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Project Notes

#129 RelaxationJouleThief

Test a “joule thief” circuit based on a relaxation oscillator.


This circuit achieves a voltage boost, enough to drive an LED with forward voltage in excess of the power supply. In fact it can continue to run the LED as the battery is depleted. I haven’t tested this yet, but apparently it should work down to a battery voltage of ~0.3V under load

The circuit is from a kit sold by Ears To Our World. Proceeds from the sale of the kit benefit the “Ears To Our World” charity which provides self-powered radios and other technology to rural, impoverished and remote regions of the world.

How it works is best explained in this excellent video by w2aew.

Here’s the essence:

  • When power is applied, the second transistor (Q2) is immediately turned on but Q1 remains off since the capacitor holds the base voltage high
  • This allows current to build in the inductor, establishing a magnetic field
  • As the capacitor charges (primarily through the 100kΩ resistor), Q1 eventually switches on
  • When Q1 switches on, Q2 shuts off. This cuts the current through the inductor, generating a flyback surge which is dumped through the LED
  • As the capacitor discharges, Q2 is eventually turns back on. When this happens, the voltage across the inductor shifts back to ground, forcing the capacity voltage negative, and in turn shutting off Q1.
  • And the cycle repeats…

The circuit I’ve built here uses some different component values, primarily because of what I had “on hand”, but also to tweak what seemed to work best with the specific LED load:

Part Original Value Actual Value Used
L1 4.7mH 1mH
R1 100kΩ 22kΩ
C1 470pF 10nF
LED 5mm white..? 3mm Red, Vf~ 2-2.2V

I’ve also put an LED in circuit directly across the power supply - just to prove the point that the battery cannot power it without help!


Here’s a trace of the circuit with 4 probes at interesting points in the circuit:

  • CH1 - output voltage
  • CH2 - Q1 base
  • CH3 - Q2 base
  • CH4 - input voltage - this is a fairly fresh 1.5V AAA




The Schematic

The Build

Credits and References

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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.