Introduction: Joule Thief With Ultra Simple Control of Light Output

About: I am a retired analytical chemist living with my wife Cynthia in Cornwall, south west England. I have held the UK radio amateur call sign G3PPT since 1961. I have been interested in computing since the days …

The Joule Thief circuit is an excellent entrée for the novice electronic experimenter and has been reproduced countless times, indeed a Google search yields 245000 hits! By far the most frequently encountered circuit is that shown in Step 1 below which is incredibly simple consisting of four basic components but there is a price to be paid for this simplicity. When powered with a fresh battery of 1.5 Volts light output is high with commensurate power consumption, but with lower battery voltage the light and power consumption drop away until at around half a Volt light output ceases.

The circuit is crying out for some form of control. The author has achieved this in the past using a third winding on the transformer to provide a control voltage, see:

Whatever control is used it should have the basic property whereby turning down the light output also turns down power consumption so that a low light setting results in low battery consumption and longer battery life. The circuit developed in this article achieves this and is much simpler in that the extra winding is not needed and yields a form of control that could be retro-fitted to many existing circuits. At the end of the article we show how to automatically switch off the circuit in daylight when deployed as a night light.

You will need:

Two general purpose NPN transistors. Non critical but I used 2N3904.

One silicon diode. Totally non critical and a rectifier diode or signal diode will be fine.

A ferrite toroid. See later in the text for more information.

One 0.1 uF capacitor. I used a 35V Tantalum component but you could use a 1 uF ordinary electrolytic. Keep the voltage rating up--35 or 50 Volts rating is not excessive as during development, and before your control loop is closed, high voltage can be applied to this component.

One 100uF electrolytic capacitor. 12 Volt working is fine here.

One 10 K Ohm resistor.

One 100 K Ohm resistor

One 220 K Ohm potentiometer. Non critical and anything in the range 100 K to 470 K should work.

PVC single cored hook up wire which I obtain by stripping down telephone cable

To demonstrate the circuit in the early stages I used a Model AD-12 Solderless Breadboard which I obtained from Maplin.

To produce a permanent version of the circuit you will to be equipped for elementary electronic construction including soldering. The circuit can then be constructed on Veroboard or similar material and another method of construction using blank printed circuit board is also shown.

Step 1: Our Basic Joule Thief Circuit

Shown above is the circuit diagram and a breadboard layout of a working circuit.

The transformer here consists of 2 lots of 15 turns of single core PVC wire salvaged from a length of telephone cable twisted together and wound on a ferrite toroid--not critical but I used a Ferroxcube item by RS Components 174-1263 size 14.6 X 8.2 X 5.5 mm. There is enormous latitude in the choice of this component and I measured identical performance with a Maplin component four times the size. There is a tendency for constructors to use very small ferrite beads but this is as small as I would like to go--with very small items the oscillator frequency will get higher and there may be capacitive losses in the final circuit.

The transistor used is the 2N3904 general purpose NPN but almost any NPN transistor will run. The base resistor is 10K where you might more frequently see 1K used but this may help when we come to apply control to the circuit later.

C1 is a decoupling capacitor to smooth out switching transients generated by circuit operation and thus keep the power supply rail clean, it is good electronic housekeeping but this component is often left out which can result in unpredictability and erratic circuit performance.

Step 2: Performance of the Basic Circuit

Some knowledge of the performance of the basic circuit may be instructive. To this end the circuit was powered with various supply voltages and the respective current consumption measured. The results are shown in the picture above.

The LED starts to emit light with a supply voltage of 0.435 and consumes 0.82 mA current. At 1.5 Volt, (the value for a new battery,) the LED is very bright but the current is above 12 mA. This illustrates the need for control; we need to be able to set the light output to a reasonable level and thus greatly prolong battery life.

Step 3: Adding Control

The circuit diagram of the extra controlling circuitry is shown the first picture above.

