Multi-LED Joule Thief

I believe it is inevitable that most newcomers to electronics will at one point or another build a Joule Thief circuit. To me it is the hello world of circuits and was one of the first circuits I prototyped on a breadboard when I first started experimenting with electronics. This multi-LED PCB is a callback to those days, something I wanted to do the first time I laid out a hand wound transformer, transistor, and LED on a breadboard.

Joule Thief

The circuit is the basic joule thief found easily on the internet with one small change. Instead of using one LED I have added 19 for increased brightness. The NPN transistor is a generic surface mount transistor I chose based on its price and similarity to the 2N2222. My favorite part of this project was finding the AA battery clips, it was great to be able to find such a good solution for connecting the battery. For the main component, the toroid, I found ferrite beads used on TO-220 package transistors. Using thin gauge magnet wire I figured I would be able to make the transformer easily. I chose an arbitrary winding number of 20 turns. The LEDs are cheap 3mm diffused white through hole LEDs. They are extremely bright for their size and power consumption.

Layout of the PCB is a breeze (almost) with only 5 unique components. My goal with the PCB layout was to use as many LEDs as possible in a PCB outline only slightly bigger then a AA battery.

I placed an initial order for the PCB using OSHpark to determine if my layout would work. I found that the LED footprints on the right of the PCB could not be populated bringing the total number of LEDs down to 17. I had completely forgot about the interference the LED leads would have with the battery clip bottom. I decided to leave them unpopulated as test points for the curious wishing to better understand the circuit. After confirming the PCB I placed the next order of 50 PCBs through a Chinese manufacture.

Assembly of the first three boards from OSHpark was simple and fun, everything went together smoothly. I finished the boards wishing I had more to assemble, that was a mistake. Assembly of the next 50 boards was not so fun. For one thing I made a mistake in my footprint for the transformer and made the annular ring for the through holes too small so that the manufacture covered the copper pad with solder mask. This required me to scrape the solder mask from each of the four holes top and bottom with an Exacto knife. I also greatly overestimated how easy it is to wind the transformer. Winding three transformers is enjoyable and finished quickly. Winding 50 transformers is excruciatingly boring and frustrating at times when the 30AWG magnet wire becomes tangled or breaks half way though. Next time I will find wound transformers, preferably surface mount. I have been looking at common mode chokes for future joule thiefs.

Wanting to know the power consumption of the circuit I hooked up my data logger to one and monitored the voltage level of the battery. I started off with an already dead AA battery and the joule thief continued to produce light for about 4 days with a final ending voltage of around 0.35V. I plan to do future monitoring of these circuits and am developing an experimentation board to monitor current consumption over different transformers. I will be sure to post my experimentation and results in the future.

I made these joule thiefs to give away to friends and family. I wanted to show off this cool circuit to those who are not into electronics and might never see one. I also wanted to demonstrate how long these will last on a AA battery, dead or new. As such I purposely did not include a power switch and as such was not expecting the request that it should have a power switch. So thinking quickly I modeled up in CAD a wedge that rotates on the positive terminal of the AA battery to break the connection with the battery clip. It was simple and with the help of my 3D printer fast and quick. No longer do I get asked why there is no switch, instead everyone is fascinated with the simple 3D printed switch.

Overall I enjoyed this project and learned a lot of things about PCB layout. The three big takeaways are to pay attention to the annular size of through holes, I do not want to scrape solder mask ever again. Also think about the footprints of the components. If I had taken a moment’s pause I would have easily been able to see that those last two LEDs would never work so close to the battery clip. The last lesson learned is about PCB assembly, it is hard and time consuming. I look forward to soldering every time I resce one of my PCB prototypes but that does not mean I will enjoy assembling 50 of the same board. Future projects will focus on surface mount parts to make assembly quicker and will definitely not use any parts I have to hand wind.

2016-01-16 11.00.26

I am selling these (and future projects) on Tindie.


2 thoughts on “Multi-LED Joule Thief

  1. Hi Sam, I wonder if you even got to do more logging? Especially on the output side: do you get the same brightness (total current) as the battery depletes? How much LED power are you getting out? Thanks! Luuk


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