Bbq made of bricks outside in family garden

Lockdown Challenge: Fire up the barbie, but get the time right

Image credit: Joingate/Dreamstime

For this week’s fun, we challenge our engineering families to build a BBQ and come up with an innovative sundial.

We’re nearing the end of our Lockdown Challenges; in fact, this will be the penultimate brace. And as the (English) summer school holidays get underway (the Scottish ones are not far from over) and every day is a sunny day, what better project than to build a BBQ. Also using the reliably relentless summer sun, our experiment from Neil Downie this week puts a modern twist on creating a sundial.

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

Lockdown Challenge #29 – Build a BBQ

It’s summer, the school holidays, and now you can invite round five people from outside your home for some socially distanced fun in the garden. More than six, if they only come from one other household. So a good old-fashioned barbie or seven might just be on the cards over the coming weeks.

Rather than just go down to the local DIY store, or wheel out the George Foreman grill on an extra-long extension lead, why not make the whole thing into a project by getting your engineering offspring to design and build your very own brick BBQ.

Get it right and you can have years of great fun and serious (or not so serious) grilling right there in your garden.

What you’ll need: bricks, brick sets, sand and cement, BBQ grill set

  • Where to put it: close enough to the house to get food to and from the kitchen conveniently, but far enough away so the smoke doesn’t blow indoors.
  • Work out the best height for the grill surface, depending on who’s going to be doing most of the grilling. The griller needs to be able to stand comfortably to reach the grill without reaching up or bending down. You don’t want a relaxing Saturday afternoon ending up with a trip to the physio or osteopath.
  • The BBQ will need a stable base so use the patio or, if you haven’t got one, lay a few slabs first.
  • Place the first layer of bricks on the ground in an E shape using the BBQ grill tray from your set to work out how far apart to put the lines of bricks.
  • Temporarily remove the bricks but leave their imprint. Make mortar from sand and cement and lay it on the ground over the imprint.
  • Lay the first level of bricks making sure the corners are at right angles.
  • Put mortar on top of the first layer and start laying subsequent layers starting at the corner.
  • Stagger the vertical joints alternately to half a brick’s width, so that each corner is vertically straight and each side is even.
  • Build the walls seven bricks high.
  • On the left hand and inner walls, two-thirds of the way up, turn the first three bricks inside to create a ledge for the charcoal tray. Half a brick should stick out.
  • Add three more layers of bricks and then include another side on the ledge to support the grill tray.
  • Smooth down the joins between the bricks for a neater finish.
  • For your work surface, put a paving slab on top of the centre and right walls. Solid enough to support a few bottles of ketchup, some rolls and a big pile of barbecued food.

Alternatively, you could go for the fire pit style BBQ. To do this:

  • Dig a hole 20-25cm deep and fill with 5cm of sand.
  • Put a metal fire ring or metal circular frame into the hole so the ring fits to the wall.
  • Build circular layers of bricks around the ring, starting each layer half a brick along from the previous.
  • Use mortar to connect the layers.
  • Either use a metal mesh to cover the pit or build a second smaller, internal layer of bricks and fit a metal fire bowl inside it to act as the BBQ surface.
  • If you want it to look really flash, build a stone hearth around the pit and a wall around the edge complete with stone seats to sit on while you’re enjoying your burgers, sausages, prawns and vegan patties!

Lockdown Challenge #30 – Sundials on your ceiling and electric sundials

You’ve had a long tiring morning cutting up a woolly mammoth into steaks, and you reckon it might be time to heat up the stones on the fire for coffee. But you are down the bottom end of the home cave, and the sundial is at the entrance. Maybe you need some new sundial tech: the mirror-spot-on-ceiling sundial.

So put a mirror – a small one, or maybe a bigger one with a cross or a circle to mark the middle – horizontally in the window of your cave. With the Sun beaming down, a spot will appear on the ceiling, moving across as the Sun moves across the sky. A few marks on the ceiling – and maybe one or two on walls – and your day can be measured. Numbered pieces of paper stuck on with Blu Tack can mark the hours or half-hours, while dots can mark the quarter hours. The mirror sundial in the pic gave the time to five minutes from 1:30 until 7:30, indicated by the centre of the square ring of light.

Sundial indications on the wall

Image credit: Neil Downie

But maybe your cave is too deep for a mirror sundial to work. Thoroughly modern troglodytes don’t have to rely on the sundial on the ceiling. Nowadays we all have multimeters in our caves, and if we also have two of those £1 garden LED lights that charge up by solar panel, then we can have an electric sundial. No need to crane your neck up at the ceiling. No need to keep those stalactites trimmed. Just run a couple of wires down from the cave entrance and you’ll have Sun-time on your multimeter…

First, take the solar panels off the garden lights. Glue a panel on two adjacent faces of a cube or cuboid piece of wood. Now glue the cube onto a wedge of about 50 degrees (it’s the UK angle of latitude). Take the output wires from the panels and connect two identical resistors from positive to negative on each one. The resistors need to be low enough that the panels can’t get up to their maximum (3V?) voltage. Something from 10 to 200 Ohms will probably do. Maybe salvage them off an old electronic circuit board. Now connect the two negative wires together, connect the two positive leads to the multimeter leads, and set the multimeter to volts or millivolts.

Electric sundial

Image credit: Neil Downie

Your electric sundial is ready to roll! Just find a spot where the Sun will go 90 degrees, from fully on one panel to fully on the other panel. Choose somewhere there is nothing which will cast a shadow over the sundial, and, more subtly, nothing which will reflect the Sun onto the sundial. Facing it due south should give you the time from 9 until 3. Why does this work? It’s all down to a peculiarity of ‘trig’ maths: the value of sin-cosin is nearly linear. The graph shows results from an electric sundial we tried in sunny summer weather.


Over the seasons, the hour-marks of your mirror-spot-on-ceiling sundial will have to shift, although they are good for a few weeks. Extend the marks into a set of radiating lines and you can have Sun time on your ceiling for much more of the year.

Try running your electric sundial against a standard clock. Take readings when the sunlight is bright. How good is your sundial time?

Electric sundial graph

Image credit: Neil Downie

For a really big challenge, put both the solar voltages into a small computer like a Microbit or an Arduino. You can compensate for variations in power from the Sun by, for instance, dividing each solar voltage by the root mean square of the two voltages. And, of course, you can program the computer to time your roast leg-of-mammoth, and tell you when 'The Flintstones' are on Cave TV. Yabba Dabba Doo!

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. For lots of other things, and a free copy of the ‘Exploding Disk Cannons’ book, visit

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