Tutorial: convert battery fairy lights to USB rechargeable

Thank-you so much for watching this video, hopefully you found it interesting. In it, I show how to covert a battery set of Christmas or other fairy lights to either run from a USB rechargeable lithium battery or to run from a USB power supply directly.

In the theory portion I demonstrated how to use the ohms law calculation to work out the appropriate size for the resistor needed to protect the LEDs from overcurrent. First you need to work out the voltage we need to drop across the resistor. Using the maximum supply voltage (for a lithium battery like I show in this video use 4.2v, if you are suing a direct USB supply then use 5v) and subtract from it the volt drop of your colour of LEDs, you can look this up, or use my cheat sheet below!
Red: 1.5v
Orange: 2v
Yellow: 2v
Green: 2v
Blue: 2.9v
Violet: 3.3v
White: 3v
(these are very uncommon for lights but…)
Infrared: 1v
Ultraviolet: 4v

NOTE! some modern LEDs do not use a direct emission and instead use a phosphoric coating to emit another colour, meaning the volt drop can differ form the colour rule of thumb, it's always best to measure or look at the datasheet for your specific LED if you have one as this rule is often wrong!

Using the formula R=V/I or resistance is equal to voltage divided by current, take the voltage we just worked out and divide it by the current you want to pass though the string, it's best to measure the current from the intended voltage and usually I like to tone it down a bit. If you are not sure what current to use a value of between 0.025 and 0.03 is a good guess, but again that is a rule of thumb and could be wrong.

Pick the value of resistor that come closes to the value we just calculated, standard values cover a huge range from hundredths of an ohm to trillions of ohms, however the ones you are most likely to need are 1Ω, 2.2Ω, 3.3Ω, 4.7Ω, 5.6Ω, 6.8Ω, 8.2Ω, 10Ω, 22Ω, 33Ω, 47Ω, 56Ω, 82Ω and 100Ω (this pattern continues, but lots of specialist values are available if you know where to look).
Putting two resistors in series with each other will cause the resistance values to add together. Putting resistors in parallel will cause them to lower, however calculating the overall resistance is complicated… the simplest way is to say if two resistors of the same value are in parallel the value will half, this doe not work if the resistors are different values. I put a 22Ω and 33Ω in series for a total of 55Ω, but I could have used 2 100Ω resits in parallel to give 50Ω.

If you are using a lithium battery you MUST use a protection board, many thousands are available, the industry standard is the TP4056 in combination with the DW01 to offer short circuit protection and overdischarge protection, the TP4056 is available on a module without this extra protection however I do not recommend this, look for a module that offers overdosage protection and has 4 pads, one set called B+ and B-, the other set called out+ and out- or load+ and load- depending on manufacturer. Modules based on other chips could be used such as the TP4057 or XC6802 or countless others.

Tags:
#DIY #tutorial #electronics #lithium #lipo #usb #christmas #Xmas #lights #tp4056

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