Joule Thief Dog Light

Joule thief dog light

My wife wanted a dog collar light for our black dog so she could see where he was in the park on those dark winter morning walks. [This is also a great little torch as well.]

You can buy them cheaply but where is the fun in that!

As I am learning electronics I had an empty solder dispenser on my desk and it seemed the ideal small container for the AA battery and LED. Then I learnt that a 1.5V AA battery can't power a LED....or could it?

I found 1up's brilliant Instructable on how to build a Joule thief which solved my problem. Big thanks to 1up and I recommend that you visit his Instructable to learn more in-depth on how to build the circuit. I could only have succeeded with his help.

I am entering this Instructable into the Make A Box competition 2016 as in essence it is all in a little box of sorts and Big or Small competition 2016 as it is definitely small...So if you enjoy this Instructable I would really really appreciate your votes.

Okay you will need-

One empty plastic solder dispenser.

An empty Golden Syrup tin, the one with the cool graphics. [Uncool ones will work as well!]

A soldering iron + solder.

An LED, I used a bright white one though a coloured one could be nice.

An NPN transistor. I used a BC547 that I took out of some old electronics though 1up suggests an 2N3904.

1k resistor.

A 10mm toroid. I have no idea what these are for, I take them out of old electronics when I do electronic dissections and they often have wires that are wrapped around them. Can anyone explain to me their use!

Thin wire two different colours. Red + blue are good I think.

An old small spring.

A Latching switch.

A piece of cord.

Hot glue gun.

I admit I am a complete electronics beginner and I rely heavily on others for the circuitry.

Please check out 1up's instructable as I followed his to build this, though I did make a change in that I had a NPN BC547 transistor in my bits box that I had de-soldered from some old electronics and it worked fine.

As my plastic dispenser is so small in diameter I had to use the smallest torroid that I had, also from an electronics dissection, 10mm in diameter.

Due to its size I could only get 5 windings of the double wire around it, though it works well.

Solder it all up.

I then soldered a little old spring onto the wire that connects to the negative end of the battery, for the battery to snugly push against and make a connection.

Cut a strip of the golden syrup tin for the conductive strip to get past the battery to the latching switch at the other end. I found that wire was too thick to get past the battery and the tin works well. As a bonus the graphics on the tin are so lovely that it adds interest to the light.

Solder the conducting strip to the wire that comes from the joule thief for the positive connection.

Test that your circuit is working!

If using as a dog collar light carefully drill a hole through the end of the dispenser from one side to the other.

Feed your cord through and hot glue the cord and holes to fix and to stop water getting in. This cord can then be tied to dogs collar.

Take a pencil and push the LED joule thief end into the dispenser all the way to the bottom. Mine was quite a tight fit and wont be coming out again!

Fit the latching switch on to the lid of the dispenser. You may need to drill out a larger hole depending on the switch diameter.

Insert a fresh battery, negative to the joule thief.

Make sure one of the latching switch lugs touches the positive end of the battery and bend the other lug slightly to touch the golden syrup conducting strip.....

....and your dog light / torch will light up!

Well My dog loves it! and he shows his appreciation by licking my face....yuck!

This is really a super simple little circuit to make and it works really well. I am a complete beginner with the electronics and as I mention in the video it seems like MAGIC!

How can a battery that can't power a LED actually be made to power a LED?

Well I decided to find out and as always there is someone out there on YouTube who knows.

I found RimStarOrg's little video that explains it well and I would recommend that you take a look if you would like to learn more about the circuit.

My understanding is that it flashes the LED by the torroid induced magnetic field collapsing and triggering a power surge through the transistor, this happens hundreds of times a second and tricks the eye in to believing that the LED is always on. Clever.

Also check out the great video by the fantastic Veritasium on how NPN and PNP transistors work at an atomic level. It has really helped me to understand how these things work within a circuit.

I made this dog collar light as part of my YouTube series Its a Rubbish Challenge where I try to make interesting things from the rubbish that we throw away. If you enjoyed this why not check it out for more rubbish challenges.

And don't forget, if you enjoyed this, please vote for this in the Make A Box competition page and the Big or Small competition page.

Thanks for visiting.