Simplest_LED_Flasher

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 conduct. 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 60 milliamps. 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

(upper trace) and the voltage across the 100 Ohm current limiting resistor in blue (lower trace).

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 tempermental, 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

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First posted in September, 2006
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