A second 2N3904 (Q2) transistor has been added with the collector connected to the oscillator transistor base, (Q1.) When turned off this second transistor has no effect on oscillator function but when turned on it shunts the base of the oscillator transistor to earth thus reducing oscillator output. A silicon diode connected to the oscillator transistor collector provides a rectified voltage to charge up C2, a 0.1 uF capacitor. Across C2 there is a 220kOhm potentiometer (VR1,) and the wiper is connected back to the control transistor base (Q2,) via a 100 kOhm resistor completing the loop. The setting of the potentiometer now controls the light output and in this case the current consumption. With the potentiometer set to minimum the current consumption is 110 micro Amps, when set for the LED just starting to light up it is still 110 micro Amps and at full LED brightness the consumption is 8.2 mA--we have control. The circuit is being powered in this example with a single Ni/Mh cell at 1.24 Volts.

The extra components are non critical. At 220 kOhm for the potentiometer and 100 kOhm for Q2 base resistor the control circuit functions well but places very little load on the oscillator. At 0.1 uF C2 provides a smooth rectified signal without adding a large time constant and the circuit responds rapidly to changes to VR1. I used a tantalum electrolytic here but a ceramic or polyester component would work just as well. If you make this component too high in capacitance then response to changes in the potentiometer will be sluggish.

The last three pictures above are oscilloscope screen grabs from the circuit whilst operational and show the voltage on the collector of the oscillator transistor. The first shows the pattern at minimum LED brightness and the circuit is operating with small bursts of energy widely spaced. The second picture shows the pattern with increased LED output and the bursts of energy are now more frequent. The last is at full output and the circuit has gone into steady oscillation.

Such a simple method of control is not completely without issues; there is a DC path from the positive supply rail through the transformer winding to the transistor collector and through D1. This means that C2 charges up to the level of the supply rail minus the forward voltage drop of the diode and then the voltage produced by Joule Thief action is added to this. This is not of significance during normal Joule Thief operation with a single cell of 1.5 Volt or less but if you do try to run the circuit at higher voltages beyond about 2 Volts then the LED output cannot be controlled down to zero. This is not an issue with the vast majority of Joule Thief applications normally seen but such is the potential for further developments that it could become significant and then resort may have to be made to the derivation of the control voltage from a third winding on the transformer which provides total isolation.

Step 4: Application of the Circuit 1

With effective control the Joule Thief can be much more widely applied and real applications such as torches and night lights with controlled light output are possible. Additionally with low light settings and commensurate low power consumption then extremely economic applications are possible.

The pictures above shows all of the ideas in this article so far brought together on a small prototype board and with the output set to low and high respectively with an on board pre-set potentiometer. The copper windings on the toroid are of the more usual enamelled copper wire.

It has to be said that this form of construction is fiddly and the method used in the next step is far easier.

Step 5: Application of the Circuit--2

Shown in the composite picture above is another realisation of the circuit this time built on a piece of single sided printed circuit board copper side up with small pads of single side printed circuit board stuck on with MS polymer glue. This form of construction is very easy and intuitive as you can lay the circuit out to replicate the circuit diagram. The pads make a robust anchorage for the components and connections to ground are made by soldering on to the copper substrate below.

The picture shows the LED fully illuminated on the left and barely illuminated on the right this being achieved with simple adjustment of the on board trimmer potentiometer.

Step 6: Application of the Circuit--3

The circuit diagram in the first picture above shows a 470k Ohm resistor in series with a 2 Volt solar cell and connected into the Joule Thief control circuit effectively in parallel with the on board trimmer potentiometer. The second picture shows the 2 Volt solar cell (salvaged from a defunct garden solar light,) wired in to the assembly shown in the previous step. The cell is in daylight and hence providing a voltage that turns the circuit off and the LED is extinguished. The circuit current was measured at 110 micro Amps. The third picture shows a cap placed over the solar cell thus simulating darkness and the LED is now illuminated and the circuit current measured at 9.6 mA. The on/off transition is not sharp and the light comes on gradually at dusk. Note that the solar cell is being used merely as a cheap control component to a battery circuit does not itself supply any power.

The circuit at this stage is potentially very useful. With a solar cell mounted discreetly in a window or on a window sill charging a super capacitor or nickel metal hydride rechargeable cell, a highly effective permanent night light becomes a possible future project. When used with an AA cell the ability to turn down the light output and then turn off the light during daylight means that the circuit will operate for a long period before the battery voltage falls to around 0.6 Volt. What a superb bespoke present for grandparents to present to grandchildren! Other ideas include an illuminated doll's house or a night light for the bathroom to allow standards of hygiene to be maintained without loss of night vision--the possibilities are enormous.