First off, let me give credit for the inspiration for this project. I had seen this article posted a few weeks back while reading through my RSS feeds by Dane Kouttron. Fantastic article and he even explains the principals behind this very simple circuit! http://transistor-man.com/lampfade.html
Here is what you are going to need:
1. (1) 100uF Electrolytic Capacitor
2. (2) 1M Ohm 1/4W Resistors
3. (3) Super bright white LEDs
4. (2) 68 Ohm 1/4W resistors (depending on your LEDs) Do the math for the resistance needed!
5. (1) 2N7000 N-CH Mosfet
6. (1) 12mm square momentary switch. Normally off.
7. (1) Tap light of your picking! I've listed the one I used.
8. A small piece (~4 inches) of wire.
9. Solder, soldering gun pliers, wire cutters, third-hand and about 20 minutes.
I strongly reccomend you check the prices at www.taydaelectronics.com for parts! I have ordered from these guys many times and I get my order quickly and accurately every time. They are located in THAILAND so shipping to US is about 2 weeks.
Step 1: Which Type of Light?
Step 2: Disassembly...
1. Remove the battery cover.
2. Remove the light bulb by twisting the circular plastic housing.
3. Remove all four screws.
4. Carefully remove the back plate. There are three springs, make sure they don't fly off!
5. Set aside the top, the large plastic retaining ring, the four screws and the three springs.
Now we must remove the existing switch which is an OFF-ON switch. These will come in handy for some future project, I'm sure.
1. Simply pull up on the switch and it should freely come out of the plastic housing it sits in.
2. You'll notice three legs on the switch, two of which are solder to red wires.
3. You need to desolder these wires from the switch.
Finally we need to pull the light base out of the plastic housing. I used a pair of pliers and gripped the metal as far down as I could and gave it a quick tug. I'll save the bulb and socket for later use in another project.
Step 3: Installing the New Momentary Switch.
I had quite a few of the 12MM square momentary switches on-hand that I had gotten from SparkFun.com some time ago. These worked perfectly for this project, you can find them here. For my switch I just needed to solder the two red wires that were connected to the old switch to one side of my new switch. I then filled the plastic housing just above the top with hot glue and positioned my new switch on top. After the glue has cooled this switch won't go anywhere!
Step 4: Installing the LEDs
You need to grab yourself (3) super bright white LEDs, the brighter, the better really. The lens on this tap light is quite opaque and diffuses the light well but if your LEDs are not bright enough you'll end up with a tap light that doesn't light up very much at all!
First you need to bend the legs on the LEDs to get them to fit nicely into the plastic housing that was used for the mini light bulb socket. We are going to reuse it for our LEDs. You can see the steps for bending the LED legs in the images below with comments on the images.
Once you have the legs bent properly and you've ensured you have all the anodes and cathodes figured out and pointing the same direction, you need to fill the plastic housing with hot glue and immediately place all three LEDs into the housing. You need to make sure that the ANODES are facing towards the solder tabs for the batteries (towards the center of the light) and the CATHODES are facing out away from the center of the light.
Once the glue has cooled, you can break out the solder and connect all the anodes together and all the cathodes together.
Step 5: Hooking Up the LEDs
Step 6: Wiring Up the Circuit
Wiring up the circuit doesn't require a PCB board, although I guess you could get a small one in this tap light if you wanted to. In the pictures below, I've outlined the process I used step by step and it was brain-dead easy.
DataSheet for my 2N7000 mosfet. Verify with your particular part's datasheet.
1. Connect the mosfet to and you capacitor together. I've trimmed the length of the legs on the capacitor a bit and pre-tinned the legs of all parts. Make sure the negative leg of the capacitor is connected to the source leg of the mosfet and the positive leg of the capacitor to the gate of the mosfet . (image 1)
3. Connect the resistors to the source and gate of the mosfet just as we did with the capacitor. You don't have to worry with polarity with the resistor. (image 3)
4. Solder the wire we previously soldered to the cathodes of the LEDs to the drain of the mosfet. (image 4)
5. Solder the positive wire from the switch to the gate leg of the mosfet. (image 5)
6. Solder the negative wire from the batteries to the source leg of the mosfet. (image 5)
Now, VERIFY every connection and ensure you have no shorts, bridges, etc! Once you have done that, do it AGAIN!
Step 7: Component Fitting
If all is good, your LEDs should all light up very brightly and stay bright for ~2.5 to ~3.5 minutes at which point they'll start to get dimmer. Wait until you have verified that they are getting dimmer and go off before proceeding. If they don't get dimmer or go off, review all the connections and ensure nothing is bridged or shorted out.
If all is well, you need to fit the components snugly into the light. (image 2) And place a small amount of hot glue on the mosfet legs to ensure that nothing gets shorted and that the components stay put! (image 3)
Step 8: Testing and Finally Assembly
1. Make sure you line up the slots in the light lens with the screw posts in the retaining ring.
2. Make sure you have put the three springs back into the holes. (2 will be in holes, 1 will be on a post) (image 1)
3. Make sure all wires are free and not pinched by the plastic retaining ring!
CONGRAGULATIONS! You now have a working auto-off LED tap light.
I hope you guys enjoy this instructable, please feel free to visit my blog to see some other neat projects I've been working on!