LED jar

I saw the Sunjar at http://charlesandmarie.com/, purveyors of interesting stuff, last week, and thought it looked like a pleasing nightlight.  I don’t feel that I need a nightlight, but I happened to notice that the empty jar from my hair, erm, product looked similar to the Sunjar’s container.  When I saw a spare electronic switch on my desk, I figured that I had the makings of a minor diversion on my hands.

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Step forward, Tresemme Smooth and Silky European Deep Smoothing Masque!  At $4.00 for a sizeable jar (alas, not quite large enough to qualify as a “jug”), this is the least expensive chemical of its consistency on the supermarket shelf.  The adjective to dollar ratio is also quite good.

Step 1: wash the jar.  While the fragrance of Vitamin H(?!)-fortified hair goop is not unpleasant, it is more suited to salons of the follicular type than the dining sort.  Oddly, the jar is double-walled, and some water got into the gap between walls when I washed it.  I had to flex the outer wall a bit to drain the water out.

Step 2: This jar has the additional advantage, for our purposes, of having the product information applied via sticker, rather than painted on.  One scientific application of pocket knife later, the large label is free, and the top label yields quickly to a fingernail.  I cleaned off the top with alcohol to get the glue residue off of the otherwise pleasingly gloss black plastic.

Step 3:  Drill a ~ 3/8″ hole in the center of the lid.  Finding the center is left as one of those irksome “exercises for the reader”.  I measured across mine a few times and marked several linear center points until they clustered.  There is probably a more elegant way to do this.  Note: don’t use a center punch; it will shatter the plastic.  In unrelated news, the lid to my currently in-use jar of hair gunk has a large swathe of packing tape across it.

Step 4:  Remove the threaded nut and lock washer from the switch.  There is a little nubbin at the top of the threads meant to restrain the lock washer.  This will be in the way later on, so cut it flush.  Do not look straight down at it as you do so, lest it bounce off of your face when it flies off.

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Step 5:  Twist the switch through the hole, leaving the button on the top of the lid, and then thread the lock washer and nut onto the switch threads from the other side, leaving it spark-plug like when viewed from below.

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Step 6:  Retrieve your electric gear bag from your car trunk, where you forgot it last night.  Wear your coat or you’ll catch a chill.

Step 7: Do some math.  You usually want a resistor in series with the LED (“throwies” notwithstanding).  The LED circuit formula to get the needed resistance is:
R = ((Battery Voltage) – (Voltage drop over LED))/(LED current spec)
The latter two terms are printed on the back of the box of LEDs.  Unfortunately for me, the box I got was an assortment, with a voltage range given.  It turns out that different color LEDs use different voltages.  Fortunately, this is usually predictable.  I looked up the value for yellow LEDs, since I wanted a yellow color.  I then did the math and then bought a “close enough” valued resistor.  Yellow LEDs with 2 AA batteries (3 volts) need about 45 ohms inline with them.  I got a 47 ohm resistor; since this resistor has +/- 5% confidence, I figured I was close enough.

Step 8: Figure out a way to hold your batteries.  I had a battery holder lying around, so I just used that.  They’re cheap enough, but you need to make sure to find one that will fit across the inside lid of the jar with the switch in the way.  Make sure to thread the bottom of the jar on when testing the size, since the male threads diminish the available space further.

Step 9:  Solder things.  In order: 2xAA positive battery holder terminal – switch – 2 yellow LEDs in parallel – 47 ohm resistor – negative terminal of battery holder.
Things to look out for here:

– LEDs have a positive leg (longer) and a negative leg.  Make sure to solder them in the correct order.
– You might want to use a length of wire in between components to ease the geometry a bit.
– Don’t lean your soldering iron against the jar lid.  It smells bad if you do so.

Step 10: Neaten up a bit.  I stuck the battery holder to the lid with some 3M double-sided sponge tape.  You can’t fasten it to the jar because then you can’t screw the lid on without breaking wires.

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Step 11: Try it out.  You might need to bend the LEDs around to get a better throw pattern.

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This won’t replace a real Sunjar, since this looks cheaper, isn’t made as well, isn’t as bright, and isn’t solar powered.  Looks neat, though.

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The switch and LEDs here are from Radio Shack.

Switch: SPST Push-On-Off switch (275-1565)
LEDs: 276-1622 (assortment)

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