Build Your Own Transit Model

Using a MakeBlock educational, programmable robot (mBot) along with some common household items, create a model that demonstrates the Transit Method  one of the most successful methods used to detect and study planets orbiting other stars. 


MakeBlock Materials

  • Educational mBot kit: mCore, chassis, AA battery holder, 
  • RJ25 cable, motor, ×2 brass studs, bolts and nuts.
  • x2 45 ° plates
  • x2 Thread Drive Beam M4x80
  • LED screen
  • USB cable

Other Materials

  • Laptop
  • Thick card
  • Tape
  • Roll 1mm wire
  • CR2032 battery
  • various single LEDs (blue, white, orange, yellow or red)
  • Screwdriver
  • Wire cutters

In the last 25 years over 3,500 planets have been discovered orbiting stars other than our Sun. We call these ‘exo-planets’. One of the most successful methods for detecting exo-planets is called the Transit Method.

The transit method is very similar to Solar eclipses. From Earth we look at a planet circling a distant star. As the planet moves around its star there is a point where it is between the Earth and the star, blocking out a little bit of the light. By measuring the dip in the brightness of the star and the length of time it takes, astronomers can learn about the planet passing in front of the star.

Members of the Las Cumbres Observatory science team, in partnership with an international group of astronomers, are using LCO to study exoplanets using Transit Method, amongst others. Using Transit Method astronomers can get valuable information about the planet's orbital path, size, atmosphere and surface temperatures.

More information about transit method can be found on LCO’s Spacebook:


Building Instructions

Follow these instructions to build a simple model that will demonstrate transit method. The model allows you to measure the amount of light collected by a light sensor and explore how the number changes when an object moves between the light source and sensor; simulating an exoplanet passing between us and its star.

Using this model you may experiment with different planet sizes, orbital lengths and orbital speeds, to see how these factors affect your measurements.

  1. The mBot core has a light sensor onboard, this will represent one of the Las Cumbres Observatory telescopes collecting light on Earth. The mBot core will need to be positioned at a 90° angle to the chassis (which will sit flat on a table or other surface) using two beams. Secure the mCore to the beams using the two brass studs.
  2. Use bolts and nuts to attach the LED screen to the chassis on the side opposite the mCore. Use the cable to connect the LED screen to the mCore in Port 2Adjust the mBot core's height, if necessary, to ensure the cable is long enough.
  3. Next place the motor onto the chassis, positioning the rotor directly in line with the light sensor. Secure the motor using tape. If necessary, tape the unsecured end of the motor to the chassis to ensure it lies flat. Connect the motor to the mCore in the slot labelled Motor 2.
  4. The separate LEDs will represent stars, choose a variety of stellar colours: blue, white, yellow, orange or red. To power the LEDs you will need a battery, such as a CR2032. Use tape to hold the LED in place on the battery, so it stays alight. 
  5. Using tape and lollipop sticks, build a structure like that seen in the image below (Image XX). Tape the battery to the bottom of the structure.
  6. The LED must sit directly above the rotor (about 2.5cm above), shining into the light sensor.
  7. To minimise the amount of unecessary ambient light entering the light sensor, cut a piece of card 1cmx2cm. Roll and tape the card so it fits snuggly around the light sensor. 

  1. Cut a piece of wire 10.5cm in length and shape it as seen in the picture below. This will be used to represent three different planetary orbits. Insert it into the rotor at point A (see image XX), securing it with blue tac if necessary.
  2. Take the card and cut out 3 circles of varying diameter. The largest should be no more than 6cm diameter. These will represent planets of different diameter choose one and hang it 4cm from B. Change the distance between the "planet" (card) and the LED by sliding it along the wire between B and C, this simulates a change in the planets orbital path and distance from its parent star.
  3. Finally, plug in the battery pack to power your model.

Programming Instructions

To programme the mBot you will need to install the mBlock programme on your computer:

Programming this model is easy, you can either download the pre-made SB2 file (mbot-transit.sb2) which can be opened in the mBlock programme and immediately uploaded to your mBot core using the USB cable, or create your own code in the mBlock programme.

Minimal requirements for your model:

  • The motor must rotate 360° at least twice. 
  • The LED screen must display ambient light measurements from the onboard light sensor.

Optional: add an extra dimension by programming the speaker onboard the mCore to beep each time the ambient light drops.

To upload your code you must connect your model to your computer using the USB cable. Ensure your model is switched and and connect the USB cable in mBlock by clicking Connect > Serial Port and selecting /dev/tty.wchusbserial142.

Next, right-click on your starter block (e.g. the mBot Programme block) and select upload to Arduino 


When you have finished building your model, and the code is uploaded, you should have a simple simulation of an exoplanetary system that can be used to demonstrate transit method. As the motor rotates, your exoplanet orbits. Once during each orbit it will pass between the light sensor and the LED, blocking some of the light. The LED screen displays the amount of light measured by the light sensor onboard the mCore, the value should visibly drop each time the exoplanet passes between the light and the sensor.

Your model allows you to alter the colour of your star, the size of the planet, the length of the planetary orbit and the orbital speed. Explore how these factors affect the amount of light detected?