Introduction: Embroidered Circuits: Ohm Sweet Ohm
This project is a great way to get started exploring the potential of combining craft and electronics. Here I am going to explain how to make your own sensor (a variable resistor as it were) then create a circuit that to control three LEDs using solderable conductive thread(!). There are a bajillion different ways to take this simple circuit and create a new design from it. Definitely do not feel constrained by or compelled to follow the design here. Make your own and then share it with me - I want to see!!
Here's more background on the project concept:
Handmade Christmas ornaments often have stories locked inside them, nostalgic promises of memories and modes of being that resurface each year. For me, a combination of kitsch and handcraft formed a powerful imprisoning mechanism. No key can stop the sentimental here. (Or unlock the perfect non-handmade gift in time for Christmas...)
For this mini-project, I wanted to play with this intersection by including an interactive layer within it. Each ornament offers a promise of togetherness to its owners (i.e. my own family who I don't see as often as I like or should). To accomplish this, the interaction mechanic engages the user in the story of the ornament. In Ohm Sweet Ohm, the hearth lights become brighter the longer the road (conductive yarn used as a variable resistor) becomes, a reminder that home is ready to embrace you however far you may be. This one was for my mom and stepdad.
Step 1: Gather Your Non/conductive Materials and Toolbelt.
To make this project, you'll need (listed from left to right):
- Calico fabric (about 6x6 inches)
- 3x3 inch wooden embroidery hoop
- Embroidery thread (multiple colors)
- Sewable coin cell battery holder
- Karl Grimm solderable (!) conductive thread (Plusea sells small quantities of these on her Etsy shop - FYI it's closed until spring 2015, but check out the comments here for a great workaround)
- Conductive yarn from Sparkfun (Nm10/3 composed of a 80% polyester 20% stainless steel blend, has a breaking load of 8094g, and has a surface resistance of <104Ω)
- Glue gun
- Soldering iron (not pictured)
- Embroidery needle (not pictured)
- Knitting needles (not pictured)
- Coin cell 3 volt battery (not pictured)
Step 2: Layout Your Design and Map Your Circuit.
A thought before you get started
Working with these materials is all about the process. One of the best things about combining electronics and craft is the dialogue that happens between them: the design of one will most certainly impact the other. The fun is in finding inventive solutions to aesthetic and engineering problems that seem to beget each other. How do I avoid a short circuit when the copper thread needs to cross another? How do I attach a component without sewing through to the other side? How do I secure connections? Draft sketches and create prototypes of your design before you begin to avoid otherwise painful shortsightedness. (Then again, these failed prototypes are the best way to learn...)
We are using a parallel circuit for the LEDs. This means that all the ground sides connect and all the power sides connect instead of daisy-chaining them (ground to power to ground etc). This will allow us to light up all the LEDs at equal brightness.
The basic idea is that we are going to build a circuit out of our thread and components, but leave one part open so we can control when it goes on and off - or lighter or dimmer in this case. Here we will control the LEDs using a variable resistor - the conductive yarn.
Designing your own sensor: How it works
The conductive yarn is more resistive than the copper thread we are using to secure the LEDs. This means the electrical current does not move as easily across it. For example, if we have a circuit with a high resistance between the power source and the LED, the LED will be dimmer than if we used the copper thread or another highly conductive material. Try making a simple circuit with the thread then with the yarn to see the difference.
This is a powerful bit of knowledge: Now that we know this, we can use the material in different ways to manipulate the resistance and control the behavior of the LEDS.
If we increase the surface area of a resistive material, we can increase its conductivity. The current simply has more space to move on. So, how can we design a variable resistor whose resistance changes as we manipulate it?
Knitting is an extremely suitable solution to create your own stretch sensors (or pressure sensors, flex sensors, and more). While at rest, the connections between the stitches are loose, meaning a high resistance. The electricity has to take a longer route around different stitches instead of moving directly for its target. When you pull the stitches together, all the stitches connect and allow electricity to pass through easily over a greater area. Here the resistance is super low and the LEDs become brighter.
The pattern for the circuit is on the next page //-------->>
Step 3: Embroider Your Design.
I used three threads on each of these stitches:
- Back stitch - words; tree
- Running stitch - snow outline
- Blanket stitch - house
- Chain stitch - roof
- Star stitch - star on the tree
I highly recommend embroidering your design before adding any electronics. This will prevent you from breaking connections through potentially destructive overhandling.
The design pattern is attached here if you want to use it. If not, make your own!
Step 4: Sew the LED Traces
- Thread the Karl Grimm thread through a large embroidery needle. The eye must be large or the thread will unravel as you try to fit it in (very frustrating). I also suggest running it through wax to prevent fraying.
