A Veritable Almanac!
The Astable Multivibrator is a basic two stage amplifier with 100% of its output fed back in phase with its input. This results in a free running oscillator whose output signal is only limited by the power supply voltage rails.
The cicuit can be used as a clock pulse generator for logic circuits.
In this instance it is going to be used to form the basis of a "Dummy" car alarm.
The circuit opposite shows the modification needed in order to make the astable function as a simple flashing light utilising an light emitting diode. This has an on-off switch to turn the alarm on.
The modification shown below allows the circuit to turn on and flash the LED when the ignition in the car is turned off.