astable multivibrator

In an astable multivibrator, neother of the two states is stable. These circuits flip back and forth between two states and form a free-running oscillator or signal source.
These timers are widely used in CMOS clocking applications, tone generators, and in making light chasers and electronic dice.

A simple astable multivibrator(alternately blinking LEDs)

5V
______________________________
     |     |           |     |
LED1 V     |           |     V LED2
     -     |           |     -
     |     |           |     |
     |     |           |     |
    | |R1 | |R2       | |R3 | |R4
    | |   | |         | |   | |
     |     |           |     |
     *-----+---. .-----+-----*
      \    |   `-+-.   |    /          
 T1    \|  |  +  | |+  |  |/  T2
        |--*-||--' `||-*--|             
      |/|    C1     C2    |\|
      /                     \
     |                       |
-----*-----------------------'
GND

legend:
- : wire
+ : crossed wires (not connected)
* : connection

Parts:
T1, T2    : NPN Transistor
LED1, LED2: normal LED
R1, R4    : resistor 330 ohms
R2, R3    : resistor 10K
C1, C2    : electrolytic capacitor 47uF

The T1 and T2 can be just about any NPN-transistor(for example the BC546B). The frequency of the flashing can be changed by using different capacitors(the lower the capacitance the faster the LEDs will flash ) or by changing R2 and R3 . The frequency is dependant on how fast the capasitors are charged; a high capacitance recharges slower, a low faster.
R1 and R2 limits the current through LED1 and LED2 and should not be switched with resistor lower than 250 ohm. Also if these gets to high the LEDs won't light up.