DIY Pressure Sensor: Rainbow Effect Haunted House

I recently learned that you can create rainbow special effects in a circuit by creating your own pressure sensor, with a small piece of resistive plastic (a conductive plastic that contains carbon). Commonly marketed as Velostat and Linqstat, pressure sensitive conductive plastic creates special fade in and fade out effects in LEDs within a circuit, because its resistance changes in direct relationship to how hard or soft you press on it. As you press down harder, the resistance in your circuit will decrease, causing more current to flow to your LEDs, making them brighten. As you reduce the pressure, the resistance will increase, causing the current flow to your LEDs to decrease, making the LEDs appear to dim.

This material gets more interesting when you have a bunch of different colored LEDs (such as Circuit Stickers, which have built in resistors) in the same circuit. If you use a piece of pressure sensitive plastic to control the current in your circuit, certain LEDs (red, orange, yellow, and green) will usually light up before other LEDs (blue, pink, and white). This will be true whether the LEDs are arranged in the order of the rainbow (ROYGBIV) or placed randomly. Reds will typically light up first, followed by orange, yellow, green, blue, pink, and white, wherever they may be placed upon a circuit!

In this tutorial, I'll show you how to make a haunted house light up, one color at a time, mimicking the visible light spectrum of a rainbow.

• drawing paper
• printer paper
• Pressure Sensitive Conductive Plastic
  • Note: Not all resistive plastics are the same; the industry standard is not as conductive as that sold by Chibitronics, because it contains less carbon. You can use a standard resisitive plastic, such as this one, but you may find that it has too much resistance, resulting in certain colors (blue, pink, & white) not lighting up as intended.

• Circuit Stickers in a variety of colors (because they are flat and have built in resistors)
• copper tape with a conductive adhesive or conductive fabric tape (I used both)

• 2 X 3V CR2032 coin cell batteries (or another 5V power source such as a USB Cord soldered to alligator clips)
• white vellum
• purple vellum
• cardboard the size of your drawing paper
• scissors
• pencil
• black pen
• craft knife
• printer
• spray adhesive
• tape double stick tape
• washi or Scotch tape
• small piece of craft foam
• (optional) cardboard cutting tool
• (optional) art supplies of your choice
• (optional) Japanese screw punch

I liked the idea of figures being revealed as lights turned on inside of darkened windows, so I decided to create a haunted house.

Using a variety of online tutorials for inspiration (my favorite was "How to Draw a Haunted House") I sketched a haunted scene with a pencil and experimented with different design elements.

In my first attempt, I sketched an owl on a tree branch and poked holes in the paper for the LEDs to shine through. I wasn’t happy with the holes in my paper and thought that the effect might be better if an owl (or monster) was merely suggested rather than sketched. Once I had a sketch that I was happy with, I went over it with a black pen.

I cut the windows and doors out of my house using a craft knife.

I placed my drawing onto a piece of cardboard the size of my drawing paper, traced the openings onto it, and cut them out with a craft knife.

Using the cutouts as a guide, I placed a new piece of drawing paper under the cardboard and sketched the locations where I wanted my LEDs to be in pencil.

You'll see a better diffusion of light if your LEDs aren't placed directly in the center of your cut-outs.

Once I knew where I wanted the LEDs to be, I thought about what colors I wanted in each location. I liked the idea of a pair of eyes and a ghost silhouette being the first items to light up and the last to turn off, so I planned to make them red.

Once you figure out what figures will be behind each window, it will be easier to construct the circuit.

There are many ways to construct a circuit. In this instance, I pressed my Circuit Stickers down directly onto the paper where I'd marked locations for the LEDs and then connected my positive and negative traces of copper tape/ conductive fabric tape on top.

Your circuit doesn't need to look anything like mine for the effect to work, as long as your positive and negative traces never touch one another. I used a piece of washi tape as a bridge when needed, to prevent a short circuit.

When planning your circuit, it’s helpful to sketch out a positive (+) and a negative (-) trace that spans the length of your sketch. It’s also helpful to draw an “X” where you want to create a switch, an area where you will be adding a pressure sensor to close your circuit to light your LEDs.

I went about creating my pressure switch by leaving a small gap in my copper tape circuit that I carefully covered with a flap of pressure sensitive conductive plastic adhered on one edge with tape. This “flap sensor” type of switch is the second example featured in the video Chibitronics: Six DIY Pressure Sensor Circuits, in case you're interested. The flap needs to be able to move freely, in case you discover that an adjustment needs to be made.

Since the conductive plastic is sensitive to pressure, pressing it down firmly over the switch gap not only closes the switch, but it also alters the resistance in the circuit. As you add pressure to a conductive plastic flap by pushing it down over the switch gap, the resistance in that part of the circuit will (in most cases) visibly decrease, resulting in a noticeable increase in the brightness of the LEDs! Of note, I found that large gaps were not as effective as narrower ones.

