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Mobile phone radar on screen

Lockdown Challenge: Turning a phone into a radar and using tech to get to sleep

Image credit: Neil Downie

Make your phone the friend and the enemy – in this week’s challenges we look at how to turn your phone into a radar, and also how to change your family’s attitudes to turning them off.

We are increasingly turning to our phones for almost everything, but using it as a radar? The iceberg's the limit, apparently, as Neil Downie’s family project this week shows how to make a radar out of a mobile phone.

Then there is the delicate matter of sleep. Every one of us is happier and healthier when we get the right amount of sleep, yet lockdown has reportedly not been good for settled sleep patterns. So why not make getting sleep ‘right’ a family project? Crispin Andrews invites you to take on that challenge.

The IET also has a host of resources, which adults can use to engage children with the world of STEM.

Lockdown Challenge #23 – Dodge those icebergs with a mobile phone radar

There you were, relaxing on the deck of luxury liner Titanic II, and some idiot runs for the lifeboats yelling “Iceberg!”. “Don’t panic!” you shout back. “We have an app for that!” You whip out your mobile phone, borrow a starting pistol. BANG! ... bang! Echo time: 4.6 seconds. “It’s half a mile away!” you calmly announce. A huge white ‘berg drifts into view as the ship turns aside. Deck chair rearranging can resume...

You need a mobile phone with an oscilloscope app. The controls of the screen vary, but you can usually get a nice trace. Phyphox is fine, as is Bolden Sound Oscilloscope above. It shows echoes from 7m (reflector) and 14m (wall). It’s also useful to have a small table.

Go outside and find an open space except for one large straight wall. Get the trace scanning across the screen in 250 or 500 milliseconds. Put the phone on the table and clap your hands. Adjust the amplitude of the trace on the phone to show the handclap beyond the screen edges. You may now see one or two echoes as well as the handclap. Go close, say 3m from the wall, with the phone microphone pointing at the wall, and you should see an echo 20 milliseconds after the handclap. Now move further away from the wall. The echo will be delayed more now. Move further and further from the wall, noting the time of the echo.

You’ll see the echo size diminishing quickly as you move further away. The intensity of light, sound or radio waves decreases as the inverse square of distance, 1/R2, which is pretty fast. But in radar, the waves have to go there (1/R2) and back (1/R2 again), so radar echoes decrease even faster, by an inverse quartic law, 1/R4. So at twice the distance, you get 16x less echo. Actually, echoes from a large wall don’t decrease as quickly, but they still go down a lot.

Try making a retroreflector for sound waves. In 2D, two mirrors at right angles to each other will reflect an incoming beam of light – or sound – right back where it came from, no matter what angle the beam comes in at. The process involves two reflections, one off each mirror. Draw this out on a piece of paper using the law of reflection (angle from the surface in = angle from the surface out) and you will see how it works. Three right-angled triangles joined at the right-angle corners to make a 3D corner do the same thing in 3D with three bounces. The retroreflectors fitted on cars and bikes work like this. Three triangles of thin plywood, with diagonals 80-100cm glued/taped together, have the same effect for sound waves. Raise the reflector from the ground, on a chair maybe, to get the best effect.

Mobile phone radar being built

Image credit: Neil Downie

The echo time is given by dividing the distance to and from the wall by the speed of sound, 770mph or 340ms-1. So at 17m, the echo will be at 2x17/340 = 100 milliseconds. The image of echoes on an oscilloscope was known in the old days as ‘A-scope’. Newsreels from the 1940s have A-scope images of wartime aircraft. Radars can see aircraft at 100km or more, and use GHz radio waves rather than sound waves. Radar displays are now mostly the Plan Position Indicator (PPI) type, which have a rotating radial line painting an image on a screen. This is the image we see today on radars scanning the air for aircraft and the sea for boats. Boats also use sound waves for a kind of radar – a sonar or depth sounder – used for finding fish as well as the seabed.


Find a really big flat wall somewhere – maybe the side of a large building. What is the longest distance you can get an echo? Is the echo better over grass or over tarmac?

Try listening for echoes just using your ears and a stopwatch. Set the stopwatch running continuously, clapping as the display passes a whole number of seconds and noting the echo time. You should be able to hear/judge times down to 100 milliseconds. A building 85m away, giving an echo at 0.5 seconds, should be measurable.

Try making sound pulses in different ways. A toy cap gun? A drum? A hammer? What works best?

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

Lockdown Challenge #24 – How to use technology to help your family sleep

Half of the UK are struggling to go to sleep during lockdown according to recent research, but with people returning to work and school, it’s more important than ever to have a good night's sleep. For most, a late night and a lay-in is no longer an option.

Anxiety, stress, fear, depression, worrying... they can all make it hard for people to sleep. As can overeating, over stimulating your brain through watching too much TV, or spending too much time on the internet.

Poor sleep also contributes to mental and emotional health problems. So, in other words, the more disrupted your sleep is, the harder you will find it to sleep. Medical studies also link lack of sleep to physical health issues like high blood pressure, heart disease, diabetes, weakened immunity and memory loss.

To help out, you need to know when to use tech and when not to.

Stay off phones and computers for at least an hour before bed – studies tell us that the blue light from computer and phone screens sends signals to our brain that we’re still awake.

Keep phones out of the bedroom; waiting on notifications from our phones keeps us in a state of fight, flight, stress and uncertainty. In this state, we cannot produce serotonin, this can only be produced when we feel cool, at ease, relaxed, or at peace. If we don’t produce much serotonin during the day, we will produce less melatonin in the evening, which is what we need to go to sleep. It’s the same with bright lights around the house. Set your phone to connect with the light dimmer in the ceiling lights then, about an hour before bedtime, dim the lights and soften the glow.

Mental health charity Mind suggests devising a pre-sleep relaxation routine that combines meditation, mindfulness, visualisation, breathing exercises and muscle relaxation. There are apps and online videos to help with this. But if you’re playing these late at night, keep the phone well away from you and make sure all notifications are turned off.

Sound can help you sleep. Certain frequencies, pitches, simple sounds and tones can induce a state of relaxation. Your brainwave frequencies fall into step with the frequency of the sounds, bringing the brain and the heartbeat down to the same rhythm as the sound itself. Binaural beats, that’s two distinct tones close in frequency, calm you down. Listen in-stereo, one tone sent to a different ear. High pitches stimulate; low pitches relax. Low, repetitive rhythmical sounds can bring about a lower brainwave frequency, an alpha, or alpha-theta, state. Again, there are plenty of videos online.

Breathing difficulties, often caused by dust, dust mites, dead skin cells and other allergens, can keep you awake at night. Use a vacuum on your bed regularly, keep surfaces free of dust and invest in an air purifier, which picks up allergens from the air when they arise.

Make your own sleep mask. Measure the size of your head across the front from temple to temple, through your eyes, then work out the range of your vision to see how big the mask needs to be – for some people, if you barely cover the eyes they can still see light coming in from under the edges of the mask. Work out how thick the material needs to be; it might be useful to have two or more layers depending on its thickness. Pin (safety pin!) or sew the elastic in place.

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