What it shows
A single cell below 1 V cannot light a white LED, which needs about 3 V. A joule thief is a tiny blocking oscillator that pumps energy from the cell in short bursts and steps the voltage up. The project builds it and measures the pulse frequency, the output voltage and the extra running time.
The science
The two windings share a ferrite core. Current rising in one winding induces a voltage in the other, which pushes the transistor further on until the core saturates. Then the magnetic field collapses, the transistor turns off, and the stored energy is released as a high-voltage spike that lights the LED. The circuit repeats this at tens to hundreds of kilohertz, so the LED appears steadily lit.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A BC547 or 2N2222 transistor
- A small ferrite toroid core and 30 cm each of two colours of enamelled wire
- A 1 kΩ resistor
- A white LED
- A used AA cell that reads 0.9 to 1.2 V
- A cell holder, a breadboard, an oscilloscope or a phone oscilloscope app with a sound-card probe (optional)
Safety
For this project: An LED glowing at full brightness for long can be dazzling: do not stare at it. Use only a single AA cell. The enamelled wire ends must be scraped clean with sandpaper, which an adult may need to help with.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Twist two enamelled wires together and wind about 12 turns of the pair around the toroid so that you have two separate windings.
Scrape the four ends bare and label them with tape: A start, A end, B start, B end.
Join A end to B start. This junction goes to the positive of the cell, and the LED will connect here too.
Connect B end to the transistor collector, A start to the 1 kΩ resistor, and the other end of the resistor to the transistor base.
Connect the transistor emitter to the cell negative.
Place the LED with its anode to the collector and its cathode to the cell positive. If it does not light, reverse the winding connections of A.
Test with a fresh 1.5 V cell, then with a flat cell at 0.9 V, and record brightness by comparing it in a dark room with a phone lux meter app.
Measure the cell current with a multimeter in series and the average LED current.
If you have a scope, measure the pulse frequency and the voltage peaks across the LED.
Write the report on how the circuit works, tabulate lux and current, and estimate how much longer a cell can run with the circuit.
What you should see
The LED lights from a cell that would not light it directly. The scope shows pulses of 3 to 10 V at a frequency of roughly 50 to 200 kHz, and brightness drops slowly as the cell goes flat.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the LED light from only 1 V?
The collapsing magnetic field in the coil produces a voltage spike higher than the cell voltage.
2Why does the LED look steady when it is flickering?
The frequency is so high that the eye cannot follow it.
3What is the use of the resistor on the base?
It limits the base current so that the transistor is not damaged and the feedback can work.
4Is the energy created from nothing?
No. The circuit extracts the remaining energy from the cell more efficiently, not more than the cell holds.
Take it further
Add a diode and a capacitor at the output to get a smoothed DC voltage and use it to charge a small capacitor.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each