The Joule Thief — Lighting an LED with 1.5 volts

I was bored, but not quite up to the challenge of debugging my existing radio project, or starting a new one. I idly began winding some wire onto a FT-37-43 toroid, and then remembered that I had never constructed a “Joule Thief”, a simple little circuit that allows you to light an LED using just a 1.5 volt cell.

YouTube – The Joule Thief — Lighting an LED with 1.5 volts.

Addendum: I mentioned that legendary hacker Jeri Ellsworth had mentioned this circuit in one of her videos: I dug around and found the video. Her circuit is nearly identical, but adds a few components to implement a simple charging circuit.

I also did some experimentation with LTSpice to figure out why the lithium coin cells I scrounged from our old “throwies” didn’t light the circuit, even though they still measured 1.5v. It appears that these coin cells might have significant series resistance (perhaps as much as 30 ohms) compared to the much lower value for alkaline batteries (a nominal 150 to 300 milliohms, according to one datasheet I found). This appears to keep the transistor from supplying sufficient current to switch. I experimented with placing a capacitor across the coin cell (various values, from 33uF to 220uF electrolytics) and found that this did cause the LEDs to blink, but at a very low rate (with a 220uF, about 1Hz). I’ll try to follow up this post with one showing LTSpice and its simulation.

Ugly is Better

So, over the weekend I assembled the HamCan, and got some pretty wonky performance. I’m going to go through it all again and see if I can figure out why the receive performance is so bizarre, (loose connection? poor adjustment?) but it got me thinking that part of the issue is that typical PCB construction doesn’t really offer much advantage for prototyping. The boards are small. They don’t allow much room for experimentation, substitution and even debugging. My early experiments with so-called “ugly construction” like what I used for the FM transmitter I built last month worked much better. The technique is not optimized for mass production, but for individual experimentation and easy substitution and testing. That seems to be a very good thing.

So, my intention is to start an “Ugly Weekender”: one of Wes Hayward’s classic QRP designs. I’ve got a box set aside where I can start putting out the components I need (I should have most of them already, some of the variable caps might require a bit of substitution or creativity). But rather than just diving in, I think I am going to try to plan out the layout of each of the two/three boards it will cover, and consider modeling portions of the design in LTSpice. I’ll try to cover each step of this construction with videos and additional files that might be of use for someone considering a similar homebrew project. I’m in no hurry to get this done: it’s a project which is designed to maximize my own learning and enjoyment, so be patient (and encouraging, it really does help).

In the mean time, here are a couple of YouTube videos that serve as inspiration for this project.

Addendum: The original Ugly Weekender transmitter appeared in the August 1981 issue of QST in an article by Roger Hayward and Wes Hayward. The receiver appeared in the June 1992 issue in an article by Roger Hayward. Both are available to ARRL members through their excellent online archive of QST backissues.