I am trying to build a circuit that will allow gadgets that are usually charged by USB to be charged wirelessly. As an example I am reverse engineering an A4tech battery-less mouse. However it is too great a challenge for me and I am seeking help from you. I thought it would be better for me to turn this into a group effort than to ditch the project. I will give a detailed description of what I have built and learnt and hopefully you can tell me where I went wrong.
Step 1: Background Information
The mouse I was talking about (see other picture) in the introduction is exactly the same as the Splashpad it uses induction to transfer power across an air gap. This is the same technology as RFID; in fact it uses this to communicate with the pad. To make our own wireless device we need to know more about induction.
Step 2: Basic Theory
It is called inductive coupling of tuned circuits (Here is some more detailed information, more tesla coil orientated but more or less the same principles. http://home.freeuk.com/dunckx/wireless/inductive/inductive.html).
We need an AC power source for the primary, any frequency would do, but to make it more efficient we need a high frequency. To explain this think of two solenoids. If we hook up the primary coil to a DC supply. Initially the current will be zero, the current will increase at a decreasing rate due to a magnetic field that is created to oppose the change, once the two reach equilibrium the current reaches a steady maximum (can be calculated by I=V/R) and the magnetic field also becomes constant. This changing magnetic field induces a current in the secondary which produces a magnetic field in the opposite direction. This is just a small blip of current that lasts just for a short period of time. We have just transferred a small amount of power. Now if we did this over and over again we would induce more tiny blips and hence more power. This is why a higher frequency would transfer more power.
The battery-less mouse has a frequency range of about 119kHz to 135kHz, which is what we will use; it is probably a legal frequency???
*The third image I grabbed from a lecture slide, the lecture had no name on it. If someone objects to me taking this slide, please let me know and I'll remove it.
Step 3: Resonator
To decrease the resistance/Impedance of the primary circuit we need a coil in series with a capacitor. So the final circuit will look something like the circuit on the left in the image below. The capacitor and inductor make something called a resonator. If the capacitor was fully charged and then connected in parallel with an inductor (right-hand side of image) we would get an alternating current flowing through the two components. This is because the capacitor discharges through the inductor, a magnetic field is created. When the capacitor is fully discharged the current stops, the energy of the magnetic field is converted back into electricity; this recharges the capacitor. This cycle then repeats many times. This type of oscillation is called resonance if the reactance's (see next sentence) of the inductor and capacitor are equal. Reactance is the equivalent of resistance in a DC circuit.
Let's take a look at my high school notes book.
Step 4: AC in Resistors
Step 5: AC in Capacitors and Inductors
The resistance is not easily calculated as AC resistance is affected by frequency, We therefore use a different physical quantity called reactance, it still has the same unit as resistance. The ohm.
It is clear that frequency affects the resistance and hence the current. See second image
Step 6: LCR (Inductor,Capacitor,Resistor) Resonant Circuit
The graph pretty much explains how the resistance of a resonant circuit decreases if the reactance's of the capacitor and inductor are equal. The frequency that this occurs at is called the resonant frequency.
Note: the circle diagrams with the arrows are phasor diagrams that represent the magnitude of the quantity at a moment in time. In the resonant circuit above the reactances of the capacitor and inductor are equal and opposite having the effect of cancelling eachother out.
The Whole picture
This theory helps us understand that if the primary and secondary circuits in our wireless circuit are at resonance we increase the effieciency. No power is lost in the inductor and capacitor.
Step 7: The Circuit (AC/square Wave Generator)
Another way would have been to use a comparator as a square wave generator (http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/square.html#c1). The maths looked complicated so I soon discarded that idea.
In the end I decided to use a 555 Timer. We have probably all heard of this universal device. As far as I know it is quite an accurate timing device and is rather simple to use. We are going to use it in its Astable configuration. Below is a small excerpt from a webpage (http://www.kpsec.freeuk.com/555timer.htm) with everything you need to know about it.
Step 8: Choosing Resistor Values for 555 Timer
Lets use the formula from the previous webpage to calculate the value of R2 (in the picture below).
(see second image)
This gives us a result of 32407.41. The closest resistor value I have is 33kOhm, which has a value of 32.5kOhm when measured with a multimeter. We will use this more accurate measurement to work out the true frequency of our circuit using the following formula
(see fifth image)
Our true frequency is 119658.12 Hz
The value of R1 has to be about one tenth of R2 to make the mark and space time approximately equal! So, 3.3kOhm
Step 9: Square Wave Generator
Step 10: AC Generator
We have alternate current through the resistor by the centre tap.
I have tested this and it does work at low frequencies for testing.
Step 11: Primary Coil
First choose a capacitor and calculate its reactance. Then calculate the required inductance you need to create a resonant circuit. I have used the values I used in my circuit.
MiscEl can work backwards. Enter the inductance and the diameter of the coil. It will then give you the number of windings needed to produce that inductance. Make this coil and attach it to the capacitor in series. Ok thats the primary circuit complete. As you may have noticed, all my work has been done on a breadboard. That's because it doesn't work yet. When I get this thing going I'll make all the printed circuit boards and modify this instructable with all the correct values and dimensions. Your help is appreciated.
Step 12: The Secondary Circuit
Anyway. I have a variable capacitor that I should use in conjunction with a fixed capacitor to allow us to fine tune our circuit. I have a variable radio capacitor with a capacitance of up to 220pf when the two internal capacitances are connected. We do this by connecting the two outside leads (A &O;) of the variable capacitor. Like in the picture below.
We also attach a capacitor in parallel with the variable capacitor to increase the capacitance. I used a green cap with the markings of 104J (100000pf). Using this information we can once again calculate the reactance's and hence the dimensions of the coil using MiscEl.
(see second image)
Step 13: Finally, the Problem
This is where I'm stuck. How can I increase the voltage across the primary coil?
Below are some photos of the A4Tech mouse. The secondary circuit of the mouse is exactly like mine, minus the rectifier which I will add later. The primary is similar too, a capacitor in series with the coil. A coil, which has very little turns and a low resistance.
Please, if you are good with electronics and physics. Could you suggest some ways that would make this thing work and point out any errors I have made?
Step 14: Update 1
For some reason I cant get the video to play. The link below is to the video above
Step 15: References So Far
This file was created on a mac with opera. If it doesn't open try opening it with a text program like notepad or textedit
Step 16: A Part IV Project Report on 'An Inductively Coupled Universal Battery Charger'
Thank you to Cerincok who brought this to my attention!