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Joule Thief

flashingforeverlight - 25

Everlasting ‘Flashing!’ Light

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A light that will shine from the moment you are born until you are 18? One single AA battery? Not possible? 

The Everlasting light is powerered using the circuit commonly known as the JouleThief. The Joule Thief came to fruition in 1999 when an article by Z. Kapernik was published in the November edition of ‘Everyday Practical Electronics’. Written for the ‘Ingenuity Unlimited’ section of the magazine, the circuit describes a clever little design that utilises almost all of a battery cell.  The term ‘Joule Thief’ was coined by Clive Mitchell or BigClive as he is known on his very popular YouTube channel. This is a great channel and I highly recommend it if you enjoy electronics.

Whereas a 1.5 volt battery might be considered ‘Used’ when the voltage drops from 1.5v to maybe 0.9v at best, the Joule Thief carries on functioning right down to around 0.3v. Some advanced versions of the circuit can even drive the cell down to 0.1v. 

The following circuit is a modified but simple build of the Joule Thief using Axial coils rather than a traditional wound coil. This makes it very simple for those new to electronics. One of the few difficulties building a Joule Thief is ensuring the coils are wound the correct way. With Axial coils this is not an issue; and so long as the coils are placed side by side, the circuit will work. 

There is a wealth of information online regarding the Joule Thief and how it works, but in essence this device is a ‘Blocking Oscillator’ that switches the transistor on and off many thousands of times per second. The coils work together to create a voltage booster, which when paired with an LED appear to show a solidly lit light. 

Ordinarily the value of the resistor feeding the base of the transistor would dictate how ‘hard’ the transistor was driven. Often a 1K resistor is used and this will provide a very bright light. In this example, a 10 MegaOhm resistor is used alongside a capacitor to extend the battery life exponentially at the expense of brightness. It’s fair to say the light produced will not light the room, but this project is not about brightness; its about endurance!

The circuit draws just under 13 microamps of power. Considering an AA battery may typically store 2 Million microamp hours of power, this would theoretically drive the LED for 153,846 hours, or 17.56 years. Of course this is very misleading. The battery data sheets that provide this information assume certain power draw characteristics that are more typical than such a low current. It is also quite possible that after 18 years the natural degradation of the battery would see more power loss than the LED load itself. Furthermore, unless extremely high quality batteries were used, its quite possibly the battery would simply leak in that timespan. 

But theoretical numbers are what Joule Thieves are all about, and so with that in mind, the project creates a light that will light for almost 18 years. We hope!

This circuit uses simple parts. A single AA battery, and a common NPN transistor. The BC337. LED colours have diferent current draw characteristics, and a green colour has been chosen in this project for its immense efficiency at a low current. The circuit is highly modifiable. Adjustments to the resistor, the inductors, or the capacitor will see different output voltages, light level and longevity. 

Keeping the inductors pressed close together will result in a fast flashing light. Creating a tiny gap will speed the flash rate up so fast it will appear as though the light is constantly on. 

This project is now available to buy in the Shop as a Kit.

All components supplied, including solder and instructions. All you need is a cheap soldering iron, some time and a small space to build. Have fun 🙂

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Daisy’s Dock Lamp

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Daisy’s Dock shone across the bay to West Egg. The slow flashing green beacon switched on and off every few seconds to guide boats in. A sort of domestic LightHouse. 

In this example we use a twin coil suppression coil wired together to form the Joule Thief. This is driven hard by a small resistor to the transistor to give a strong current to its output which is then regulated to 5v. 

The output is fed to a 555 IC timed to roughly 3 seconds on with an almost 50% duty cycle. 

The light is extremely bright and can be seen over 200 meters away. 

The 555 is not known for its efficiency and will drain the single AA battery in under a week. The same circuitry could be better designed using other IC choices but for the sake of simplicity we stuck to the 555 here. 

 

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The light shows a simple example of using a 5v circuit at 1.5v. 

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This project will soon be available to buy as a kit. Please visit the store to purchase.

