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
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
charging again, thus starting a new cycle.
image, showing the voltage across the 330 uf capacitor in yellow
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.
power supply was set to 14 volts for this measurement and the LED
was an early device
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
oscillation and shorter flashes.
The 2N2222 NPN transistor seems to work reliably in this circuit.
transistors may be more temperamental, and others might not work at
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.
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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