Basically, you put together LEDs and 7-segment displays in such a way that models a chemical molecule. Each component represents an atom and everything is shaped such that it resembles the actual geometry of the molecule.
They make excellent gifts because they can be very personalized. You can choose a molecule that keenly describes the recipient of the gift, making it more special.
Step 1: Find Your Molecule
Wikipedia is a good place to find molecules because they have pictures of the actual structure of the molecule as well as a brief description of its properties.
If you are giving this as a gift, choose a molecule that best describes the recepient. For example, the first molecule I made was acetylcholine which I gave to my friend Coline. The second one I did was phenethylamine, a substance dubbed as the "love chemical".
Here are some other suggested molecules
1) serotonin - antidepressant
2) tryptophan - makes you sleepy
3) dopamine - "rewards" and pleasure neurotransmitter
For this instructable we will be constructing the estradiol variant of estrogen, the female hormone
Step 2: Buy Your Parts
For most of the part, since you'll most probably be doing an organic molecule, carbon atoms will define the structure. Use an LED to represent carbon atoms. On a personal note, I tend to associate carbon with the color green, so I used green LEDs. Buy as much green LEDs as there are carbon atoms in your molecule
For the other elements present in your molecule, you may use a different colored LED, but i find it better to use a 7-segment display with the element symbol written on it (more on this later). Atoms such as oxygen and nitrogen are quite nicely represented by 7-segment displays.
Note also that hydrogen atoms are not explicitly represented in the sculpture. Why? Practicality. A typical organic molecule may contain up to 30 hydrogen atoms and that's too much. I find it best to include hydrogen only on those "special groups" like OH (hydroxide) and NH2/NH3 (amine). I tend to associate hydrogen with the color white so I used white LEDs (which are kinda expensive).
So for the estradiol project we'll be needing
18 green LEDs (for the 18 carbon atoms)
2 common anode (ELS402) seven-segment displays (for the 2 oxygen atoms)
2 small white LEDs (for the 2 hydrogens in the hydroxide (OH) group)
You'll also be needing a battery clip, 1kOhm resistor, soldering wire and iron. You might also want to buy an enclosure if you'll be giving this as a gift.
Step 3: Start Building Your Molecule: Carbon Backbone
Take two LEDs and put them up together as shown in the first picture. You can have the LEDs form whatever angles, but since this would be part of the benzene ring, they are angled at 120 degrees. Squeezing their legs together like this fixes them in place so it'll be easy to solder them.
Note: be sure to solder together legs of the same length. What I mean is to solder them in such a way that they are parallel. For convention, I put the longer (positive) leg on top.
Also be careful not to let the solders touch, effectively shorting them out. Check if it works by applying voltage across the legs. All the components should light up.
Continue the process until you get the desired structure, stopping every so often to see if it still lights up. Next I'll show how to put the 7-segment display as well as share the technique I used on those "tricky" parts.
Step 4: Start Building Your Molecule: 7-segment Element
On a common anode 7-segment display, you have 8 pins that correspond to the 8 "lights" (7 segments + 1 decimal point) and 2 supply pins (they are redundant).
1) Hook up the supply pins to a positive source
2) Connect the remaining pins to negative (ground) to light up the corresponding segment. See second picture for an illustration.
So if you were to write the letter O, You ground up all but the one pin that corresponds to the middle segment. If you were to write the letter F you ground up all the pins except the bottom and the two right side segments.
Here's the technique that I used to connect the 7-segment displays (refer to the first picture)
1) Bend the supply pins flat inward.
2) Insert a wire (perhaps a resistor or LED leg) through the bent pins and solder into place. (fourth picture)
3) Cut off the pins that you don't want to ground ground (the one that corresponds to the segment that you don't want to light up)
4) Bend the remaining pins together towards the center.
5) Place another wire this time on top of the remaining pins. Solder into place. (fourth picture)
You'll end up wit a 7-segment display with two wires sticking out of it. Connect this into your molecule as you would do with an LED. Be careful with the polarity!
Step 5: Start Building Your Molecule: Third Dimension
To add an LED that sticks out, I find it best to bend the legs of the LED as shown in the first picture. Slot the LED into place in your molecule and you're good to go. You can use the same technique to attach LEDs at different angles. You can see the LED sticking out of the molecule on the third picture.
*NOTE: I have just realized that my model is not quite geometrically accurate. I should have been forming tetrahedrons on the second and third benzene rings. The technique outlined in this step could be used to make those tetrahedral shapes.
Step 6: Add the Finishing Touches
-Take note of polarity: Solder the red wire to the positive side of your molecule. Black wire goes to the ground side. NOTE: You should put the resistor between the molecule and the red positive wire to prevent burning out your LEDs. I actually forgot to do this, but it's still going to be pretty safe since you got a lot of LEDs in parallel sharing the current.
2) (optional) Put it inside an enclosure. The enclosure that I got has a screw-on lid. It's a nice touch to have the recipient of the gift use a screwdriver to "unwrap" the gift
Note that I didn't put any batteries with it. I also did not tell her what it actually is. She had to figure out that she needs to clip in a battery, and she had to do some research as well (she actually mentioned the Merck Index).
As a cherry on top, I've asked HER to take a picture of the sculpture all lit up. I'll post it here after she does.
EDIT: see the last picture with it all lit up!