Build a Model Satellite with Arduino

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.

Build Your Circuit

1. Read about the engineering design process and think about constraints and criteria for your satellite before you proceed. See the link in the Bibliography.
2. Assemble your circuit as shown in Figures 3, 4, and 5. You can also access a Tinkercad Circuits version of the simulation here. The circuit uses a part called an H-bridge to control a motor so it can spin in both directions and act as a reaction wheel.
   1. For now, keep both your Arduino and your breadboard flat on the table.
   2. Save space on the top half of your breadboard. You will be taping your motor and 9V battery there later.
   3. Note that your breadboard may have the left/right orientation of the positive (+) and negative (-) buses reversed. Always base your connection on whether it should be positive or negative, not the left/right position.
   4. Put the H-bridge chip in the breadboard so the notch is facing up (toward row 1). The chip has 16 pins, labeled counter-clockwise starting at the top left. Use jumper wires to connect the pins as follows:
      1. Pin 1 to positive bus.
      2. Pin 2 to Arduino pin 4.
      3. Pin 3 to motor positive wire.
      4. Pin 4 to negative bus (also called ground).
      5. Pin 5 to negative bus.
      6. Pin 6 to motor negative wire.
      7. Pin 7 to Arduino pin 2.
      8. Pin 8 to positive bus.
      9. Pin 12 to negative bus.
      10. Pin 13 to negative bus.
      11. Pin 16 to positive bus.
      12. Pins not listed here remain disconnected (they can be used to control a second motor).
   5. Note: this configuration will power the motor from the Arduino's 5V pin (the Arduino will be powered by a 9V battery). Normally, you would need a separate, external battery for a motor, because it will drain the 9V battery very quickly. However, for this project, you want to keep the satellite lightweight, and using the 9V battery is OK for quick demonstrations. See this video about how to power an Arduino project for more details.
   6. Connect a photoresistor and 10kΩ resistor in series to form a light sensor:
      1. Place the resistor's legs in two different rows of the breadboard. Make sure they are not in the same row as any of the H-bridge pins.
      2. Place one leg of the photoresistor in the same row as one of the resistor's pins. Place the other leg in a separate row.
      3. Connect the free end of the resistor to the negative bus.
      4. Connect the free end of the photoresistor to the positive bus.
      5. Connect the middle (the row where the two parts meet) to Arduino pin A2.
   7. Repeat step 2.e on the other side of your breadboard (across the gap in the middle), but connect this sensor to Arduino pin A3. 
   8. Connect power to your breadboard:
      1. Connect the opposite positive buses together.
      2. Connect the opposite negative buses together.
      3. Connect the Arduino's 5V pin to a positive bus.
      4. Connect the Arduino's GND pin to a negative bus.
   9. Optionally, connect your two solar panels. Important: only connect your solar panels directly to the Arduino if their maximum output is 5V or less. Higher voltages can damage your Arduino.
      1. Connect each solar panel's negative wire to a negative bus. 
      2. Connect the solar panels' positive wires to Arduino pins A0 and A1. 

Image Credit: Ben Finio / Science Buddies

Figure 3. Close-up view of the breadboard.

Image Credit: Ben Finio / Science Buddies

Figure 4. Zoomed-out view of the circuit showing the optional solar panels.

Image Credit: Ben Finio / Science Buddies

Figure 5. Schematic view of the circuit.

3. Download the Arduino example code. Read through the commented code so you understand how it works.
4. Upload the code to your Arduino. Open the serial monitor (Tools→Serial monitor) and look at the light sensor values. Make sure your body is not blocking or shading the photoresistors when you do this.
5. The values may fluctuate slightly even if you are not moving. That is OK. Enter the approximate values you see for the light1Baseline and light2Baseline variables in the code, then re-upload the code.
6. The motor should not spin. Now, try either covering one of the photoresistors with your finger or shining a flashlight directly at one of them but not the other. The motor should spin. 
7. Cover the other photoresistor (or shine a flashlight on it). The motor should spin in the other direction.
8. Try adjusting the deadband variable in the code. This will change the range of light sensor reading differences in which the motors will not spin. You do not want this value to be zero—since the sensor readings are slightly noisy, this would cause the motor to constantly spin back and forth when the readings are close. 

Build Your Satellite

So far, you have built a working prototype circuit on a breadboard. Your goal for this project is to build a working model satellite that can demonstrate reaction wheel control around a single axis. This is an engineering design project, so there is no single right or wrong way to do it. Figures 6 and 7 show two possibilities for mounting your satellite. Figure 6 shows the satellite hanging from a string. Figure 7 shows the satellite mounted on a vertical wooden dowel through a clear plastic straw. Both methods allow the satellite to rotate, but each one has advantages and disadvantages. This section of the procedure will show you how to build the satellite shown in the figures, but you do not have to follow this exact design.

