At Christmas we put lights on everything. We light up our trees, our houses, and even our lawn ornaments. So why not put lights on the presents as well.
Most people don't want power cords running to their presents. So in this project, I am going to show you how to make a light up Christmas present that is safely powered by induction. That way there are no visible power cords and your present isn't tethered to the wall. It will just light up when you place it under the tree.
Step 1: Select an Inductive Charger
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: https://www.instructables.com/id/Wireless-Ipod-Charger/
Step 2: Select a Set of Christmas Lights
Step 3: Check the Power Ratings of the Charger and the Lights
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 Supply
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. https://www.instructables.com/id/Convert-Battery-Powered-Electronics-to-Run-on-AC/
For this project we will be using the diode method.
Step 5: Connect the Lights to the Receiver Unit
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.
Lastly you need to connect the battery pack of the lights. The anode of the diode connects to the negative terminal of the battery pack 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 Charger
Step 7: Modify the Box So That It Will Work With the Inductive Charger
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 8: Secure the Receiver and the Battery Pack to the Inside of the Box
Step 9: Route the Lights Outside 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.