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Simplest LED Flasher Circuit
This may be the simplest LED flasher circuit you can build, with the notable exclusion of LED's with integrated flashing circuits
This might be a good replacement for the LM3909 in some applications.

In action.

Take a close look. Only the emitter and collector leads of the 2N2222 are connected.
The base lead was cut off.
The LED is from a string of Christmas
lights and it has an integrated 100 Ohm resistor.

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As simple as it gets and still works

This LED flasher occurred to me while reading about negative resistance in transistors. It was reported that Leona Esaki, who was at Sony at the time, had been surprised to see a negative resistance region while investigating production defects in transistors which lead to his thesis work on the Esaki or Tunnel Diode in 1957, which eventually lead to his receiving the Nobel Prize in 1972.

The 330 uf capacitor is partially discharged through
the transistor, LED, and current limiting resistor.

In this implementation, a common NPN transistor is used.  In the circuit, a 1k resistor charged the 330 uf capacitor until the voltage became large enough to get the emitter-base junction to avalanche. In the oscilloscope image,  it can be seen that the peak voltage (yellow trace) was a little bit less than 9 volts. At this point transistor turned on quickly and partially discharged the 330 uf capacitor through the LED and the 100 Ohm current limiting resistor. The current wavform, which is the voltage drop across the 100 Ohm resistor, is shown in the blue trace on the scope image. Peak current was 26 milliamps, and the transistor continued to discharge the capacitor until conduction suddenly ceased at 6 milliamps (Many thanks to Luke in Australia for pointing out the correct current). After the transistor stopped conducting, the capacitor began charging again, thus starting a new cycle.

 Oscilloscope image, showing the voltage across the 330 uf capacitor in yellow
Channel two (blue trace) of the scope is really 20 volts per division; the scope doesn't
know that I used a 10X probe). Thanks to a fellow named Luke in Australia for pointing this out.
The power supply was set to 14 volts for this measurement and the LED was an early device
manufactured by Hewlett Packard, and does not contain an internal current limiting resistor.

If the resistor that charges the capacitor is too low in value (or if the power supply voltage is too high), the current through the transistor will not become low enough for the transistor to turn off. If the resistor that charges the capacitor is too high in value (or the power supply voltage is too low), the  capacitor will not be able to charge to a high enough voltage to enable the transistor to turn on.  This is because the transistor draws as small amount of current before switching on.

I've tried this with red LED's with and without integrated current limiting resistors, and on some while LED's. This circuit can be built without the current limiting resistor, but if you choose to do so, please be aware that the peak currents may be high enough to shorten the life of the LED.

The capacitor value isn't critical. A lower value will result in faster oscillation and shorter flashes.

The 2N2222 NPN transistor seems to work reliably in this circuit. Other transistors may be more temperamental, and others might not work at all.

A fairly straightforward explanation of this phenomenon and a collection of additional llinks is given on the web page referenced below.
A Negative Differential Resistance Oscillator with a Negistor

Enhanced Simplest LED flasher (December, 2009)

While working on something unrelated, I realized that by adding a second transistor to make a latch when combined with the negative-resistance transistor, I could get a brighter, longer flash.

With the 330 Ohm resistor between the base and the emitter of the 2N2907, the LED stays illuminated until the capacitor volage gets so low that the LED current drops below about 200 microamps. This means that the capacitor is discharged much more deeply on each cycle, yielding more lumin-seconds (my apologies to metrologists) per flash can be powered from approximately 12 volts; the voltage has to be high enough to cause the emitter-base junction to avalanche and the charging resistor, shown as 10k in the schematic below must be high enough so that the current through the resistor when the capacitor is fully discharged is not enough to hold the circuit "latched". It might still the the simplest circuit in terms of discreet components. In any case, this is now one of my favorites.

The enhanced version is no the simplest,
but it give really impressive performance.

YouTube demonstrations

In the years since this web page was first published, a number of demonstratons of this circuit have appeared on YouTube. That note that in the fist video, the demonstrator describes inserting the transistor backwards, but actually installs it properly.

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Contents ©2006, 2009, 2010, 2011  Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.or
First posted in September, 2006, updated in May, 2009, December 2009, correction August 2010, updated December, 2010, September 2011.
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