Introduction: 1W LED Lantern Using 2 AA Batteries
This is my first project using a 1W LED. By using a DC-DC boost module, I am able to power the LED with two 1.2V rechargeable AA batteries. You can omit the DC-DC module and use four AA batteries instead if you wish but check your resistor values.
Because of the current limiting integrated circuit (LM317), you can feed 3-40V, although the LED needs 3.2-3.4V at 320 mA.
I am really pleased with the result of this project. I plan on using this when camping, or any time I want some extra portable light.
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Step 1: Materials
Forward Voltage: 3.2-3.4V
Current: 320 mA
DC-DC Boost Module (eBay) (for 2 AA batteries. Omit this module and use 4 AA batteries - check your resistor values)
Input voltage: 1V~5V
Output voltage: 5V
Output Current: 500mA
LM317 (used as current regulator)
2 x 8.2 ohm 1/2W Resistors (actually 8.8 ohm when tested due to manufacturing tolerances)
AA Battery Holder
2 x AA Batteries
Tall Food Container (4 cup / 946 mL) - Find these in your grocery or $1 store.
For such a simple project, there are a few key things which make this work:
1. LM317 Adjustable Regulator - Probably more commonly used as a voltage regulator, you can use just one resistor to use it as a fixed current regulator. We are using 2 1/2W resistors in parallel so we can achieve a 1W power handling.
2. DC-DC boost that is described as:
"input any DC voltage of 1V~5V, output 5V DC voltage is stable, the input to the output current of 500~600MA with two AA batteries, a single AA battery power supply output current 200ma about for mobile phones camera, single-chip, digital products supply."
I know this is a somewhat specialized part but I have seen circuits for this type of boost circuit, and for a little over $1, it's probably not worth building yourself. You can use four AA batteries if you don't want to use the boost module.
Step 2: Calculate
The LM317 is being used to limit the current that the LED needs, which is 320mA. The LM317 accepts an input voltage of 3-40V and can generate a fixed current based on the resistor value you choose. We will be feeding it around 5V at approx 500 mA, generated by 2 AA batteries and our DC-DC boost circuit. Because the LM317 will provide stable current with such a large voltage range, you can power this LED with a wide array of batteries without risk of damaging the LED. Be aware of proper heat sinking.
Here is a handy online calculator for the LM317. Notice you only need to specify the target current.
Enter 320 mA as the target current for the LED (as provided in the specs for the LED) and you get a 3.9 ohm 1/2 W resistor as the value to use. I wanted to use a 1 W resistor so I connected two 8.2 resistors in parallel (actually 8.8 when tested), which effectively provides 4.4 ohms (actual) of resistance at 1W. Sparkfun has a great tutorial on resistor theory.
The image is the formula from the LM317 data sheet.
Step 3: Build the Circuit
I breadboard everything first to confirm my current output of 320 mA with my resistors and my two AA batteries. Once that was done, all that is left is to build it.
Start with building the LM317 on your circuit board. Refer to the datasheet or leave questions in the comments section.
For pin identification, see note in photo.
Pin 1: Adjust
Pin 2: Vout (will refer to LM_out)
Pin 3: Vin
Both resistors connect from pin 2 (LM_out) to the output, which will be connected to the positive node of the LED. Pin 1 connects to the output with a jumper wire. Positive end of the LED connects to the same output as the jumper wire and the two resistors. Negative end of the LED connects to ground. I just made a small arch with the LED at the top of it. Check the notes on the photos and let me know if you have any questions about the circuit.
Next, wire the DC-DC boost module. One end is marked for USB output and nicely accepts a female USB jack. We are only using the outside pins marked 5V and GND. Connect 5V to Pin 3 of the LM317. Connect GND to the ground on the circuit board (shared by the LED ground wire).
The other end of the DC-DC module is marked IN+ and IN- which is the module power input from the battery. IN- connects to the negative wire of the battery holder.
Make sure you mount the switch on the lid before you continue. Just cut a hole in the lid to fit the switch you're using. I'm using a SPDT rocker switch.
IN+ connects to one end of the switch (test first if you switch is marked with on/off symbols). Finally, the middle terminal on the switch connects to the positive wire of the battery holder. I'm using a SPDT switch so the other end terminal on the switch is not connected.
Step 4: Assemble
Craft board is great if you don't have a 3D printer. It's strong, lightweight and can be used to create supporting structures for your circuits by using hot glue to bind them together. Follow the photos for one way to assemble your circuit. Feel free to leave questions for me. I would also like to know if anyone has better ways to assemble your electronics projects. Your construction may be different if you don't use the boost module and use 4 x AA batteries instead. I would like to better secure the battery holder. I think if I were to drop this, the battery holder with batteries could do some damage to the craft board structure.
Enjoy! I would love to hear from anyone who makes a similar lantern and how you designed it and where you use yours.
Leave your ideas and feedback in the comments. Again, if you like this Instructable, please vote for me in the Make it Glow contest!
Step 5: Improvements
I wanted to add a solar cell to the lid to recharge the batteries but it would take longer than a day to recharge them to full capacity so I decided I would plan to build a separate solar battery charger instead.
I also wanted to add a PWM (pulse-width-modulator) to dim the LED but that can be added later.
With an internal switch or creative switch mounting, this could be a waterproof lantern. Overall, I am very pleased with this lantern. You can get widely different effects if you use a cool white LED versus the warm white LED. I can see many uses for this and at only a few dollars to make and no moving parts, you don't have to worry about abusing them a little.
This could also make a good emergency light since it will accept a wide range of input voltages (although if you're using the DC-boost, you need to keep the power input under 5V).
Step 6: Test Results/Conclusion
2x AA batteries with DC-DC boost: Approx 6 hours
4x AA batteries without DC-DC boost: Around 12 hours. Overall performance with two AA batteries is similar to four AA batteries. This means you can build and use this with 'jellybean' parts (aside from the LM317 regulator, which is still needed), but without the DC-boost, you cannot get the LED to light at all with only two batteries.
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