This is my first Instructable and my first summer project. This is a Pressure Sensitive LED Card driven by a 3 Volt battery and a basic transistor circuit which senses the resistance between the two probes (the foil). Luckily as you press harder the transistor allows for more current to reach the LED this is a neat effect and something which I thought my girlfriend would appreciate. As such I decided to make this simple circuit I was studying into a card for here and in the process document and write up the process.

I included an EAGLE Schematic file of the schematic and a pdf version. Furthermore I have those files hosted on my github account. The schematic was taken from the circuit used in MAKE Magazine's Introduction To Transistors video. The video is a good basic introduction to transistors. However, I don't really understand the inner workings of transistors so I am going to avoid in depth discussions of transistors, instead resorting to links to explain a little more about transistors.

Step 1: What You'll Need

You will need the following
  1. 20MM Coin-Cell Battery Holder 
  2. 3V 20MM battery  (I used Energizer CR2016, but a thicker battery would be preferred. I needed to tape the battery into the casing to maintain a good connection)
  3. 3mm Red LED (color, size and quantity may vary depending on your card)
  4. 2N3904 NPN Transistor (any low power BJT NPN Transistor SHOULD work)
  5. 1/4 Watt Resistors: 220 ohm, 1k ohm, 100k ohm
  6. A SPST Toggle Switch
  7. Aluminium Foil
  8. Electrical Tape
  9. Cardboard
  10. Nice Paper
  11. Tracing Paper
  12. Wire (22 gauge is fine)
  13. Glue
  14. A Card Design
  15. Solder and Soldering Iron
I got most of my parts from Sparkfun, Radioshack and Michaels (most of them could have been acquired from Radioshack alone).

I included a csv with the bill of materials (including Supplier Part Numbers) and heart.svg the base card I used (Inkscape will open the file for editing).

Step 2: Construct the Demo Circuit (optional, But Suggested)

This part is optional but I would really suggest demoing the circuit first. This will gain you a better understanding of the circuit (and transistors!) and make sure you know what to solder when you get to constructing the card. I HAD a bunch of pictures of this stage, but they all got deleted... As such I am going to reference you back to the MAKE Magazine video on transistors which has a nice breadboard layout of part of the circuit (you just need to add in the switch from the schematic).

Video with breadboard layout in it (thanks to MAKE): http://youtu.be/-td7YT-Pums

I plan on updating this as soon as I get the time together to reconstruct the breadboard layout (with a video).

Step 3: Design the Card (or Wing It)

Now is the point in time where you should design your card. Where the LED will be, where you are going to position the electronics, etc. Remember the paper is not going to be super sturdy so keep that in mind when making the design. Also this would be a great time to get any color you want to put on the paper on the paper.

I laid out mine in Inkscape (heart.svg is my quick layout). I did a lot of improvising while building it and I wish I mapped out the location of my transistor and a few other things a little better. Keep in mind your on-off switch as this should be labeled correctly (mine was not). Have fun here.

Suggestions for layout:

1) Mark off your edges (don't use solid black lines like me, use crosshairs in the corners to mark the corners or dotted lines. Grey lines work too).
2) Keep the design simple, if you want to write on the card leave room for this (you can also write on the back.
3) You can lay this out in pencil. If you do I suggest laying it out on a separate piece of paper and marking the edges with a computer and printer. Then you can transfer.
4) Have fun and get your message across. It doesn't have to be a cutesy one, it could be a get well card or something. Have fun!
5) If you want me to host it for others to download, please email it to me (happyrobot.labs <at> gmail.com)

Step 4: Put the Card Together

This part requires a lot of ingenuity because your design might not be the same as mine. I will give you a basic outline of what should be done. Heat up and tin your soldering iron, because you are going to need it (try not to burn the paper).

First cut out everything you need to cut out. It is ok to poke holes in the paper with the leads of the transistor and LED, but it might distort the paper. Now put your 3 main parts and secure them to the paper with a little bit of glue (also bend the LEDS). My switch had a washer and nut on it, so I used those to secure it to the paper (which worked brilliantly). As you put in each component I suggest labeling each lead and terminal so you know what goes where (also have your schematic ready). Now you can start wiring things together be careful to not mess up any of the connection, especially those on the LED and transistor (unless you want to buy a new transistor and LED).

TIP: I actually put each part in and drew my connections with a pencil on the back of the paper so I knew what had to be connected where (this does not have to be super neat, because we are going to cover the back).

Solder the connections together using wire where needed. This can be tricky, make sure to make smooth connections and cut the extra leads. Leave the two probe wires unconnected on one end (make sure that one can reach the left side of the card and the other the right side of the card). Now take your aluminum foil and wrap it around the card. You are going to need to try to solder onto the aluminum (if you use copper foil there is no issue here), which is neigh impossible. However, I found that if you heat the foil for long enough it will eventually accept the solder a little. Use electrical tape to secure this solder connection a little better and put more foil around it (you can glue the foil down to the paper and to itself.

YAY! We got the electronics connected. Make sure it works.

Now you are going to need to do any decorating to the card you want to do (you can also delay this for after the reinforcement step).

Step 5: Decorate the Card

I used some tracing paper to make the LED disperse over a larger area (especially in the dark) and a hi-lighter to color in the heart and give it a nicer look when unlit and also contribute some color to the matter.

I took a sheet of tracing paper and cut out about 4 squares. I took one and cut it a little small and put it on in somewhat of a heart shape (this is very difficult) using glue. I then repeated this with increasingly larger squares of tracing paper until I got something which resembled a heart a little. Using a sharpie I tracked out the original heart and colored it in using a pink hi-lighter.

Tada! Done with decorating!

Step 6: Strengthen the Card

Lets do one last thing: reinforce the card. Let's face it, the card is currently a piece of somewhat flimsy paper with tissue paper glued on top, this card (in its current state) wont last long. However, if we reinforce the card its life span will increase tremendously.

So lets get down to it. For this step you will need:

Cardboard, Scissors, and an x-acto knifeGlue, and another piece of paper (don't bother cutting this one to size yet)

Take the card and place it onto the cardboard. Carefully press the card down to make an imprint of the on-off switch onto the cardboard. Then, using an X-ACTO knife cut out a nice slit for the on-off switch (I suggest wrapping the on off switch in electric tape to shield the solder joints. Now glue the card down to the cardboard and let it dry. Once dry cut the cardboard down to size and try to avoid cutting off the aluminum foil.

Repeat the same process with the card stock. 

Now you have a nice sturdy card that lights up at variable brightnesses at variable pressures! It is like magic (or electronics)!

Step 7: Write It and Ship It

Here is where I am no longer useful. Have fun and take this project to the next level!

Take pictures, video of reactions, write cute notes and let me know how it goes. Send one to a loved one and make their heart glow.
Brilliant. You're covered for pretty much all major card-giving holidays.

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