Connect your circuit to power and ground to ensure that your connections work, the LEDs are placed where you want them, and the pressure sensor switch is working correctly. I added pieces of conductive fabric tape to the ends of my positive and negative leads so that I could clip them with alligator clips and power my artwork via USB. You can alternatively create battery holder that connects to your circuit with alligator clips and magnets, if you do a little fiddling;here's a tutorial you might consider.

Once you've connected to power and ground, you should be able to press down on the resistive plastic flap and see the LEDs light up in this order (red, orange, yellow, green, blue, pink, and then white). If you are seeing too much resistance in your blue, pink, and white LEDs, the pressures sensitive plastic that you are using may not be conductive enough. One way to test out this theory is to place a piece of copper tape over the switch gap and see if the lights all turn on at the same time. If they do, then the issue may be that the resistive plastic you are using has too much resistance.

As mentioned earlier, the Pressure Sensitive Conductive Plastic sold by Chibitronics is more conductive than the standard resistive plastic products on the market. When testing my circuit, I discovered that the blue, pink, and white LEDs were encountering too much resistance, resulting in a much dimmer LED effect than I wanted, even with firm pressure. When I swapped out the Velostat that I had been using, substituting it for a square of the Chibitronics version of the conductive plastic, the LEDs brightened up considerably. I did, however, discover that by adding a piece of the less conductive resistive plastic between the copper tape switch and the more conductive Chibitronics flap, that I got a wider range of brightness values when pressing on the switch than using only the more conductive material. (This effect is similar to what happens in the DIY Pressure Sensor example in Chapter 5 of the Circuit Sticker Sketchbook (Template 5).

I found copyright friendly illustrations of skeletons that I then edited, resized, and printed onto white vellum paper. Pixabay is a good place to find images. Getting them to fit the spaces that I’d cut out took a little experimentation. If you try this method, rather than drawing your own, you may want to do a few test prints on printer paper before making a final print on vellum.

To get an idea of how each skeleton would look in the openings I cut out, I held my haunted house sketch up against a glass window, positioning each skeleton illustration in front of them. I then traced an outline slightly larger than each window cutout onto the vellum, leaving a border around each image for narrow strips of double stick tape. It’s important to avoid getting tape on what will be the exposed part of the vellum cutouts, as it may be visible when the images are backlit.

I taped one sheet of purple vellum to the back of my haunted house sketch (leaving the sides and bottom free from tape, so that I could move it out of the way when I needed to. I then cut and adhered the skeleton illustrations (printed on white vellum) to the back side of that. I used a Japanese screw punch to poke holes in the cardboard, so that a silhouette of eyes would peek through when lit up. I also added a silhouette of a ghost to the back side of my sketch (requiring me to make an additional cut-out in the cardboard).

While I'm not an expert at explaining how forward voltage works, I believe that this example will help explain how the rainbow effect works with a bunch of colored LEDs in the same circuit.

I created this chart based on voltage ranges I found online (so they are approximate).

Looking at the table, you will notice that the forward voltage is lowest for red, orange, yellow, and green LEDs, and that it’s highest for blue, pink, and white. The amount of current running through an LED and resistor circuit depends on both the forward voltage for that particular LED color and on the resistor value. Given the same resistor value, circuits using LEDs with a higher forward voltage (blue, pink, and white) will have less total current. Less total current means dimmer LEDs. As a result, when I press down gently on the pressure sensitive conductive plastic, red and orange will show up first. As I press down harder and the resistance goes down, the other colors will show up in order of their forward voltage.

If you take a look at the gif I've included in this section (or the video above) you can see this slow fading rainbow effect in action, starting with red eyes, a red and orange ghost, and yellow windows. Next, you'll notice the green eyes, a blue door, a pink window, and then several white ones. (In areas where light from more than one LED is glowing, you may notice the appearance of other colors).

After creating my circuit, I decided that I wanted to add visual details to my sketch. I made a few changes to the original drawing (keeping the cutouts the same) and painted in some details using gouache paint.

I created a final version of my sketch on watercolor paper, adding details with gouache paint. Once it dried, I adhered the sketch to the cardboard with the cutouts using a spray adhesive and briefly placed it under weights. I also used spray adhesive to mount the paper with my circuit onto another piece of cardboard. I then positioned the artwork layer on top of the circuit layer, adding a couple bits of craft foam to the outer edges to minimize contact between the art layer and the circuit layer. I discovered that placing the foam on top of the conductive plastic switch flap helped guarantee a more solid connection when pressing the cardboard down (you can see this in the video). In order to maintain access to the circuit layer, I created a taped hinge at the top of my piece rather than adhering the art layer permanantly to the circuit layer.

In a related (but different) tutorial that I posted, I created an interactive graveyard, with each tombstone connecting to a different window in the haunted house.

If you are interested in learning more about ways to use pressure sensitive conductive plastic in your circuitry projects, to create an array of different special effects, consider visiting Haunted House Pt. 1: Intro to Pressure Sensors.