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Inventors Kit 7: Sunlight indicator

Posted by Matthew Little on

In this activity we are going to investigate how much sunlight the is each day. To do this we are going to built two different types of sunlight indicator. These will give an idea of how bright the sun is and can be used for further experiments!

Solar Dial

We are going to need the solar PV connected to the motor again! So please wire your kit up like this:

When the sun is bright the motor will spin very quickly. This is too quick for us to easily see and count how many times it has spun around.

So we need to reduce the speed of the motor spinning. To do this we need to introduce pulleys. We are going to use the fast spinning motor spindle as a pulley to turn a larger pulley. The larger pulley is made from a jam jar lid which we will add a dial onto. We will use an elastic band to connect the two pulleys.

The reduction in speed between the motor and the dial will be the ratio of the circumference (the distance measure around) of the motor spindle compared to the circumference of the jam jar lid (used as a dial). So if the motor spindle has a circumference of 2 mm and a jam jar lid has a circumference of 100 mm, when the change in speed will be 100 mm / 2 mm = 50 times. So the jam jar lid dial will go 50 times slower than the motor.

Basically you need to know that going from small to big pulleys will decrease the speed, going from big to small pulleys will increase the speed. Connecting between two pulleys of the same size will not change the speed.

We will need to following:

  • Jam jar lid (bigger is better!)
  • Hammer & nail
  • Sticky tape
  • 2 x old (unwanted) CDs
  • Bamboo stick
  • Elastic band (quite a big one - you might need a few to try out)
  • Small cardboard box

First we need to make a hole in the jam jar lid. You MUST ask an adult to do this for you. Use the nail in the centre of the jam jar lid and tap with a hammer over a piece of scrap wood. This will make a small hole in the jam jar lid. We need to hole to be just slightly bigger than the bamboo stick.

Then we need to tape the jam jar lid onto one of the CDs. Wrap tape around the lid and then cut slots down through the 

Stick the jam jar lid to one of the CDs.
Stick the other CD onto the top - I just used wrapped sticky tape to do this.
Cut a piece of the bamboo and put it though the jar lid and CDs.
Make a carboard pointer so you easily see what is happening. Mount this through the carboard box. It needs to spin freely.
Stick the motor onto the side of the box. I used some more carboard to hold the motor in place and then wrapped it in sticky tape.
You will need a thin elastic band. It need to be just the right size - if its too loose then it will not spin but if its too tight then there is too much friction on the motor so will not spin. Try different sizes until you get it right.
The elastic band goes over the spindle of the motor and the jam jar lid. The CDs keep the elastic band in place, so it stays on the jam jar lid.
The finished indicator! Mine needed good sunlight to start spinning, but it went quite quickly when there was good sunlight. You can test this with a battery to make sure it is spinning correctly.

You could turn this in a "citizen science" project. Record how many times the dial has rotated in a set time (maybe 60 seconds). If you measure this every hour you are awake and write this down. You can then plot how the sunlight varied through the day. Was it affected by clouds? How did it compare to the weather forecast in your area?

Solar Indicator

This was a bit fiddly and difficult to make! I wanted to make something that moved up and down to indicate the solar power available (rather than spinning around).

My solution was to have an indicator dial with the solar motor to pull it up (when there is bright sunlight)  and a weight to pull it down (when no sunlight).

In this example I used some cotton which was attached to the motor spindle (using the motor connector and some tape. This pulled up the indicator when there was enough sunlight for the motor to spin.

The battery taped to the indicator was weight to bring the indicator back down.

This was not a huge success for me! It only moved a small distance until the indicator hit the motor spindle. This could be used as a 'trigger' to set something off when sunlight hits the solar panel. Maybe it could be a 'suns up' alarm clock!

Try out different ideas and see how well they work. Try building something but don't worry if it does not work first time: look at what is happening and try to understand why and how to fix it. No inventions ever work first time! Prototyping and trial and error are very important parts of the inventing process!

Here are some links relating to sunlight indicators and sundials:

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