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.
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.
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 avelanche. 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.
