Step #1: Select an Inductive ChargerPrevNext
- For this project, you need an inductive charging system. This kind of system uses a transmitter circuit to convert electricity from a wall outlet into electromagnetic energy that is wirelessly sent to the receiver. The receiver then converts the electromagnetic energy into electricity that it uses to charge mobile devices.
- I am using the PowerMat inductive charger. In this model the transmitter is a thin plate that the receiver sits on. The receiver is a small disk with a mini USB power cord that plugs into the device that you want to charge. When the transmitter detects the presence of a receiver unit, it automatically turns on and begins sending power to the receiver.
- You don't need to use this exact model. Other inductive chargers can also work. If you are interested in building an inductive charger from scratch, you can check out this Instructable from user Inducktion: http://www.instructables.com/id/Wireless-Ipod-Charger/
Step #3: Check the Power Ratings of the Charger and the LightsPrevNext
- You want to make sure that the inductive charger is capable of powering your lights. The charger should have its power rating labeled somewhere on the transmitter or the receiver. If it isn't written on the charger itself, check the manual. My charger is capable of outputing 1 Amp (1000mA). If there is no label at all, you can reasonably assume that it is at least 500mA (the capacity of a USB 2.0 port).
- Then check the power rating of the lights. If it is not labeled you can estimate the current requirements based on the number of LEDs. Most small LEDs require 20mA per light. So multiply the number of lights by 20mA. The lights that I am using have 36 LEDs wired together in parallel. So they should require 720mA.
- My charger can output 1000mA and my light only require 720mA. So there shouldn't be any problem with the power supply.
Step #4: If Necessary, Adjust the Voltage of the Power SupplyPrevNext
- A charger typically outputs about 5 volts (mine actually measured 5.16 volts). However, most battery powered LED lights run on 4.5 volts. If you try to run a 4.5 volt LED on 5 volts it would probably work, but it can significantly reduce its lifespan. Fortunately, there are a number of ways that you can make small adjustments to the output voltage of the charger.
- The simplest way to reduce the voltage is to add a diode. Most silicon diodes create a voltage drop of about 0.7 volts (0.3 V for Germanium diodes and 0.2 V for Schottky diodes). So by adding a diode, you can drop the 5.16V output of the charger down to 4.46V. This voltage will work fine for the lights. Just make sure that the diode is rated high enough for the current used by your lights. In this case the lights use 720 mA. So I used a diode that was rated for 1A.
- Another method is to add a resistor. To determine the value of the resistor that you would need, use these formulas:
Resistance = [(Voltage of the power supply) - (Voltage of the lights)] / (Current of the lights)
Resistor Wattage = [(Voltage of the power supply) - (Voltage of the lights)] * (Current of the lights)
So in this project, the power supply voltage is 5.16V, the voltage of the lights is 4.5V and the current of the lights is 0.720A. This means that I would need a resistor with a value of 0.92 or about 1 ohm and a wattage of 0.48 or about 1/2 a watt.
- A third option is to use a variable voltage regulator such as a LM317. This lets you use a pair of resistors to set the output at any voltage that you want. For a simple tutorial on how to use a voltage regulator, you can check out another of my projects where I show how to use a voltage regulator to adjust the output of a DC power supply. http://www.instructables.com/id/Convert-Battery-Powered-Electronics-to-Run-on-AC/
Step #5: Connect the Lights to the Receiver UnitPrevNext
- Next I connected the receiver unit to the lights. I cut off the USB connector from the power cord. Then I stripped off the insulation and separated the two internal wires. One wire was the negative output and the other wire was the positive output.
- Then I added a diode to lower the output voltage. I soldered the cathode (the side with the stripe) of the diode to the negative wire.
- and the positive wire from the receiver connects to the positive terminal of the battery pack. The easiest way to do this is to solder the wires directly to the terminals on the battery pack.
- When you have made all the connections, place the receiver on the transmitter. If everything is working properly, the lights should turn on just as they would if they were being powered by batteries.
Step #6: Test the Range of the Inductive ChargerPrevNext
The ideal place to mount the receiver is inside the box. This keeps it concealed and out of the way. Unfortunately, the cardboard creates a gap between the transmitter and the receiver. This can cause problems for some models. So you need to test your inductive charger to see how big of a gap there can be between the transmitter and the receiver and still turn on the lights.
Step #7: Modify the Box So That it Will Work with the Inductive ChargerPrevNext
- If the operating range is less than the thickness of the bottom of the box that you wanted to use, then you will have to modify it. You have several options as to how you can do this.
- It is unlikely that the charger will be able to work through two layers of thick cardboard. If your box has a bottom that is made from two sets of folding flaps, you should either remove one or turn the box so that it sits on its side. That way you only have to work through a single layer of cardboard.
- If the charger can't work through even one layer of corrugated cardboard, you can make it thinner by removing the corrugation. Trace an outline of the receiver unit on the inside of the box. Then using a sharp knife cut through the first layer of cardboard. Now carefully tear out the cut section. After the top sheet is removed carefully cut the ridges of the corrugated material and remove that also. This should leave you with a single thin sheet of cardboard on the bottom of the box. Fit the receiver into the cut section and test it to see if it will work now.
- Another option is to completely remove a section of the box and patch it with tape. Trace the outline of the receiver. Then cut this section out with a sharp knife. On the bottom side of the box cover the hole with tape. Now the charger just has to work through a single layer of tape.
Step #9: Route the Lights Outside the BoxPrevNext
- Now you just need find a way to get the lights outside of the box. You can run it through a gap in the flaps at the opening. You can cut a slit in the corner. You can also just poke a hole in the side of the box.
- After feeding the lights through the box seal any opening that you made with tape. This also helps to keep the wire securely in place.
Step #12: Alternate DesignsPrevNext
- You can use this kind of wireless power system is a lot of ways. You can use it with other kinds of packages such as gift bags. You can use it to power things like music players. You can add rechargeable batteries so that it can remain powered after you remove it from the tree. Use your imagination. If you can think of some good alternate applications, leave a comment and share.
- It is easy to make custom colors for your lights. Just get a string of white lights and color them with various colored markers. This lets you make colors and patterns that aren't sold in stores.