The 555 timer IC is an integrated circuit (chip) used in a variety of timer,
pulse generation, and oscillator applications. The 555 can be used to provide time delays,
as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package.
The LM555 from Texas Instruments is a highly stable device for generating accurate time delays or oscillation. The circuit may be triggered and reset on falling waveforms, and the output circuit can source or sink up to 200mA or driver TTL circuits.
If the 555 timer circuit is connected as shown in the Figure shown below (pins 2 and 6 connected) it will trigger itself and free run as a multivibrator. The external capacitor charges through R1 + R2 and discharges through R2. Thus the duty cycle may be precisely set by the ratio of these two resistors.
In this mode of operation, the capacitor charges and discharges between 1/3 VCC and 2/3 VCC. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage.