Introduction: Fairy Light Battery Saver

About: Always curious...always learning.

CR2032 batteries are great, but they don't last as long as we would like when driving LED "Fairy Light" strings.

With the Holiday Season here, I decided to modify a few 20 light strings to run off of a USB power bank.

I searched online and found that not all USB power banks will remain powered on with such a small current draw.

Through testing and with a few iterations, I found a working solution that I think others may want to try.

Besides a typical continuous run time of 60 to 80 hours between charges, fewer CR2032 batteries will need to be purchased and recycled!

Please be sure to follow this through, or skip to the end to see the final version...

I wanted to save the best for last!

Bob D.

Step 1: Gathering the Required Parts

Only a few components are required, and they will all fit in place of the two CR2032 batteries in the battery box.

1x 3,350 mA - 4,440 mA USB power bank ( or similar) - from Walmart or Amazon

1x 20 LED light string - many types available on Amazon

1x 2N2222A or 2N4401 transistor - I confirmed both types work well.

2x 1N914A or 1N4148 diodes - I confirmed both types work well.

1x 3,300 ohm 1/4 watt resistor

1x 16 ohm or 2x 33 ohm 1/4 watt resistor - for Version 1 and 2

1x 10 ohm 1/4 OR (1/2 watt preferred) resistor - Version 3.

1x 270 ohm 1/4 watt resistor - version 2

1x salvaged USB A connector and cable - we will use the Red + and the Black - leads, and insulate the white and green data wires.

Step 2: Tools Required

Soldering station and solder.

Cutters, wire stripper, surgical clamp, precision screwdrivers.

Heat shrink tubing and heat source.

Hot glue gun and glue stick.

Digital meter or two for current, voltage and resistance testing.

Files round and flat.

Step 3: Schematic Diagram and Parts Layout - Version 1 and 2

Like most things I build, I am always thinking of ways to reuse as many things as I can. I enjoy a good search on Amazon, and the excitement any time a new parcel arrives...but using parts that I have on hand is a great feeling.

This was one of those builds, so I decided to use a basic constant current LED driver circuit I had recently learned about online.

The key component that determines the current delivered to the LED lights is the emitter resistor. To simplify the explanation here, I am going to state that the voltage drop across the emitter resistor is quite constant at 0.5 vdc thanks to diodes 1 and 2 connected to the base as a voltage divider.

In Version 1 and Version 2, I experimented with a 15 mA to 30 mA LED drive current to the LED string.

The math calculation for the emitter resistor resistor required:

0.5 volts / 0.015 amps = 33 ohms


0.5 volts / 0.030 amps = 16 ohms

In Version 2 the main difference is the 270 ohm resistor added to increase the total circuit current draw to just over 50 mA to keep some battery banks from shutting down after about 30 seconds.

In Version 3...I will wait until later to talk about this modification.

Step 4: Disassembly and Preparation

Remove the 4 screws that hold the cover together, set the batteries aside and let's get started.

We need to bend the tabs in order to create more space for the components. Needle nose pliers or a surgical clamp work for this task.

Next we need to remove the connecting bar that joined the two batteries. I trimmed the plastic nubs, and popped the bar off as it is no longer required.

Heat up the soldering station, and remove the switch and LED wires at the points noted in the picture.

I noted that the anode + lead has a white stripe for future reference, and put the LED lights aside for now. We will need to reattach them later, and make sure they are connected correctly.

I also added the switch and the connecting bar to my parts never know when they may be useful for another project!

Step 5: Populating the Battery Box - Refer to Version 1 or Version 2 Schematic

Here is how I assembled the components:

Reminder: the cathode negative (-) is the end of the diode with the black band.

-join D1 and D2 in series and solder ( I added a small piece of clear heat shrink as well).

-clip the anode lead of D1 and the base lead of T1 as close as possible to still allow a solder connection, and solder them.

-with T1 flat side face down, position cathode of D2 so it can be soldered to the negative USB - rail (where we bent the tab).

-trim the cathode lead to size, and solder.

-locate the 16 ohm or 2x 32 ohm emitter resistor(s) required, and solder between the T1 emitter lead and the negative USB - rail tab.

-I added a small piece of clear heat shrink to the 3K3 resistor, and then fit it between the T1 Base / D1 anode junction and the USB + rail tab. Then solder in place.

