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#437 LED Drivers: QX5252 Joule Thief

Using a QX5252/YX805 LED driver chip as a joule thief for driving an LED from a low-voltage battery.


Here’s a quick demo..



I recently “discovered” solar LED driver ICs that are mainly designed to drive solar-powered lights, such as the QX5252 and YX805. They come in TO-94 and DIP-8 packages, and most applications only require an additional external inductor.

I’ve since used these chips in some circuits like the LEAP#436 BEAM Vibrabot.

The QX5252 and YX805 turn out to be identical in form and function, but since I was more successful in finding resources for the QX5252, I will mainly refer to that product.

The core features include:

  • a switching circuit for voltage boost from 0.9-1.5V to the 2-3V required to drive an LED (similar to a joule thief circuit)
  • charging a rechargeable battery from a solor cell, with integrated Schottky Diode for reverse polarity protection
  • over-discharge protection
  • light control switch, only available in DIP-8 package - requires external photocell to achieve a similar result with TO-94 package

Key specifications:

  • Operating voltage: 0.9V-1.5V
  • Output current: 3mA-300mA

The primary configuration is as follows:

  • rechargeable battery e.g. NiMh is the main power source for the LEDs
  • a fast oscillation (100kHz +) on the LX pin turn the inductor into a boost converter to drive the LEDs at sufficient voltage
  • a solar cell will charge the battery (no fancy charging circuitry)


The Simple “Joule Thief” Configuration

As a test and demonstration, I’m ignoring the solar charging aspect here, and just use the chip as a “joule thief” to drive an LED with a battery that has a lower voltage than the forward voltage of the LED.

To make things a little interesting, I’ll build this as a free-standing circuit that just clips onto the battery.

The battery I’m using here is an AG8 (aka LR1120, 191) rated at 1.55V, and I confirmed it cannot directly drive the LED I’m using - a nice blue 0805 SMD (and blue typically has much higher forward voltage than other colours).

Only 4 parts are required:

Item Description
IC1 QX5252 (or YX805)
L1 inductor
LED1 The LED - blue 0805 SMD
BAT Battery - AG8 1.55V

The only question to answer is the size of the inductor:

  • some versions of the datasheet mention 68µH as a guide. That works.
  • or it can be calculated: LED Power = 2 * VBAT / L * 10^-6 (formula from QX5252 datasheet)
  • or one can experiment..

In truth, a wide range of inductance will work, with the considerations:

  • too low, and it may not generate sufficient voltage to drive the LED
  • too high, and it may fry the LED (if the battery can deliver enough power)

Here are some examples at both ends of the spectrum.

First, with a larger inductor of 100µH, voltage output from the boost converter (at LX, driving the LED) looks like this. This is a good curve, peaking at just over 3V which is ample to drive the LEDs (Vf = 2.7V). The switching frequency is around 130kHz, and duty cycle over Vf is perhaps 20%. This results in a nice strong LED illumination.


Dropping the inductance to 1µH and the result is quite anaemic. Sufficient voltage to drive the LED is only generated for a very short period. So while it does still manage to illuminate the LED, it does so quite weakly.





Carefully put together so that it forms a clip over the battery:




Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.