Gravity Assisted Blinking Coin Cell Christmas Tree

The Blinking Coin Cell Christmas Tree runs entirely off the power of a coin cell with a bit of a gravity assist.

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The Blinking Coin Cell Christmas Tree runs entirely off the power of a coin cell with a bit of gravity assist. No Arduinos or 555 timers were harmed in the making of this project. It is fun for kids and can be made in an afternoon from parts commonly at hand or easily obtainable.

The Christmas tree is made from foam board with copper tape used to connect a 2032 coin cell to different LEDs as it rolls down a ramp.   The slope and length of the rails plus the tilt of the tree determine how fast the coin cell rolls and long the LEDs will be lit.  Tilting the tree back allows the coin cell to "stick" to the foam board as it rolls and keeps it from falling off.

CR2032 coin cells start out at approximately 3.2 V and are sufficient to provide the forward voltage required of common LEDs.  The internal resistance of the coin cells is such that current limiting resistors are not required.

The positive side of the coin cell also wraps around the sides and the bottom side will ride along the rails and the coin cell rolls down the ramp.  This then is the positive rail.  The negative side of the coin cells projects and bears against a strip of copper on the tree itself and forms a negative rail.  The negative rail is interrupted from time to time which causes the LED to turn on and off.

And here it is working...

  • 1 × Foam Board Obtainable at office supply and craft sotes. Other materials could be used but may be more difficult to cut. Make sure the material is thicker than the coin cell.
  • 1 × LEDs - various colors 5mm through hole works well
  • 1 × CR2032 Coin Cell 3..2 Volts needed to light the LEDs.
  • 1 × 0.25 inch Copper Tape Any conductive surface could probably be made to work but copper tape is easy to apply and solder to.
  • 1 × Glue Any type suitable for paper. Super glue was used for this project

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  • Some Thoughts

    Frank Milburn12/28/2017 at 02:10 0 comments

    If making this project again there are several things we might try differently:

    • Design a uniform end return that would slow and redirect the coin cell at the end of each ramp.  The end return being used now is a bit of a hack
    • Catch the coin at the bottom and have it do something instead of just roll out (e.g. light and hold the top LED)
    • Look at adding a motorized coin cell return to the top of the tree
    • Consider other features - like nails used similar to a Pachinko machine or a display to count trips

    Credit to Adrian for the idea of a rolling coin cell.  In fact, credit to Adrian for all the real thinking....

  • Mechanical Return of the Coin Cell to the Top

    Frank Milburn12/28/2017 at 01:46 0 comments

    I have a six year old to pick up the coin cell at the bottom of the tree and return it to the top.  If you don't have one yourself, you might consider a mechanical return mechanism.  We considered several approaches but did not pursue them to completion due to said six year old and laziness but are confident you could do it.

    Each approach used a motor removed from a small camera.  The motor was attached to a salvaged gear mechanism from an old inkjet printer.  In each case the coin cell was able to move the cell the required height but the number of repetitions was limited and the mechanisms add complications that require further engineering.

    Approach One - Winch

    A spool was glued to one of the gears and heavy thread used to move the battery up and down.  A completed mechanism might look something like this:

    Approach Two - Pulley

    Similar to above but use a pulley so the motor can be offset with possible geometry advantages.

    Approach Three - Belt

    Approach Four - Mechanical Linkage

    Something like this:

  • Coin Cell Bounce

    Frank Milburn12/28/2017 at 01:27 0 comments

    As expected, there is a lot of bounce as the coin cell rolls down a track.  I was curious to see how much.  Here is an oscilloscope trace from one such experiment.

    Fortunately, the LEDs couldn't care less about this.  I did try putting a rather large cap between the rails which removed the bounce entirely.

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  • 1
    Step One - Determine the best slope for the rails

    Cut a piece of foam approximately 10 mm (3/8 inch) wide and apply copper tape to it.  Attach it to a cardboard box or other scrap surface and experiment to see what slope and angles work with your materials that keep the coin rolling at a moderate speed.  My experiments yielded 10 degrees.

  • 2
    Step Two - Sketch out the tree

    Sketch out a tree with LED placement to start with the following things in mind:

    • The slope of the ramps is determined in the previous step - mine was around 10 degrees
    • The foam used for the rails will extend the width of the tree
    • Negative rails to light the LEDs around 5 to 10 cm (2 to 4 inches) in length seem to work well
    • The drop between one rail to the next must be sufficient for the coin cell to pass
    • Where possible, keep the LEDs near the end of the negative rail (makes soldering faster)
  • 3
    Step Three - Cut out the tree and rails

    The tree can be cut out with a sharp hobbyist knife if made from foam board. 

    From the left over foam board cut out the foam rails.  Rails that are approximately 10 mm (3/8 inch) deep work well.  Attach copper tape to the edge of each rail and wrap it around the edge.

    Optional:  At this point the rails could be attached to the tree with blue tack or other non permanent adhesive to make sure the slopes and angles are as desired.

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