This fun and easy to make desk toy was inspired by the Hypnotoad from the TV series Futurama. The Marvelous LED HynoFrog uses slow color changing LEDs to create a mesmerizing effect. The Marvelous LED HypnoFrog employs a magnetic reed switch to turn it on and off: when the frog is placed on top of a magnet, the eyes will illuminate and start changing colors; when removed it will stop.

Step 1: Parts List

The following parts are needed to make The Marvelous LED HypnoFrog:

Hollow soft rubber toy frog (the one I used was a Toysmith "Frog Squirter" that cost $3 at a local toy store)

Electronics

Two slow color changing LEDs (available from eBay. I got mine from eBay seller "bestshop2008hk")
Normally open magnetic reed switch (I obtained mine from Jameco Electronics. The switches are part number 171872 and cost $0.85 each)
560 ohm 1/2 watt resistor (available from RadioShack or other electronics suppliers)
9 volt battery clip (available from RadioShack or other electronics suppliers)
9 volt battery
Red and black very flexible thin wire (6" each)
Black electrical tape
Heat shrink tubing (optional)

Tools

Soldering iron and solder
Wire cutters / wire strippers
Box cutter or Xacto knife
Awl or other sharp point tool

Step 2: How It Works

The Marvelous LED HypnoFrog consists of a simple circuit of two slow color changing LEDs placed in the eyes and body of a hollow soft rubber toy frog.

The circuit is very simple as shown in the schematic below. Two slow color changing LEDs are wired in parallel with a 560 ohm 1/2 watt resistor between the positive side of the battery and the LEDs. A standard 9 volt battery is used to the power the LEDs.

Because the LEDs don't change color at exactly the same time and because they are wired in parallel and draw current at different rates, the LEDs will get out of sync and you'll see different colors at the same time and some may flicker and vary in brightness. This produces the hypnotic effect.

A normally open magnetic reed switch is used to turn the LEDs on and off. When the magnetic reed switch is in a magnetic field, the switch will close and the LEDs will illuminate and start changing color. When removed from a magnetic field, the switch will open and the LEDs will turn off.

The short video below demonstrates how the magnetic reed switch operators and how you can amaze your friends with The Marvelous LED HypnoFrog. A magnet has been placed under the right hand side of the blue paper surface where the frog sits.

Step 3: Open Up the Frog's Abdomen

The first step is to open the abdomen of the frog. Using a box cutter or Xacto knife, carefully cut into the bottom of the frog to create a large flap. The opening must be large enough for you to get two fingers inside to push the eyes in place and large enough to slide a 9 volt battery into the abdomen.

Step 4: Prepare the Frog's Eyes

Use an awl or other sharp pointy tool to poke holes in the frog's eyes. The safest way to do this is to poke the hole from the inside of the frog as shown in the photos below. The holes should be about 5 millimeters in diameter.

Step 5: Wire the LEDs

For each LED:

Solder one end of a red wire to the long wire on the LED (anode) and solder one end of a black wire to the short wire on the LED (cathode) and cut off the excess LED wires as shown in the first three photos.
Using heat shrink tubing or electrical tape cover and insulate the bare wire and soldering joint as shown in the fourth, fifth, and sixth photo.

Step 6: Insert LEDs Into the Eyes

Insert the LEDs into the eye holes. This requires some dexterity so be patient: it may take you a few tries to get them in position. Cut the wires coming out so there's approximately 2 1/2" sticking out. Strip the ends of the wires.

Step 7: Wire the Circuit

Construct the circuit as follows:

Solder the two black wires from the LEDs and one end of the magnetic reed switch together as shown in the first photo.
Solder the other end of the magnetic reed switch to the black wire coming from the 9 volt battery clip as shown in the first photo.
Solder the two red wires from the LEDs and one end of the resistor together as shown in the second photo.
Solder the other end of the resistor to the red wire coming from the 9 volt battery clip as shown in the second photo.
Wrap electrical tape around the resistor to insulate the soldering joints and prevent a short cicuit when the wires are in the frog's abdomen. See the third photo below.
Put the 9 volt battery clip on the battery and place a piece of electrical tape over the top to prevent shorts as shown in the third photo.

Step 8: Insert the Wiring Into the Frog

Almost done! Insert the wiring into the abdomen of the frog as follows:

Gently coil the wire and insert in the body of frog as shown in the first photo below. Try to place the magnetic reed switch as close to the bottom of the frog. Don't bend the magnetic reed switch - it's glass and will shatter if bent.
Push the battery into the frog's abdomen as shown in the second photo.
Use a piece of black electrical tape to hold the flap on the bottom of the frog closed as shown in the third photo.

That's it! You're now ready to be mesmerized by The Marvelous HypnoFrog!

Step 9: Voila!

You've now finished constructing The Marvelous LED HypnoFrog! Place the frog on top of a magnet or bring a magnet close to the frog and the LEDs will illuminate and start changing color. Don't stare too long or you may become hypnotized and succomb to the frog's will! :-)

The photos below show the frog on a flat surface with a magnet hidden underneath the right hand side.

Enjoy your Marvelous LED HypnoFrog!