9V LED Flashlight




Introduction: 9V LED Flashlight

Build a working flashlight using a 3D printed case, electronic components, and a soldering iron. The case has a belt clip and a hole for a lanyard or key ring. All the parts may be purchased through eBay in quantities that make this an inexpensive project for your group - make717 builds these at community maker faires for less than $1.50 each.

Designed by MDIM for make717.

Step 1: Theory, Don't Skip It!

An LED gives off light when electricity passes through it. It can run at almost any voltage as long as the current stays within the design limits of the LED. Our circuit uses a 9 volt battery. To keep it from burning out the LED, we use a resistor to limit the electrical current. A switch turns the LED on and off.

The schematic shows how the parts relate. We will construct this circuit using "dead bug" techniques, so there is no need for a printed circuit board.

The math (from http://www.electroschematics.com):
(Source Voltage – LED Voltage Drop) / Amps = OHMs

Amps = mA/1000

If your LED has different ratings, you can use this formula to adjust the value of the resistor.

The wrong resistor could cause your LED to burn out!


Source Voltage = 9 volts

Voltage Drop = 3.1 volts typical for a blue or white LED

Desired Current = 13 milliamps

(9 – 3.1) / ( 13 / 1000 ) = 452 ohms; 470 ohms would be the nearest standard value.

Our LED specs:

Water Clear White 5mm

3.0-3.2V Voltage Drop

20mA Desired Current

Using the math from above, (9-3.0) / (20/1000) = 300 ohms would be an appropriate resistor.

Step 2: 3D Print the Flashlight Case

An STL file for FlashLight Gen 4 is provided. Print this STL file in PLA. It takes about an hour with a typical printer. You may finish the rest of the project while the model prints. The flashlight is designed to be printed with the flat top down (belt clip up) as shown.

TIP: if you are doing a group project, print multiple copies at once. At make717, we print 9 flashlights at a time. Print a pile of them in advance, and let your group replenish the supply while they complete their projects. This reduces assembly time to about ten minutes.

PRO TIP: The flat side is a great place to laser engrave someone's initals - at make717 we add this step to the project when hosting a group at our space, where the laser is right at hand.

Step 3: Electrical Parts List

As shown above:

  • Battery Clip, modified from a hard plastic T-clip.
    Copy and paste these keywords into the search box at eBay:
    9V 6F22 Black Battery Snap Connector clip Lead Wires Holder T-type Hard
  • Switch.The project is designed around a generic flashlight switch.
    Copy and paste these keywords into the search box at eBay:
    1A 30V DC 250V black Latching on off mini torch push button switch
  • 300 Ohm Resistor, 1/4 watt.
  • LED, Water Clear White 5mm.
  • 9V Battery (may be heavy duty or alkaline).

make717 does not endorse any vendors - use the one you like best!

Step 4: Modify the Battery Clip

Our battery clip is modified from a hard plastic T-clip, which comes in bundles of 100:

  • Pry the plastic cap from the clip using a small screwdriver.
  • Clip the two wires inside close to where they are crimped to the contacts.

TIP: Do this as busy work in front of the TV, and bag the clips for use later. The wires make great jumpers for your Arduino project.

Step 5: Prep the Switch

Bend one leg of the switch to match this drawing.

Step 6: Prep the Resistor

Bend one leg of the resistor to match the drawing. You can trim the leads to match the print, or solder it in place (next step) and then trim the ends, whichever works best for you.

Step 7: Solder the Resistor to the Battery Clip

Feed the bent lead of the resistor into the negative terminal (-) of the clip. That's the smaller of the two terminals. There should be a hole in the plastic at this end of the clip; the resistor goes through this hole as shown.

Solder in place.

TIP: Limit the solder time to 3 seconds or you may melt the plastic on the battery clip.

Step 8: Solder the Switch to the Battery Clip

Feed the bent lead of the switch into the positive (+) terminal of the clip. That's the bigger terminal. Also the one not connected to the resistor.

Solder in place.

TIP: Limit the solder time to 3 seconds or you may melt the plastic on the battery clip.

Step 9: Bend the Switch Into Final Position

Bend the switch until it touches the three posts on the battery clip. It should be centered between the battery contacts.

Step 10: Solder the LED in Place

Look at the picture, and make the connections to the LED:

  • Solder the long lead of the LED to the switch.
  • Solder the short lead of the LED to the remaining lead of the resistor.

TIP: Depending on how you prefer to work, it may be easiest to connect the short lead to the resistor first, and then trim the long lead to fit the switch. Or connect the long lead to the switch first, using it to position the short lead in contact with the resistor lead.

PRO TIP: The photo shows a small alligator clip holding the LED in place while the short leg is soldered. Saves burnt fingers and a lot of cussing.

Step 11: Test the Assembly

Snap the electrical assembly onto your 9V battery.

Press the button on the switch a few times. The LED should turn on and off. If it does not, go to the troubleshooting section and figure out what’s wrong.

