555 Astable Mode

In the 555 astable mode, the IC puts out a continuous stream of rectangular pulses having a specified frequency.

555 Astable Mode

Resistor R1 is connected between V_cc and the discharge pin (pin 7) and another resistor (R₂) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node.

Hence the capacitor is charged through R₁ and R₂, and discharged only through R₂. Pin 7 has a low impedance to ground during the output low intervals of the cycle, therefore capacitor C is discharged.

In astable mode the frequency of the pulse stream depends on the values of R₁, R₂ and C:

Particularly with bipolar 555s low values of R ₁ must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above.

How the Astable Works

The first cycle will take appreciably longer than the calculated time. This is because the capacitor must charge from 0V to 2/3 of V_cc from power-up on the first cycle. However it only needs to charge from 1/3 of V_cc to 2/3 of V_cc on subsequent cycles.

Less than 50% duty cycle

To achieve a duty cycle of less than 50% a small diode (that is fast enough for the application) can be placed in parallel with R₂. The cathode side then goes to the capacitor. This bypasses R₂ during the high part of the cycle so that the high interval depends approximately only on R₁ and C.

The presence of the diode causes a voltage drop that slows charging on the capacitor. The result is that the high time is longer than that expected and that often-cited.

ln(2)*R₁C = 0.693 R₁C. The low time will be the same as without the diode as shown above. With a diode, the high time is:

Where V_diode is when the diode has a current of 1/2 of V_cc/R1. This can be determined from its datasheet or by testing. As an extreme example, when V_cc= 5 and V_diode= 0.7, high time = 1.00 R₁C which is 45% longer than the “expected” 0.693 R1C.

At the other extreme when V_cc= 15 and V_diode= 0.3, the high time = 0.725 R₁C. This is closer to the expected 0.693 R₁C. The equation reduces to the expected 0.693 R1C if V_diode= 0.

Reset mode operation varies between manufacturers

The operation of RESET in this mode is not well defined. Some manufacturers’ parts will hold the output state to what it was when RESET is taken low. Others will send the output either high or low.