# The 2022 Summer STEM Challenge

Image credit: Neil Downie

Our STEM Challenge series is back for the summer. So why not get your young engineers involved in the engineering fun with a couple more projects you can do with the whole family.

## STEM Challenge #55: the paddle steamer

The famous Tesla electric car company takes its brand name from the wacky but hugely inventive Nikola Tesla. Beginning in Victorian times, Tesla discovered and invented a tonne of stuff like the AC electricity system we use today, radio, radio control and – what we are interested in here – the ability of rotating smooth discs to produce a jet of liquid.

Normally a paddle steamer is propelled by one or two huge wheels with their lower rim in the water. The wheels have blades on them which push the water backwards. But amazingly, you don’t have to put blades on the wheels to make them ‘grip’ the water. You can use a plain smooth disc and rely on the ‘grip’ of viscosity.

As a wheel spins in water, it will pull along a thin layer of water next to the wheel, and that layer will pull along the next further out layer of water, and that layer pulls the next one, and so on. The forces which pull those layers is governed by a characteristic of a liquid called viscosity. Treacle and syrup have high viscosity, while water has low viscosity. But not so low that a smoothwheel steamer won’t work. In fact, I’ve even tested a smoothwheel steamer on a shallow tank of liquid nitrogen. (LIN has an almost unbelievably low viscosity – 20 times less than water. The LIN boiled and steamed vigorously, the wheels casting droplets of steaming LIN into the air as they turned, and the little boat was jostled around. BUT – amazingly – it worked!)

Roughly speaking, the propulsion force you get from a smoothwheel is proportional to viscosity, to the area of the wheel in the water and the speed of the wheel. The disc should probably sit with around 10-20 per cent of its diameter in the water. Too shallow, and there won’t be enough area in the water, while too deep and quite a bit of power will be wasted, carrying water upwards or forwards rather than backwards.

Here’s how to make a smoothwheel ‘Tesla’ paddle steamer you can take down to a lake or pool to check out...

Firstly, you need one or more thin smooth discs: something like a CD or a little smaller, maybe a plastic lid with the rim trimmed off. Find materials to make a boat hull about four times as long as the disc diameter: wood, balsa maybe, or expanded polystyrene. You’ll then need to attach the disc to an electric motor. Then a battery or two in a holder wired to the motor with a switch. With a straight motor, without a reduction gearbox, use lower than normal battery voltage: 1.5V instead of 3V, for example. Maybe the easiest configuration is to attach the motor and disc so that the disc rotates in a slot in the hull. An alternative design is to use two discs, one either side of the hull, like traditional paddle steamers with side paddlewheels. You could use two motors, or a gearbox like a model car, with an axle a little wider that the boat’s hull.

A rudder for directing the boat in a straight line – or a radio-controlled rudder – is an obvious thing to add. Not so obvious is a splash plate. With the disc spinning fast, some of the water will be flung off the disc in the forward direction, which we don’t want. A deflector plate at the front of the disc will stop that happening.

Now what else could you try? Well, we can expect from the propulsion force physics that smoothwheelers will probably go faster with more wheels, or more speed on those wheels. But try it – maybe it’s better than you think – or worse! And can you steer a smoothwheel steamer by having two wheels which you can radio-control separately? Finally, what about a full-size smoothwheeler you can ride around on? A catamaran made from two canoes and electric bike components for powering the smoothwheel in the middle maybe?

## STEM Challenge #56: laser gargoyles

Shine a torch into the dark scary woods and you will occasionally get a fright when a pair or two of red eyes stare back at you. Now you can amaze your friends by waking up some scary ‘animals’ of your own at the bottom of your garden. The gargoyles will stare back with luminous eyes when you rouse them with a little laser pointer or a torch. The trick is done with an unbelievably simple circuit.

We all know about feedback. We’ve all heard the screaming noise coming from the loudspeakers at an event when someone puts their microphone too near the loudspeaker. But feedback is a Really Useful Thing in electronics. A feedback circuit takes its input, amplifies it to the output, and then adds part of that output back to the input. As we see with the loudspeaker and microphone, this can make an oscillation, which, designed well, is useful. But it can also make a 1-bit memory, a ‘latch’ circuit, which we can use for a gargoyle that can be ‘woken up’ by a light from a laser.

Put a clear LED so that it stares at a light-dependent resistor (LDR) and connect them in series to a battery and you have a feedback circuit. Here’s how a LEDLDR (ledd-ledder?) works: some incoming blink of light through the back of the LED causes the LDR to lower its resistance. Current flows, and that makes the LED emit light, which causes the LDR to lower its resistance, making the LED emit more light, which makes the LDR lower its resistance... and so on, until the LDR is maxed out at a few milliamps. It’s a LEDLDR latch!

You could wire all sorts of things in series with the LEDLDR trigger. Standard LDRs will only conduct 50mA or less, but this is ample to light up a little bunch of LEDs.

The LEDLDR unit is simple to make; it’s simply an LED facing an LDR, fixed in place with tape or maybe the two parts glued into the ends of a piece of 5mm tube facing each other. The 5mm diameter LDRs are pretty standard, but LEDs come in many shapes

and sizes, and many will work. For the LEDLDR itself, best to go with ultra- or hyper-bright red, yellow or green to start with, LEDs of more than 1000millicandelas and water-clear encapsulation. You can use further LEDs of any sort for the eyes of your animals.

You’ll need to shade the LEDLDR with a black tube (say, 20mm diameter, 20cm long) so that only your laser will trigger it. Seal the LEDLDR into one end of the tube using black tape, with the clear back of the LED facing down the tube. Look up the open end while holding against the sky, and check for light leaking in. Now hook up the eye LEDs in series and connect to a PP3 9V battery clip via a switch. You can put the black tube near the gargoyle, or maybe use it as the nose.

Now try it out. Plug in the battery. If the gargoyle eyes come on, switch off for a second or two. That should make the eyes dark again, unless there is enough daylight getting down the nose to put them on again. Check that a pulse of light will wake up the LEDLDR. Then switch off again, go to the other end of the garden and wave your laser pointer near the gargoyle’s black tube sensor. With a bit of luck, the Laser Gargoyle will wake up and stare back at you with its scary red, yellow, or green eyes.

Test out the range – it should be very long indeed, 100m or more. The limit is probably how well you aim the laser pointer. You could also try a small LED torch, with a magnifying glass lens to concentrate the light from it.

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