The simplest led flasher (one transistor)
This circuit consists of negative resistance in transistors.

A common NPN transistor is used.  In the circuit, a 1k resistor charged the 470u uf capacitor until the voltage became large enough to get the emitter-base junction to avalanche. At this point transistor turned on quickly and partially discharged the 470 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, Peak current was 26 milliamps, and the transistor continued to discharge the capacitor until conduction suddenly ceased at 6 milliamps. After the transistor stopped conducting, the capacitor began charging again, thus starting a new cycle.By changing C1 frequency change.

  A number of demonstratons of this circuit have appeared on YouTube.