- We only want enough visible on the front for us to solder the LEDs. The remainder should remain on the back where we will connect it the circuit.
- Stitch the top first (this is your power/+ line) then the bottom (ground/-). Leave enough space between them to be able to solder your LED. Tip: A little bigger is easier to manage on the soldering end of things than a little smaller.
- Leave about 5 inches extra on the right side (if you are looking from the back) and about an inch on the left. We will connect the thread on the right side later.
- Hot glue the ends on the left side and trim the excess. If you don't, it might create a short circuit, an obnoxious enemy that is usually at the root of many a creative circuit/computational craft project.
A short circuit happens when the power and ground touch. Too much electricity (specifically current) will run through the circuit back into the battery and cause the system to stop working.
You should take care to hot glue all properly soldered connections or lose conductive threads. This being said, TEST the circuit before hot gluing a soldered connection. It might not be as strong as you think and having to remove the hot glue is extremely annoying, as you might imagine.
You may need to hot glue the thread on the right to prevent the threads from touching once you start to add the battery and sensor.
Step 5: Tin the Traces and Solder the LEDs
- In general, there is a small depressed triangle on one side that usually connotes GROUND/-.
- Double check which side of your surface mount LED is ground.
- Before we get started here, remember that you want to solder the correct sides of the LED to the correct traces! I usually have to keep a diagram with me at all times for this.
- Then touch this to the thread until it transfers. Do this to every exposed trace on the FRONT side.
- Touch your soldering iron to the solder so a little bead of it is on the tip.
I was elated to discover the hack I'm about to share. Soldering LEDs this small is a cumbersome task, but this should help a lot.
- Grasp the GROUND/- side of the LED in an alligator clip as shown above and place it directly next to one of the top traces.
- Hold the soldering iron on the trace - the tinned piece of copper thread - for a few seconds. (Any more and you might burn the LED.)
- Make sure the solder is solid before taking the alligator clip away. You can give it a gentle tug to make sure. If it's not sticking, add another small blob of solder on the tip first.
- Do this for the remaining sides of the LEDs until they are all attached.
Step 6: Knit the Stretch Sensor
- For this size ornament, I cast on five stitches and knit about 15 rows, alternating rows of knit and purl stitches. Change the length according to how much variation you want to see.
- If you're not a knitter, crochet totally works and lose braid with threads doubled up would do the job as well.
Step 7: Attach the Sensor and Test It
- Rethread the copper thread from the bottom line. This is your GROUND line.
- Sew one end of the knit sensor to the fabric. Make sure it creates a secure connection between the thread and the yarn.
- Using a new piece of copper thread (~6 inches), secure the other side of the sensor leaving at least 3 inches left over. We will connect this to the battery holder in a sec.
- For now, attach it to an alligator clip that is connected to the GROUND side of your battery holder.
- Now grab the top copper thread holding the LEDs and attach it to an alligator clip connected to POWER.
- Being careful not to create a short circuit, gently pull the sensor. You should see the LEDs get brighter the harder you pull. If this doesn't happen, make sure everything is properly connected.
- Once it is, move onto the next step!
Step 8: Sew in the Battery Holder
- Now it's time to secure the battery holder. Let's look at it first. The side with the extended arm over the middle is the POWER side. The side with no arm is ground. We want to place the battery holder so POWER is on the RIGHT side.
- Place a small amount of hot glue on the back of the battery holder, then place it as shown above.
- Solder the copper thread from the POWER line to the right side of the battery holder. I like to put a little bit of hot glue on one side to hold the thread down while I solder it.
- Solder the copper thread from the GROUND line to left side of the battery holder.
- Once it is working, hot glue the connection to secure it.
Step 9: Adhere the Wool Roving
- Take a bit of wool roving (or some other diffusing material) and use a small amount of hot glue to adhere it to the front of the LEDs.
- Secure it with a star stitch over the tree and a straight stitch to create two windows.
Step 10: Test and Debug
- Test it! You should be testing all along to figure out if something isn't working - this is the easiest way to pinpoint a problem.
- And even if you have, a problem can still always pop up :) Here are some potential problems and solutions:
- Short circuit: Two copper threads are touching that shouldn't be. It can be the teeniest tiniest wisp of copper or the whole thread. Hunt around for any possible connections. Sometimes I find it helpful to label my copper traces to keep them straight.
- Do you have a bad connection? A component might not be not soldered properly or the sensor might not be connected tightly enough. Go back and recheck your solder jobs or tightened the thread on the sensor. If you have a multimeter, use it. This can be tricky if you have already hot glued, but sometimes it's possible to peel it off.
Step 11: Wrap It!
Package it up with an extra coin cell or two. A sturdy small box will help the beneficiary of your lovely work store it properly.
Now go party. You just made something awesome!