Here’s a guide to building a USB powered LED lamp compatible with Voltaic Systems products. This is a quick and easy way to convert the solar energy we’ve stored in our Voltaic Systems batteries into light. We’ve used ours when building some cables in low light conditions, but it could also be useful on a camping trip.

Note: As with all our DIY projects, try this project at your own risk. Voltaic Systems makes no guarantees against damage to the Voltaic battery or your LEDs. Use caution in your wiring.

Instructions: Solar Lamp

    Parts List:

  • 3x White LEDs
  • 3x 8 ohm resistors **
  • Electrical Wire ( we recommend 22AWG stranded)
  • Craft Wire (we recommend 18 AWG)
  • Shrink Tube
  • USB male plug
  • USB Battery Pack
    Tools:

  • Soldering Iron
  • Pliers
  • Wire Cutters
  • Wire Stripper
  • Heat Gun


There are two pins on each LED, one is the positive (+) pin or anode, and the other is the negative (-) pin or cathode.


Attach the resistors to the anode pins of the LEDs. Then cover the resistors and LED pins in shrink tube to prevent short circuits in the future.


Twist together and solder the resistor pins so that the common connection is in the middle of the three LEDs. Trim back excess pin material.


Solder a red wire (for positive connection) to the bundle of resistor pins and cover this connection with shrink tube.


Solder wires to the cathode pins of the LEDs. Then twist and solder the other end of these wires together. Solder a white wire (for a negative connection) to the bundle of LED cathode wires. Cover this joint with shrink tube as well.


Cut a length of craft wire.


Wrap one end of the craft wire around the LED assembly. Use shrink tube to keep the electrical wires fixed to the craft wire. Wrap the electrical wire around the craft wire.


We used a USB connector from one of our output wires. The red wire is connected to pin #1 of the USB plug, and the white wire is connected to pin #4. Connect the wires from the LEDs to the wires on the USB plug. Cover the connection in shrink tube. Wrap the craft wire around the USB plug and twist to lock into place. You may consider using an epoxy or other adhesive to make this connection if wrapping and twisting isn’t enough.


Finally, turn on the battery and connect the light!

Performance
For a small 3-LED lamp, it outputs quite a bit of light. The quality leaves a little to be desired (it is a very cool 7500K) and the output is too focused to be a general use lamp. It works really as a reading light and as a focused work light for detail work. The LED Lamp is designed to use 500mA at 5V or 2.5W. The V11 capacity is 11Wh, so you can expect this LED Lamp to run for about 3.5 hours before the V11 needs to be recharged.

** Calculating the Resistor Size
To calculate the resistor size we determined that we wanted to draw no more than 500mA from the V11. Because the LEDs are wired in parallel the total share of 500mA for each is about 167mA (500mA / 3). For our lamp design, each LED is its own circuit consisting of a 5V power supply, an LED and a resistor. Our power supply is limited to 5V and the LEDs require 3.7V to begin conducting fully, which leaves a 1.3V drop across the resistor (according to Kirchoff’s Voltage Law). Because the resistor is the element which limits the current flow in the system, we must calculate its minimum size given the voltage drop across it and the current restriction we set above. Using Ohm’s Law (V=IR), we can solve for Resistance (R) using Voltage (V) =1.3V and Current (I) =.167A. We get about 7.8 ohms (R=1.3V/.167A). The closest standard resistor value we had in our lab was 8 ohms, which is what we recommend for your project.

Increasing the resistance will make your lamp less bright. It is not recommended to reduce the resistance below 8 ohms. This may cause damage to the LEDs or any USB supply you connect your lamp to.

Once you have assembled the LED lamp cluster, try inserting different resistor values between the USB connector and the red wire leading to the LEDs. Using a switch, it is possible to choose between full brightness and a dimmer output.

5 Responses

  1. Along

    just wondering, How to calculate the resistor size, if we only use 1 3.7v LED light in the case above? is it 2 ohm? But will the LED can take 500ma?

    Reply
  2. Clark

    I wanted to use a USB cord from an old USB mouse to rewire a study lamp and then connect 3 LEDs as instructedd above. Wondering if the wires will be able to carry enough current. I will not need the green and white wires and the stranded shielding. Not sure how to prevent short of the green/white/shielding. If I connect my lamp to a USB from a laptop pc, want to make sure I dont blow out the PC. Do you have a USB cord that I could use that only has the +5v and ground wires connected? I need about a 2 1/2 to 3 foot long cable.

    Reply
  3. Clark Pontius

    Thanks for this type project. Although it may be easier to purchase some ready made stuff, it is imperative that people get back in touch with “tinkering” and this is just the type of project to start with. As for me, I will still have to scrape together enough money for the parts. But now I have instructions!  Thanks!

    Reply
  4. Chad Armstrong

    An interesting project. I love this little demonstrations on this blog.

    However, I found it easier (and likely cheaper for me) to purchase a USB-powered LED light and then plug it into a USB battery pack like the V11 or the iGo Charge Anywhere.

    Reply

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