Image Credit: Ben Finio / Science Buddies

Figure 6. Model satellite hanging from string. The advantage of this approach is that the satellite can rotate freely with very little friction. The disadvantage is that torsion in the string tries to rotate the satellite back in the opposite direction when it turns.

Image Credit: Ben Finio / Science Buddies

Figure 7. Satellite mounted on a wooden dowel through a straw. The advantage of this approach is that the satellite can rotate freely about the dowel without any restoring torque. The disadvantage is that there is friction between the straw and the wood at the base due to the weight of the satellite. This particular satellite was too heavy to rotate in this configuration, however, it may work without the added weight of the solar panels.

To build a satellite similar to the one shown in this project, follow these steps. Refer to Figures 8 through 11.

1. Temporarily disconnect wires that go from the breadboard to the Arduino.
2. Peel the adhesive backing off the breadboard.
3. If you will be mounting solar panels, attach popsicle sticks to the adhesive backing so they stick out to the sides.
4. Attach a straw vertically in the middle of the breadboard. This is useful for a later step, even if you do not plan to mount the satellite on a wooden dowel.
5. Using double-sided tape and pieces of cardboard as needed, attach the Arduino to the back side of the breadboard (Figure 8).
6. Mount the motor and 9V battery to the empty area on the breadboard using tape or glue (Figure 9).
7. Press a cork onto the shaft of the motor to give it added mass.
8. Bend the photoresistors around the edge of the breadboard so they face "forward" (toward the Arduino side).
9. Reconnect wires between the Arduino and the breadboard, carefully wrapping them from front to back.
10. Attach a small piece of cardboard between the photoresistors (you can glue or tape it to the straw). This helps block light that is hitting one photoresistor from hitting the other one (Figure 10).
11. Drill a small hole in the top edge of the breadboard and screw in a small eye hook (Figure 11).
12. Tie a string to the eye hook.
13. Clamp the other end of the string to a shelf.
14. Your satellite is now ready to test! Make sure it can rotate freely, and then move on to the next section.

Image Credit: Ben Finio / Science Buddies

Figure 8. Front view of the Arduino side of the satellite. Wires plugged into the Arduino wrap around to the breadboard on the back.

Image Credit: Ben Finio / Science Buddies

Figure 9. Back view of the satellite. The motor and 9V battery are attached to the breadboard.

Image Credit: Ben Finio / Science Buddies

Figure 10. Bottom view of the satellite, with a cardboard divider between the photoresistors. 

Image Credit: Ben Finio / Science Buddies

Figure 11. Top view of the satellite with an eye hook screwed into the breadboard.

Test Your Satellite

1. Disconnect the USB cable from your Arduino if you haven't already. The USB cable will prevent the satellite from spinning freely.
2. Make sure the string is completely untwisted and the satellite is not rotating on its own due to torsion in the string.
3. Plug the 9V battery into the Arduino using the snap connector and barrel jack. This will power on the Arduino and cause the program to immediately start running.
4. The lighting conditions may have changed since you now have the satellite hanging vertically compared to when your circuit was sitting flat on the table. Ideally, your motor should not spin by default. Try to make sure no asymmetric shadows or light sources are falling on the photoresistors. You may need to adjust the lighting in the room, for example, by turning a lamp on or off, or by opening or closing window curtains.
5. You may need to recalibrate your baseline light sensor readings while your satellite is vertical. This may require a longer USB cable so you can connect your satellite to your computer while it is hanging. 
6. Once you have calibrated your sensors so the motor does not spin when the satellite is just hanging there, try aiming a flashlight at one of the photoresistors. The motor should spin, causing the entire satellite to rotate in the opposite direction and point towards the light source.
   1. Note: if your satellite rotates the wrong way (spins away from the light source), temporarily disconnect the 9V battery. Switch the motor's positive and negative wire connections to the H-bridge. This will reverse the motor's spin direction and should fix the problem.
7. If you shine your flashlight on the other photoresistor, does the satellite spin in the opposite direction?
8. If you shine the flashlight evenly on both photoresistors at once, does the satellite hold still? 
9. If you have trouble with your satellite's performance, what changes could you make to the physical design or the code to make it work better?
10. How could this behavior help a real satellite aim its solar panels towards the sun?