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The Internet

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As a massive fan of the IT Crowd it made sense to create an identical replica to the box that provides the entire internet to the world. Ideally situation on the top of Big Ben (for best reception) this wireless device is to be handled with absolute care at all times. It’s a perfect talking point when giving a speech as employee of the month.

This project begins with a black box. Simply determine the centre point and drill a hole and fit an led mount. The light is a standard 5mm diffused LED.

Circuitry is a little fiddly. The ideal circuit here is a high resistor, high capacitor joule thief to create a single flash around once per second. However red LEDs do not like this setup due to their low forward voltage. To create a bit more back pressure a single BAT43 diode and small cap across the output is inserted to allow the light to flash in a predictable and reliable way. Without it the light can become erratic. It is also more sensitive to bizarre behaviour as the battery voltage lowers.

The final product is a small flashing light that blinks around once per second. I estimate this should run for many many years without replacing the battery. It’s a circuit that could be used in many applications from toys to alarms.

This product will be available in kit form from the shop soon.

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IKEA Yellow House Nightlight Power Supply

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This item was brought to my attention by an old friend who had one of these for his son. This small light emits a soft yellow glow at night and is controlled manually by a switch on the underside.

The batteries are 2 x AA and these power a single coil boost circuit which when tested without a load create around 12 volts. The small current produced means you can connect an LED directly to this clipping the voltage down to around 4volts.

The main problem with this circuit is power consumption. My friend informed me he was changing the batteries around every 10 days.

Existing configuration. Single coil and solar light type IC

I redesigned the circuit to override this circuit and instead use a single AA dual coil booster.

The device has an inner diffuser cover and an outer ‘house’ cover. This creates a soft glow.

Once apart the switch could be reused to power the device and a simple link wire placed across the battery holder to convert the unit to a single battery.

The new circuitry fitted in the opposite corner of the housing.

The new circuit incorporated a potentiometer to adjust how hard the JTPS was driven. The adjusts the current available on the output and allowed the device to be ‘tuned’ for efficiency.

Old vs new.
Old vs new.

As you may be able to see above the new power supply creates a very slighter lower light. On a simple light meter this equates to around 5% less light.

Positioned side by side both with fresh batteries I compared the run time of both and found them to be almost the same. Albeit this time the device has only one battery. The new circuit actually lasted longer than the old circuit even with a single battery but in the last 3-4 days of the test the light lost about 30% of brightness.

The new power supply in effect doubled the usable run time of the light.

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Light for Will and Dan

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I was chatting to an old friend who has two young boys. Both own a night light from Ikea. The light in question is a small yellow house (SOMMAR product code) that glows using a single yellow LED. This light draws 20ma and drains the battery flat over a period of around 10 days. You can see my efforts to extend the length of battery using a JTPS (Joule Thief Power Supply) in another article. I was able to double the length of the run time of the Ikea light but further improvements were difficult with such a bright diffused light.

I decided to design and build a new night light. The new nightlight would be a small black box with drilled holes to spell out the boys names using 5mm low energy blue LEDs. The bulbs draw extremely low current and with just 1ma across the entirety of the bulbs. The light would run 24 hours a day for up to a year.

I built three of these lights. One would spell Dan, one Will and a final test unit spelling Kev to test the battery power and longevity once the other two were given away.

Paper layout of drilled holes.
Holes drilled
Led Mounts fitted.

The JTPS was a basic dual coil 1M / 10PF Capacitor supply with a Zener 5v regulator across the output. Driven by a low power BC337-40 transistor, this fed the LEDs in parallel from a single AA battery fitted in a battery holder unit.

LED’s soldered in parralel
JTPS on 330uH coils
Heat shrink circuit board and glued insulated LED and battery holder fitted.
Final product with Homemade Button finish!

The final product is the basis for many other projects that require long duration lights. The brightness is sufficient for a nightlight and is actually a little too bright if aimed directly at the user. On a C or D size cell this light could run for several years.

Will and Dan with their new lights!
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LED Love Light!

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The finished LED sign

The LED Love Light consists of a PCB populated with dozens of 3mm red LED bulbs. These are driven by a variable power joule thief power supply regulated at five volts and powered by 1 or 2 AA batteries.

The light will run for several weeks non stop or months when used intermittently.