-for Version 2 - fit and solder in place the 270 ohm resistor between the USB + and the USB - rails.

-it is now time to dry fit the USB cable, and plug in the glue gun.

-you will have to snip and file a bit to allow the USB cable into the battery box (where the switch was originally located) patient here.

-with the red and black leads routed, solder them in place.

-now is the time to hot melt glue the USB cable to the base of the battery box. Hold the wire in place while the glue hardens. Add a few drops of glue to hold the green and white data wires out of the way while you are at it.

-I wanted the LED string to protrude in a straight line opposite the USB cable entry point. This meant I had to again snip and file the battery box to fit the wire in place.

-dry fit the striped Anode + LED lead and solder to the USB + rail.

-dry fit the Cathode - LED lead to the T1 collector lead. Solder, and add a piece of heat shrink to insulate the connection.

-Inspect all connections, and if all looks good it is time to connect it to the power bank.

Step 6: Version 1 Testing and Version 2 Modification

Version 1 Testing:

I used a Hype HW-440-ASST power bank which worked consistently (did not shut down) while powering the string of 20 LEDs.

Note:The calculated run time (fully charged) would be 4,400 mAh / 30 mA = 145 hours

I then tested Version 1 with the ONN ONA18W102C power bank, which would auto shut down after 30 seconds.

Version 2 Creation and Testing:

I then put together the same Version 1 circuit on a breadboard, and added the additional 270 ohm resistor to the USB + and USB - rails. This increased the total circuit current draw to 50 mA. The ONN ONA18W102C would then remain powered on consistently. This became Version 2 which will work for most USB power banks.

The calculated run time (fully charged) for the ONN ONA18W102C power bank would be 3,350 mAh / 50 mA = 69 hours. This will deliver at full brightness during this entire time.

Original battery ratings and thoughts:

The CR2032 batteries are rated at 3 vdc with a capacity of 240 mAh, and the site boasts that they will last 72 hours with continuous use. The internal resistance of the CR2032 battery limits the current to the Fairy Lights, and that is why there is no limiting resistor in the original design. However, all the sites that I look at indicate that the CR2032 doesn't like to discharge at such a high (30 or so mA) rate.

I can't confirm for certain at this point, but I recall the lights looking noticeably dimmer after 3 evening (of 4 hour duration). There is no way that you are getting "magic" out of these batteries. I confirmed through testing that the lights look very dull when the batteries hit 2.5 vdc per cell.

I will have to do some real life testing and update this post at a later date, but I think that the 3,350 mAh @ 5 vdc power banks should totally outperform the 240 mAh @ 6 vdc ( 2 batteries in series) CR2032.

Besides, the goal here was a longer run time, and ultimately fewer CR2032 batteries being "expended" and recycled.

Going Farther:

You guessed it...Version 3 is conceived, so keep reading!

Step 7: Fairy Light : Version 3 With Two Strands of LED Lights

Version 3 uses the additional current that was being diverted (wasted) into the 270 ohm resistor in Version 2.

Since we were targeting 50 mA as the total current draw to keep the average power bank powered on, we can make an improvement. I did a test where I powered a light string with 15 mA, and a 2nd light string with 30 mA and asked my wife if she could notice the difference. She looked back and forth several times, and indicated that she could not really see and difference.

This experiment confirmed a better solution would be to power two (2) Fairy light strings in parallel, and drive them with 50 mA of current. You can see in the attached schematic for Version 3, that all that was required was to change the emitter resistor R2 to 10 ohms, and connect a 2nd light string in parallel.

To calculate the power through R2 with Ohm's Law:

P = E x I

E = 0.5 volts (across R2)

I = 50 mA (through R2)

0.5 x 50 = 0.025 watts

We can safely use a 10 ohm 1/4 watt (250 mW) resistor for this application.

Image 2 shows the test circuit draws 50 mA as calculated.

I added a few images of the build process to show the cable routing.

Version 3 completed and testing on my bench.

Step 8: Version 2 and Version 3 - the Final Product

Here is Version 2 and Version 3 in operation on my bench.

Closing note:

This was a fun build, with lighting that I can use for any season throughout the year.

The best part being I don't have to order and wait for CR2032 replacement batteries anymore!

Thank you for following along, and Happy Building!

Bob D.

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