Build a Jumping Robot

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

Remember, the steps in this procedure are a general guide. You can modify them as you decide how to build your own robot. You may also wish to build parts of the robot in a different order. Adult supervision is recommended for steps that involve drilling, cutting, or using hot tools like a soldering iron or hot glue gun.

1. If you are using rechargeable batteries, make sure you charge them so they are ready when you are done building your robot.
2. Start by making a small hole in the center of each bottle cap. The hole should be about the same diameter as the motor shaft, so the bottle caps can be pressed snugly onto each side of the shaft (Figure 4). You can make the holes using a drill bit or a sharp knife. Once the bottle caps are snugly pressed onto the motor shaft, secure them with glue. After the glue dries, you should be able to spin the bottle caps and make the motor shaft spin along with them. They should not spin freely on the shaft.
   
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 4. Bottle caps pressed onto the motor shaft and glued in place.
3. Build a base for your robot. Figure 5 shows a design made from a cut-up popsicle stick, with two "feet" that are glued together using a third piece.
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 5. Example robot base.
4. Cut two pieces of wooden dowel, each roughly 10–12 cm long.
5. Cut two pieces of straw, each roughly 3–4 cm long.
6. Make sure the straw pieces can slide smoothly up and down the dowels without getting stuck. If they get stuck, you may need to use sandpaper to smooth out the dowels (or get smaller dowels or bigger straws).
7. Now for the tricky part. You need to glue a straw piece to one side of the motor, such that when you place a wooden dowel through the straw, it is in contact with the bottle cap, as shown in Figure 6. You will repeat this on the other side of the motor. It is important to make sure the straws and dowels are aligned and parallel to each other, and that the dowels are in firm contact with the bottle caps.
   
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 6. Straws glued to the sides of the motor such that the wooden dowels touch the bottle caps when placed through the straws.
8. After gluing both straws to the motor, you need to glue the dowels to the base of the robot such that the motor can slide up and down the dowels, as shown in Figure 7. It is important to make sure the dowels are well-secured and parallel to each other. You should attach an additional crosspiece (made from a dowel or popsicle stick) at the top to hold them together. You may wish to use additional pieces at the bottom to help secure them to the base and hold them at an angle. What angle, you ask? Why, that is up to you! The dowels should not point straight up, because you want your robot to jump up and move forward when it jumps. Make sure the motor still slides smoothly up and down the dowels before you continue.
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 7. Dowels attached to the robot base, with a crossbar connecting them at the top. The motor slides smoothly up and down the dowels.
9. Loop a rubber band over the top crossbar and connect the ends to opposite sides of the motor. You can do this by, for example, gluing a small piece of toothpick through the hole at one end of the motor to make a hook, then wrapping the rubber band around the other end of the motor (Figure 8). Make sure the rubber band is not too tight or too loose. It should pull the motor up toward the crossbar. When you press down on the motor, the rubber band should stretch without breaking. When you release the motor, the rubber band should pull it back up without getting stuck. If it does not work, try a different rubber band.
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 8. Rubber band attached to the motor.
10. Now you need to cover half of the outer surface of each wheel with a high-friction material, like a strip of rubber band or a bead of hot glue (Figure 9). The strips should be aligned on the two wheels. Wait for your glue to dry, then make sure you can spin the wheels by hand, and that the high-friction surface "grips" the wooden dowels. This part of the robot is very important. If there is not enough friction, the wheels will just slip against the dowels. If there is too much friction (for example, because the high-friction layer is too thick), the wheels can get stuck.
    Image Credit: Ben Finio, Science Buddies / Science Buddies
    Figure 9. Strips of rubber band glued along half of the outer surface of each wheel.
11. You are ready to connect a battery and test your robot! Note: If your motor stalls (gets stuck), it may start to get hot. Disconnect the battery if your motor gets stuck.

    The simplest way to connect the motor and battery is to plug the motor's pins directly into the battery's connector (Figure 10), but you can also build a more permanent circuit with a switch (see step 13). The motor will start spinning as soon as the pins are connected. To turn it off, just disconnect one of the wires. Make sure your motor is spinning in the right direction—it should pull itself down along the dowels, not push itself up. If your motor spins the wrong way, reverse the connections with the battery wires. Ideally, the motor will pull down and stretch the rubber band. Then the smooth plastic part of the bottle caps will come into contact with the dowels, allowing the motor to slip. The rubber band will contract, pulling the motor up and causing the robot to jump. You want to make sure your robot can move with the added weight of the battery, so you can use double-sided tape to attach the battery to the motor, as shown in Figure 11. (We do not recommend using hot glue on lithium batteries.)

    It is OK if your robot does not work on the first try! This is where the engineering design process really starts to come into play. Move on to the next step for more information.

    Image Credit: Ben Finio, Science Buddies / Science Buddies
    Figure 10. Motor wires plugged directly into the battery connector. If your motor spins the wrong way, you can reverse the red and black wires.
    
    
    Image Credit: Ben Finio, Science Buddies / Science Buddies
    Figure 11. Battery taped to the bottom of the motor.
12. If your robot worked on the first try, great! You are ready to try to improve its performance. If not, do not worry. It can be difficult to get the robot to work at first. The design is particularly sensitive to the gap between the bottle caps and wooden dowels and to the thickness of your "grippy" layer. If the gap is too big, the wheels may simply spin against the dowels without enough friction to pull the motor down. If the gap is too small (or the grippy layer is too thick), the wheels may get stuck. There are several things you can adjust without having to take your whole robot apart or start over:
    1. Experiment with different rubber bands.
    2. Try changing the thickness of your grippy layer. You can try adding more material (like another layer of glue or rubber band), or cutting/peeling off some material. You can also pry off the bottle caps and start over with new ones.
    3. You can connect one more battery in series (Figures 12 and 13) to give your motor more power. This may be useful if your motor is getting stuck. We do not recommend connecting more than two batteries in series, as this can exceed the motor's power rating and may cause damage.
    Image Credit: Ben Finio, Science Buddies / Science Buddies
    Figure 12. Circuit with two batteries connected in series. Note that to connect two batteries in series, you connect the red (positive) wire from one battery to the black (negative) wire of the next battery.
    
    
    Image Credit: Ben Finio, Science Buddies / Science Buddies
    Figure 13. Picture of two batteries connected in series (using a jumper wire) and connected to the motor.
13. You may wish to build a more permanent circuit with a switch that lets you easily turn your robot on and off without disconnecting wires. Depending on the parts and equipment you have available, there are different ways to do this. For example, you can use a mini breadboard with a breadboard-compatible switch (see our breadboard tutorial if you do not know how a breadboard works), or you can solder the circuit if you have a soldering iron.
14. Once you have a working robot, you can continue to apply the engineering design process to improve its performance. Can you establish metrics to measure your robot's performance, like how high or how far it goes in a single jump? You can set up obstacles for the robot to jump over, or use a phone to film it from the side in slow motion and set up a ruler to measure the jumps. There are also many different things you can change about your design. Some of them can be changed without building a new robot, like the rubber band or the number or location of the batteries. Other things, like the angle of the wooden dowels, may require building a new robot. Keep going, and maybe one day you can build a world record-breaking jumping robot!