Have you ever looked into your mobile at night, maybe on a walk or outside enjoying the stars and gone instantly blind? You have just lost your night vision temporarily. But how does night vision work and is it possible to look upon, let’s say a printed star chart, without losing all of that precious low-light adaption your eyes do so well?
Yes it is – using red light!
There is a splendid Wikipedia article on low-light adaption, but I will just use this graph to illustrate why it is good for your night vision to use red LEDs that irradiate a wavelength of approx. 625 nm. At this wavelength, the rods that are used in low-light vision, are very insensitive to light and don’t lose their photoactive molecules that generate your vision. But the cones, that give the color impression for red, can see quite well.
Now how do we get red light from LEDs to go without spending a ton? We do it ourselfs of course, using inexpensive materials and stuff we already have. What do we need?
1. A power source – since everyone has a cell phone that charges from 5V supply, I'm assuming you have a powerbank for recharging your phone (e.g. Anker Astro E1 for 15$ will power your light for up to 50 hours).
2. A soldering iron, soldering flux and soldering tin and some basic working tools (wire cutter, flat pliers)
3. 10 red LEDs. A pack of 30 comes in on 5.25$ on amazon.com
4. 5 resistors of 33 or 47 Ohms, depending on your power source. Costs should be below 10ct per piece.
5. Disc of acrylic glass or plywood of 3 to 5 mm thickness and 7cm diameter
6. A can of Pringles or another tube-like object of 7cm+ diameter
Step 1: Designing a Simple Circuit for Your LEDs
A quality power bank you already have for your cell phone has the advantage of supplying a stable 5 V voltage, and you do not have to deal with voltage drop when the battery is low. So a simple circuit design is sufficient. For 10 LEDs on our acrylic glass disc, we can connect 2 LEDs in series with an additional resistor to match the LEDs operating voltage. For the LEDs i used, that is 2.2 V per LED, giving 4.4V combined. Now we still have to drop at least 0.6 V to not burn our LEDs, which we can do by adding a 33 Ohms resistor in this series. When there is the specified current of 20 mA going throgh the LEDs, the resulting voltage drop will be (33 Ohms times 20 mA equals) 0.66 V, and now we are below the max. specified voltage. If the LEDs you already have or bought have a lower operating voltage, you will want to go for 47 Ohms resistors or even higher using the same simple math.
To have 10 LEDs shining on our disc, we will use 5 strands of 2 LEDs and 1 resistor in series.
In addition to the image showing the circuit, I have also added the circuit file form LT Spice IV for your use in case you would like to modify the circuit :-)
Step 2: Arranging the LEDs on the Disc
I had prepared in adavance an acrylic glass with 10 holes of 5mm. There is 10 mm of free space on a circle between the LEDs, which makes for an inner LED circle that hase a diameter of approx. 48 mm. Using this arrangement, it is possible to use the pins of the LEDs and resistors for all soldering work without needing additional wiring.
You can cut your disc and the holes best using a laser cutter, or might as well use a mechanical drill of 5 mm size.
Apply all-pupose glue to the base of the LEDs and put into the disc. Make sure to alternate the position of plus and minus (the long and the short pin) from pointing inwards to outwards on every other LED. Now you have to wait a while for the glue to harden before the next step - soldering part 1.
Step 3: Soldering the LEDs Into Series
Bend the minus pin of one LED towards the plus pin of its neighbour, so that 2 LEDs eacht are connected in series.
For soldering, add soldering flux at the point of contact and solder with sufficient amount of soldering tin.
After you're done soldering the 5 minus pins to 5 plus pins of the particular neighbours, cut off excess pin length with a wire cutter.
Well done, your work should now look more or less like this!
Step 4: Complete the Soldering
Now that you have a stable base construction, you can add the 5 resistors into your circuit. I arranged them in a pentagon-shape to match the plus pins sticking up from the diodes. You may have to try several combinations of bending points and angles, but approx. 45° degree and two bends worked for me.
Now bend the 5 minus pins inwards - very close to their LED bases - so that they all meet at the center. Apply enough soldering flux and use plenty of soldering tin to connect them all.
Your old USB cable now comes into play, and you have to solder the red wire (+5V rail) to the top of this awesome shape, and the black wire (ground) to the center. If you care for a test drive, now is the time :-) First light is there!
Step 5: Casing
Here comes our low-budget casing!
Use a cutter to half a standard Pringles can, leaving the top for use. To make a new bottom, we need to get rid of some material from the mantle to allow for folding in. As the tube has a diameter of 7.25 cm, its circumference is approx. 22.7 cm. I cut 10 triangles (matching the 10 LEDs) of 23 mm base length. When you have cut away the upper triangles, you can fold in the spikes and glue them together. Make sure to cut away a little more material for your USB cable. Adding a little bit of all-purpose glue to the top of the LEDs, you can attach the transparent lid of the chips can to your LED disc. It fits perfectly to the can again, if you center your LED disc precisely!
And do not forget to enjoy your chips during all that work :-D
Step 6: Power Up, Enjoy!
You can now plug the USB cable into your power bank and enjoy the light show. Have fun with your night light, and to successfull star-gazing!
Step 7: Space Contest
I hope you liked this short tutorial and therefore maybe help me out a bit.
This instructable will enter the 'Space Contest' here on instructables.com and it needs your love and votes! So please click on the orange litte flag on the right top corner of this tutorials to vote and tell your friends about it. Your support is highly appreciated <3 Thank you!
Wish me luck ;)