NOTE: This instructable covers a harmless prank device, not a weapon or a destructive device. It utilizes toy strip or ring caps used for children's cap guns, found in many toy and grocery stores.
The "exploding pen" is a common item found in novelty and magic stores. These ugly prank devices somewhat resemble a crudely molded pen, and contain a small mousetrap-like device that sets off a toy cap when the pen's cap is removed. As a kid, I was very excited to acquire one, and quickly improved it by housing the initiating device inside a real pen. After pranking a couple of my friends with it, one of my friends was struck on the face by a tiny spark. He was not injured in any way, but this incident caused me to shelve my pen for safety reasons.
The primary design problem with the original "exploding pen" is that the detonation of the toy cap is relatively uncontained, allowing a shower of tiny and usually harmless sparks to fly in all directions. I have included a picture of my old pen firing. The odds are low, but occasionally, these sparks can connect with the target of your prank and cause minor pain or even a freak accident like this one.
My redesign has addressed these safety concerns on two fronts. First, by enclosing the toy cap and initiating device inside the body of a pen, almost all sparks are contained. Second, simple body mechanics make it difficult and unnatural to click a ballpoint pen while pointing it at your face.
I decided to modernize the design by housing it in the ubiquitous Bic Clic Stic, often available for free as a disposable advertising medium. I give you the improved, safe "exploding pen".
Step 1: Tools and materials.
- a "Bic Clic Stic" pen (commonly given out by businesses for free as advertising or easily found online)
- a short section of 1/4" drip irrigation tubing (or improvise with any other cylinder that can fit inside the pen body)
- 1x 5/32" pop rivet
- any brand of lighter containing a piezoelectric igniter (cigarette-type lighter, not a BBQ-style lighter as these usually contain much larger piezoelectric igniters)
- toy caps
Other materials and tools that I found useful:
- super glue
- 1/8" (3.18 mm) drift punch
- razor saw
- mill bastard file or swiss files
- 13/64" (5.16mm) drill bit
- 7/32" (5.56 mm) drill bit
- small hammer
Step 3: Build the cap retainer.
Next, cut a short segment of drip irrigation tubing to the same height as the inner cylinder of the rivet collar. Partially drill out the interior diameter of the drip irrigation tubing with the 13/64" bit (to at least a 3/16" depth). The unenlarged portion of the drip tubing interior should friction fit around the bottom of the inner cylinder of the rivet collar, and the enlarged portion faces the top to accept a cap.
The end result is an assembly that can easily slide in an out of the pen body to facilitate a cap reload.
Step 4: Prepare the piezoelectric igniter.
Point in a safe direction and click it, and a small metal cap lodged in the central channel of the igniter should fly out. Ream out the end of the channel until the captive reset spring inside can slide free.
Carefully file the cut end of the igniter down until it matches the third picture.
File the corners of the igniter down until it can freely slide in and out of the pen body.
Step 5: Build the initiator.
Cut the plunger at the middle indentation as shown in the third picture, and click it back into the plunger shell with a small amount of super glue.
As in the fourth photo, file down the corners of the ingiter plunger until it fights tightly in the pen plunger, then press together with super glue.
Line up the pen clip tube with the igniter and plunger, and mark and cut the tube of the pen clip flush with the igniter body. Compare to an intact pen to ensure that the pen plunger remains at the correct depth. Save the portion that you cut off to use in the final pen assembly.
Drill out the interior diameter of the pen clip tube to 7/32", or until the pen plunger can pass freely through to complete the initiator.
Step 6: Final fit.
Step 7: Load and test.
The design of the cap retainer can be endlessly modified based on the materials you have available - the final picture is of some of the past designs that I have experimented with. My design goal was to create a mechanism that does not significantly change the weight or shift the balance of the pen, and also safely allowing the expanding gasses to vent out of the tip of the pen, thereby containing most of the sparks and avoiding excessive fouling to the striker mechanism.