Lockdown Challenge: ‘MegaWatt Sunbeam Balloon Exploder’ and Raspberry Pi gadgets

Even in places like Britain, on the same latitude as the frozen wastes of Canada, our friendly local star is powerful on a sunny day. At 500W per square metre, the sunlight falling on a 9cm magnifying glass is 3W. Doesn’t sound much? Ah, but what counts is the intensity of light, the power per square metre. If you can concentrate that 3 watts into a 2mm spot, then you have not 500Wm², but 100Wcm². Doesn’t sound much? That’s a megawatt per square metre!

You will need a mirror (e.g. a 20cm or so bathroom mirror), a magnifying lens, balloons and wood. First, check out the focal length of your lens. Focussing a distant light source onto a wall or something is the easiest way. The Sun is 93 million miles away, so that works, but a light bulb or a window 5m away will do and won’t burn the wall. Now find a wood base 5 or 10cm wide and a few centimetres longer than the focal length. Attach a couple of pieces of wood, or shelf brackets or something, to hold the balloon at one end of the base, and drill a hole to take the handle of the magnifying glass. Be a little generous on the focal length, because the balloon surface will stick forward of the brackets and because half of sunlight is infrared with a slightly longer focus.

Inflate a dark-coloured balloon and hold it with tape or elastic bands to the brackets. Put your Sunbeam Balloon Exploder outside somewhere, pointing at a sunny spot 5 or 10m away. Now take your mirror to that sunny spot and bounce a sunbeam toward the balloon. With luck, the balloon will explode moments after the sunbeam from your mirror alights on the lens.

Something that will help you with your aim is that the Sunbeam Exploder will retroreflect – shine a beam back to you – if the mirror is fairly near your eyes.

If you run out of dark balloons, you will need, like Long John Silver (Arrr, Jim Lad!), a black spot. Take a wide felt-tip pen and ink in a large circle where the focused spot will fall. This will increase the light absorption and get more like the full power of the sunbeam burning the rubber.

If you have another lens or two, try making more Sunbeam Exploders and put them in a row. Sweep the sunbeam across them and you can explode them all in rapid sequence.

Why can’t you focus the Sun to a point with a lens? The lens is a bit dodgy, maybe? Nope. The wavelength of light is too big? Nope. Its because the focal spot is not a point, it’s actually a tiny image of the Sun. The Sun is 0.5 degrees across, which means that at, say, 22cm, it will be 2mm. This means that if you use a smaller lens, but one which has a proportionally smaller focal length (technically, the same ‘f-number’), then you get the same light intensity as a big lens, which brings us to our first lockdown challenge: how small can you go with the magnifying lens? 6cm, 3cm?

• How far away can you go and still explode a balloon? 20m? 30m? More?
• How quickly can you explode a balloon? One second, half a second, less?
• How many balloons can you explode in 10 seconds? Two, three, five?
• Can you explode a double-balloon (one inside the other)? Bigger bang, if you can!
• Where is the thinnest (easiest to burn) part of the balloon?
• Can you explode a balloon inside the house rather than outside? What about exploding a balloon inside the house from outside? Don’t forget, you can relay a sunbeam via two mirrors – or more – if you need to.

If you liked this, you will find lots more fun science stuff in my books like The Ultimate Book of Saturday Science (Princeton University). For lots of other things (and a free copy of the Exploding Disk Cannons book), go to www.saturdayscience.org.

Lockdown Challenge #17 – Get your fingers in the Raspberry Pi

If you’ve got a Raspberry Pi - or something similar like an Orange Pi Prime, a Banana Pi Prime or an Odroid XU4 - but you’ve always been too busy to set it up to do all that really cool stuff around the home that everyone was saying it could do when you bought it, well, now’s the time.

It could even help solve some of those real lockdown challenges, like who makes the tea, feeds the cat and keeps the beer cool. As always, it’s a great way to teach children computer programming skills and how those skills can be of direct, everyday use in their own lives.

Automated tea maker

No more squabbling about whose turn it is to make the tea. Tell the Raspberry Pi for how long and at what temperature you want your tea brewed and the Pi will activate the connected kettle, measure the temperature and lower the tea in with a servo motor.

Feeding your cat

Cats always seem to suddenly get hungry at the most inconvenient of times – usually at five in the morning, or when you’re on the phone and definitely whenever you’re in the middle of cooking your own dinner. If you’ve got more than one cat, it’s even worse, as it's usually a sprint to see which cat can get into prime pestering position, which more often than not means twice as much commotion.

Until now, that is. Lots of clever cat lovers, it seems, have been using their Raspberry Pi’s to keep their pets full and contented.

Some cat feeders use emails to trigger the feeder, on-demand, or an IFTTT applet to set up an automated schedule.

You’ll need a servo motor to spin the food dispenser, a power adapter, some copper piping and some everyday household tools.

Smart beer fridge

Now, just for a change, how about something for the adults?

If you've got a mini-fridge that you're using to store your cold beer and wine, a Raspberry Pi could help you keep tabs on how many bottles are left in it, how many you’ve drunk, and whether or not anyone who isn’t supposed to has been sneaking in there to help themselves when no one is looking.

You can get the information sent to your computer, laptop, or phone and get notifications when the fridge is getting low. You can even get the Raspberry Pi to monitor the temperature inside the fridge.

Solar-powered garden camera

You might have noticed more wildlife about since the lockdown started. We’ve had ducks, pheasants, partridges and jays in our garden, to go along with the more regular visitors, such as robins, magpies, pigeons and squirrels. That’s just in the daytime. The other day, I caught a glimpse of an owl at dusk. Many wild animals are nocturnal and there’s no telling what might turn up in the garden at night. Unless of course, you attach a camera and a solar battery to your Raspberry Pi; leave it outside during the day to gather power, and then set it up in a secluded spot - preferably quite high - where it has a good view of the garden but the animals themselves can’t see it. Add motion-detection software to get automatic shots. Even in built-up areas, you’ll be surprised by what you see.