Revitalising a Sun Jar and the Joule Thief Revisited

A joule thief

If you know a little about LEDs, you may be wondering how a 1.3V rechargeable AA battery can switch on LEDs that require a forward voltage of around 3V. Well, the QX5252F solar light component works with the inductor to create a joule thief (formally called a switched-mode power supply). Cleverly, the QX5252F senses the voltage on the solar cell to decide whether it is dark or not; that is, when the voltage across the solar cell goes low (because it is dark), the QX5252F switches on the LED. So removing the solar cell completely gives a circuit that will always be on.

If the idea of a joule thief is new to you, I have written a comic describing how to make one that includes a short description of how the circuit works, and Paul Gardiner has an article describing how to use one as the basis for a KS3 night light project.

What this means is that another use for the QX5252F could be to create a simple joule thief circuit, for example to give a ‘dead’ battery an extension of life by effectively raising the voltage it is able to provide, or to power a simple LED torch (which needs up to 3V to turn on the LED) from a single 1.5V battery.

A joule thief created with the QX5252F is no better or worse than any other one, but the QX5252F is a cheap component and coupled with a cheap radial leaded inductor (about 70p- and no need to wind your own!) makes for a very neat, low cost and easy to build circuit.

Fig 12 a simple joule thief circuit

Figure 12 shows a joule thief circuit based around the QX5252F on a breadboard. This is simply the circuit shown in Figures 3 & 4 with the solar cell removed.

A little bit of experimentation showed that with the 220μH inductor the AA cell voltage can drop to about 0.87V before the LED becomes so dim as to count as ‘off’. I should note that from a battery starting voltage of around 1.1V it took around two weeks for the light to drop to this level. Adding a second inductor in series with the first produces a higher inductance (the total inductance of inductors in series is the sum of the individual inductor values – just like the case with resistors). This allows the battery voltage to drop even lower. I added a 470μH educator to make a total of 690μH and immediately got a bright glow again from the LED.

Download a ZIP of the Circuit Wizard file for the Sun Jar and also the resulting Gerber files.