Step 12: Assemble the Electronics to the Plastic

Assemble the case:

  • Slide the LED into the groove in the flashlight case (1).
  • Press the switch into the hole in the top of the case (2).
  • Snap the battery clip into the case (3).

The video shows how it's supposed to work.

Step 13: Done!

Check the flashlight for function.

If your flashlight doesn’t work, consult the trouble shooting section (next step).

You’re done!

An alkaline battery should last for about 25 hours. The LED will get dimmer as the battery runs down. You can use any 9v battery as a replacement.

Step 14: Troubleshooting

LED doesn’t light?

  • Check the LED leads - (+) goes to the switch, (-) goes to the resistor. The flat side of the LED body is (-).
  • Check the resistor - is it connected to the (-) contact of the battery clip?
  • Check the switch - is it connected to the (+) contact of the battery clip?
  • Check all solder joints.
  • Check for crossed leads (short circuits).
  • Using a multimeter, check the LED for continuity. It should conduct in one direction only.
  • Using a multimeter, check the switch for continuity. If it does not conduct electricity, press the button and test again. Expect a 5% failure rate on these switches, usually from overheating during assembly.
  • Using a multimeter, check the resistor for continuity. It should conduct electricity, and if you test its value, it should read 300 ohm.
  • Using a multimeter, check your battery. It should produce at least 1.5 volts. At 1.25V, the battery is officially dead.

PRO TIP: When building with a community group, assign one person to run the test equipment and send troubled flashlight builders there for service. That keeps the line moving.

Step 15: Bonus Step: a Jig for Assembling Flashlights

Because make7i7 does these in volume, we use a jig to simplify assembly. It's easy to make your own:

  • Find a chunk of aluminum for a heat sink. Our jig uses a slice of 1x3 heavy-wall tubing, about 1-1/2" (38mm) wide.
  • Modify a 9V battery clip by trimming away the plastic until it lies flat.
  • Use 2-56 miniature machine screws through each battery contact to fasten it to the heat sink. We clamp the contact to the sink, then drill and tap using the center hole of the contact as a guide.
  • Label the jig to reinforce correct placement of the electronic components. The LITTLE contact should be labeled SWITCH. The BIG contact should be labeled RESISTOR/LED. This is the opposite of the actual assembly, because the battery clip mates to the contacts on the jig.
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    8 Discussions


    Question 1 year ago

    How long does the flashlight last before emptying the battery


    3 years ago

    Two-thirds of your power is being wasted in the resistor.

    (Don't take this the wrong way, but you need to pay attention to electrical theory !-)

    Three 3V LEDs in series can use exactly the same current as one 3V LED and a resistor, and you get much more light for your buck.

    Ebay has bags of 100 white 5mm LEDs for a little over a dollar.

    Reply 3 years ago

    I always welcome comments from people who write their own Instructables. I invite you to write one appropriate for ages12-18 on how
    to calculate the values for this circuit using three random LEDS.

    (Don't take this the wrong way, but you need to pay attention to educational theory !-)

    We use a resistor to teach what a resistor is for. Using more diodes might be an efficient circuit (though I question its resiliency in a power fluctuation) but when teaching, separating functions makes the game a little easier for teacher and student. The math is more important than the light output.

    (Don't take this the wrong way, but you need to pay attention to Design for Manufacturing !-)

    We make design choices with purpose: our budget was a strict $1.00/unit, and we've eased that only because
    batteries that met the budget had an unacceptable failure rate. Resistors are still cheaper than LEDs. More LEDs would require more plastic for mounting - more time on the printer, which costs money, too. And since there is money in the soldering jigs, you can't alter them without justifying that cost.

    The polarity of one diode is a manageable issue when building kits with 200 impatient kids, most of whom have never held a soldering iron. Also, make your circuit work with my construction jigs, so kids don't burn themselves trying to assemble the LEDS. There is enough variation in handmade circuits to make fitting the plastic over one LED a challenge; threading three LEDS would certainly be interesting. Show me how you would do it.


    3 years ago

    Nice, compact idea. You could "double" your light for "free" by using two leds and dropping your resistor to 120 ohms. It would require a slight change in your enclosure design.


    Reply 3 years ago

    A worthy suggestion, but this project was designed to a strict $1.00/unit budget, and the extra LED and plastic would impact the bottom line. The current cost - about $1.25 - reflects name-brand batteries because generics don't keep on the shelf.

    Also, extra LEDs deplete the battery faster. Some of our builders had never seen a 9V battery - do you think they'll be able to find one when it's dead?

    For those of you not constrained by costs, go ahead and mod away! Although the correct resistor value would be 150 ohms, since you always want to err on the side of protecting the LED.


    3 years ago

    nice job, enjoyed reviewing it!


    3 years ago

    I did always like the lego assembly instructions, with their 3D renderings for each step, so thank you for bringing the same style to instructables! Although it's a simple project, the way all parts come neatly makes it outstanding from the crowd of 9V flashlights :)


    3 years ago

    Good tutorial, thanks for sharing :)