antenna to some parts of this circuit might cause interference to communication.

This is intended to demonstrate the principles of transferring energy across a short distance without direct connection. Actual devices used for similar purposes have a more complex circuitry intended to overcome some of the limitations of this simple circuit.

I found that only red LEDs could be lit using this circuit. Red LEDs have the smallest requirement of voltage to turn on: a little more than one and a half volts. All other LEDs have much higher threshold voltage and so might not work with this circuit. So stick to red LEDs for experimenting at first, when you can successfully light a red LED you might try other colours to see if they may be made to light, as well.

Increasing the area of the loops, and the number of turns of the loop, would increase the efficiency of energy transfer. But then the oscillator circuit would need more components to work, and thus will need more work to wire up and operate. Such circuits are available on the web, just do a google search on the keywords "wireless LED".

## Recommendations

• ### Internet of Things Class

18,865 Enrolled

## 44 Discussions

The only thing I find hard to comprehend about this circuit - is that the RF that your circuit is emitting (used to power the LED) is AC. A LED will only run on DC, so unless there is a rectifier in place I don't understand how this works.

The L E D (Light Emitting Diode) is itself a diode. It passes current only in one direction.

It will conduct during one half cycle of the induced rf voltage. It will be off during the other half.

So the current in the inductor will be a pulsating direct current, and the Light Emitting Diode will emit light whenever it passes current. It does dual duty, rectifying the rf and emitting light in this application.

no resonance

A commendable efforts. Well done. However, I am not sure what is wireless in this design. The circuit is still hooked onto 6v source. My idea of a Wireless POWERED System is not what is displayed here. This unit is powered by the attached 6v supply.

Anyway it is a nice attempt! Cheers

Sorry but I can tell this is a poor design just my looking at it.. no wonder some people comment it doesn't work..

Those are not even a true coils.. might a well be a single unwound wire..
cap and inducer isn't even necessary for a transmitter.
This can definitely be made simpler...

can you give me some pointers on how to make the simpler design of yours, thanks

So what you are sayin is that this is a fake???

I tried VERY hard to duplicate this with very hard to find...imported BF494 transistors, 104 z cap etc. etc. etc...........nada....!

suggestions? rick?

The music in that video is the tune of "Never Gonna Give You Up" by Rick Astley. It was a favourite trick long ago in the internet to trick people into clicking on the link of the video, disguised as something else.

That comment was just to acknowledge the source of that music.

I'm still trying to figure out if there's anyway possible to make a "nano" sized (approx 3-4mm square) receiving device that could light a single LED. Any ideas or suggestions?

2 replies

I do believe it to be very possible heck take a smt led solid wire make a small coil solder to led tune transmitter circuit to pulse a optimum frequency and wala micro wireless led to make even better attach a smt capacitor and since you want it that small you could take tiny magnets and a dry erase board make a loop of wire the size of the board use about 10 to 20 loops place behind the board and have a very cool bultin board with wireless led lighting

I've seen these micro inductors that someone told me about but unfortunately I'm not quite savvy enough to figure these things out on my own very well.

I have two applications I'm trying to work with. One would put a single led approx 2-3 inches away from a standard 110vac power line and I'd like to make an led light up wirelessly (without needing big components)

The other application is probably more challenging because it needs to be incredibly small - at least the receiving led portion.

If you can, I'd like to see what you'd suggest to make this work.