This is a classic 2 transistor astable multivibrator. Many other NPN small signal or switching transistors can be used, including 2N4401, PN2222 or 2N2222 using the circuit on the left. The circuit can also be inverted using PNP transistors such as 2N3906, 2N4403, PN2907, or 2N2907 as shown to the right.
The 470 ohm resistors determine the LED brightness. Lower resistance means higher current, and more light. LEDs that require more current or have a higher operating voltage (such as green and yellow) may work better with 300 ohms.
The RC time constant of the 39K ohms resistor and the 10uF capacitor determines the on time for each side. (The two sides do not need to match - vary the RC time constant for one side to get a lower or a higher duty cycle). With the values shown, the flash rate is about 1 cycle per second at 50% duty cycle.
from http://www.reprise.com
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