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).
Brilliant. You're covered for pretty much all major card-giving holidays.

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