Santa Claus

Christmas STEM Challenge: Detecting Elves and Santa Claus

Image credit: Dreamstime

This weekend, why not make your very own Santa Claus & Elf detector just in time for the festivities.

If you are a light sleeper, and the reindeer make a clatter on the roof or Santa calls out to Rudolf or Donner and Blitzen, then you’ll be all right. When you hear Santa Claus and co, just open the window and look out. But suppose it’s quiet, or you are dozing – you’ll miss the chance to see the magic of Christmas. That’s why you need your very own Santa Claus & Elf Detector.

Now here is a curious thing. A high-sensitivity multimeter, with its leads connected to nothing, set to AC, will in general display a reading, often 100mV or more. Just the probe wires lying around will pick up a signal. And that signal isn’t constant – move a bit closer to the meter to read it and the reading will change, like as not. What is going on here? Quite often, it is the magnetic field around wires carrying current – wires in the wall, wires to lights in the ceiling, wires in transformers will all do it to some extent. This AC magnetic field will induce a small voltage in nearby wires – like the leads of your multimeter.

It’s sometimes called ‘pickup’ because it is picked up from other electrically powered devices. And sometimes ‘mains hum’, because it is often at mains 50Hz frequency, or a mains harmonic like 100Hz. Pickup or hum varies if the wires move – both receiving or transmitting wires, or if the current in the transmitting wires changes. It’s complicated and unpredictable, so pickup or hum is a nuisance that you want to be rid of. Shielded cables, twisted wire cables, hum filters, and much other ingenuity are devoted to getting rid of it. But, amazingly, we can make it do something for us.

You’ll need to find somewhere where Father Christmas will pass through – like the mantelpiece at the bottom of the chimney – or the door to the room where the Christmas stockings are. Then you need to connect the multimeter probe wires to two pieces of metal which are as large as possible and on either side of the passing place. The two pieces must be isolated from each other – check that the resistance between them is >10MegOhms. If there isn’t anything convenient to clip to, just use a piece of wire to extend the probes. Put the multimeter somewhere its leads won’t be trodden on.

Santa detector tech by the door

Image credit: Neil Downie

Finally, you’ll need electronics. But don’t panic, all the electronics you need are lined up and ready to go inside two handy boxes: a multimeter and a mobile phone. The multimeter should be one that can transmit readings out to the phone on Bluetooth, so that you can monitor the multimeter from a distance and – even better – plot a live graph of the readings. Most phones will work, but you need to load the multimeter software onto the phone.

Set up the multimeter to a high-sensitivity AC scale, and then push the Bluetooth button to connect to your phone. Switch the Bluetooth on the phone to receive it. Then set the phone up to display or plot the readings, and that’s it, you’re done.

Wait a metre or two from the passing area and look at the reading. Now try stepping through the sensitive area and see what you get on the graph. With a bit of luck, you’ll see it go up or down, and more-or-less stay there as long as you stay in the sensitive area, and then resume its resting value when you are away from the doorway. Your body is stopping – or enhancing – the hum in the wires going to the multimeter.

Santa and elf detector up close

Image credit: Neil Downie

If it doesn’t work at first, try, try, try again. Try different places, clipping onto different pieces of metal or wire. And, equally important, test the signal with different things in the room switched on. Lights, TVs, power supplies (which have transformers or similar inside) and most powered things will affect the readout to some extent.

Try switching them on and off until your system works – and then don’t change anything. But wait a minute: maybe your bedroom is too far for the Bluetooth signal to reach your phone. Nil desperandum! You can hook up the detecting wires to a coax or shielded cable and run that upstairs to the multimeter.

If you want to check the system out in more detail you can load the readings to a spreadsheet. Send the file from the phone and convert the text in the columns into numbers if necessary. Then plot time or reading number on the x-axis versus millivolts on the y-axis.

Diagram of Santa and Elf detector

Image credit: Neil Downie

Happy it’s all finally working? Then drink up your Christmas Eve nightcap, sit upright so you don’t nod off, and keep your eyes on that multimeter, ready to look out and see Saint Nick with his sleigh and his elvish pals. Maybe you’ll hear him as he leaves exclaim: “HAPPY CHRISTMAS TO ALL, AND TO ALL A GOOD-NIGHT!”

If you liked this, you will find lots more fun science stuff in Neil Downie’s 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 In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project at

There is a back catalogue of STEM-related challenges from the past two years to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM. Also, check out the IET’s Santa STEM workshop.

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