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 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
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
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
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.
Step 6: Conclusion
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".