Figure 10 shows the 555 connected as an astable multivibrator. Both the trigger and threshold inputs (pins 2 and 6) to the two comparators are connected together and to the external capacitor. The capacitor charges toward the supply voltage through the two resistors, R1 and R2. The discharge pin (7) connected to the internal transistor is connected to the junction of those two resistors.
When power is first applied to the circuit, the capacitor will be uncharged, therefore, both the trigger and threshold inputs will be near zero volts (see Figure 11). The lower comparator sets the control flip-flop causing the output to switch high. That also turns off transistor T1. That allows the capacitor to begin charging through R1 and R2. As soon as the charge on the capacitor reaches 2/3 of the supply voltage, the upper comparator will trigger causing the flip-flop to reset. That causes the output to switch low. Transistor T1 also conducts. The effect of T1 conducting causes resistor R2 to be connected across the external capacitor. Resistor R2 is effectively connected to ground through internal transistor T1. The result of that is that the capacitor now begins to discharge through R2.
As soon as the voltage across the capacitor reaches 1/3 of the supply voltage, the lower comparator is triggered. That again causes the control flip-flop to set and the output to go high. Transistor T1 cuts off and again the capacitor begins to charge. That cycle continues to repeat with the capacitor alternately charging and discharging, as the comparators cause the flip-flop to be repeatedly set and reset. The resulting output is a continuous stream of rectangular pulses.
The frequency of operation of the astable circuit is dependent upon the values of R1, R2, and C. The frequency can be calculated with the formula:
f = 1/(.693 x C x (R1 + 2 x R2)
The Frequency f is in Hz, R1 and R2 are in ohms, and C is in farads.
The time duration between pulses is known as the 'period', and usually designated with a 't'. The pulse is on for t1 seconds, then off for t2 seconds. The total period (t) is t1 + t2 (see fig. 11).
That time interval is related to the frequency by the familiar relationship:
f = 1/t or t = 1/f
The time intervals for the "on" and "off" portions of the ouput depend upon the values of R1 and R2. The ratio of the time duration when the ouput pulse is high to the total period is known as the duty-cycle.
The duty-cycle can be calculated with the formula:
D = t1/t = (R1 + R2) / (R1 + 2R2)
You can calculate t1 and t2 times with the formulas below:
t1 = .693(R1+R2)C
t2 = .693 x R2 x C
The 555, when connected as shown in figure 10, can produce duty-cycles in the range of approximately 55 to 95%. A duty-cycle of 80% means that the ouput pulse is "on" or high for 80% of the total period. The duty-cycle can be adjusted by varying the values of R1 and R2.
