Here we will explain how 555 Timer works in Astable multivibrator mode with simulation. You will also find equations relating to the timing and frequency of output signal generated in this mode. This is required because when you want to generate periodic signal you want to know pulse duration and frequency of the output signal.
555 Timer can be used in Monstable, Bistable and Astable multivibrator mode. Monostable mode was previously explained in how to simulate 555 timer as monostable multivibrator tutorial. Learning how the 555 timer Astable mode works can be useful in variety of engineering projects. There are many astable multivibrator application such as creating a Voltage controlled Oscillator(VCO), FSK(Frequency Shift Keying) generator, square wave generator etc.
Working Mechanics of 555 Timer in Astable Mode
Astable mode of 555 Timer is one in which the circuit of the timer is not stable in either high or low state but transitions between them periodically. This is why they are called oscillator(relaxation oscillator) or free running non-sinusoidal oscillator. The circuit diagram(from datasheet) for operating in the Astable mode is shown below.
For astable operation the Trigger and Threshold inputs are connected together and this is what causes the self-tiggering and therefore oscillation. Periodic charging of capacitor C via resistors Ra and Rb and discharging via Ra and internal discharging transistor to ground is what causes the oscillation. This is what is explained next in more details. To understand this see also the internal circuit diagram of the timer shown below.
Initially when the timer is turned OFF, there is no voltage across capacitor C and therefore is in discharged state. At this stage, the Trigger(TH) input voltage is less than 1/3Vcc. After the timer is turned ON the capacitor is slowly charged through resistor Ra and Rb to 2/3Vcc. During this charging from 1/3Vcc to 2/3Vcc the output is HIGH. When it reaches 2/3Vcc, the internal Flip Flop turns on the internal discharge transistor(see the internal circuit diagram below) because of the internal comparator 1. When the transistor is ON, the current flows from Vcc through the resistor Ra then through discharge transistor to ground. Thus, the capacitor starts to discharge. This discharging continues until it reaches 1/3Vcc. During this discharging from 2/3Vcc to 1/3Vcc the output is LOW. When it reaches 1/3Vcc, the Trigger input causes to trigger comparator 2 which in turn causes the internal discharge transistor to cutoff. Hence stopping discharge. Therefore the circuit starts again to charge the capacitor C through Ra and Rb. This is then followed by discharging mechanism which has just been explained. These cycles keeps on repeating again and again creating oscillation.
The internal circuit is as shown in the figure below.
Equations:
Charging Time(Tc) is given by,
\(T_c = 0.693(R_A+R_B)C\) ------------(1)
Discharge Time(Td) is given by,
\(T_d = 0.693R_BC\) --------------------(2)
One Cycle Period(T) is thus given by adding (1) and (2),
\(T=T_c+T_d\)
That is,
\(T=0.693(R_A+2R_B)C\) ------------------(3)
Frequency of Oscillation(Fosc) is thus,
\(F_{osc}=1/T\)
or, \(F_{osc}=1/0.693(R_A+2R_B)C\)
That is,
\(F_{osc}=1.44/(R_A+2R_B)C\) ---------------------(4)
Duty Cycle
\(Duty Cycle = \frac{R_A+R_B}{R_A+2R_B}\) ------------------------(5)
Circuit Simulation of 555 timer in Astable Multibrator mode
Following is the circuit schematic drawn in Proteus simulation software. We have in this equation used R1 of 5KOhm and R2 of 3KOhm and a capcitor of 0.1uF. The 0.01uF between supply and ground was used in accordance of 555 timer datasheet. It is used to reduce noise that might get coupled into the circuit.
We can add oscilloscope to view the waveform.
The following oscilloscope display shows the voltage across the capacitor in the upper channel and the signal output from the timer in the lower channel.
As you can see the voltage across the capacitor charges during which the output is high and when it discharges the output is low. This is as expected from the Timer in astable mode of operation.
We can use the above equation outlined and calculate the pulse width for HIGH and LOW signal level.
Video demonstration of Simulation of 555 Timer in Astable multivibrator mode
Hopefully this explanation of working mechanism of 555 Timer in Astable multivibrator mode was useful to you. We will show you in the next tutorial how to use DIY Oscilloscope using Arduino and Matlab Simulink to display the square wave generated here using 555 Timer in astable mode.
Leave your questions or queries if you have any in the comment box below.
