Astable Multivibrator is one of the many uses of 555 IC. Astable means the output is not stable in any position whether it is high or low. It continuously fluctuates between high & low state so we can call it an Oscillator. Here we demonstrate how to design a rectangular wave oscillator using the most popular 555 timer IC and calculate other components value for a required frequency.
Lets see the pin allocation of 555 IC. It also shows the internal block diagram and sometimes it is very helpful to understand the operation. Image courtesy http://electronics-tutorials.ws.
To operate the 555 Timer as an Astable Multivibrator only 2 resistor and 2 capacitor are required. And these components are connected as below.
Frequency of oscillation is determined by R1, R2 and C1. This circuit is oscillating at about 70.5 kHz and R3 (800Ω) is the load resistance. The formula to calculate frequency is:
That’s all, now choose the parameter as you need and modify for the values of components available in market. Note that, Capacitor C2 is used to eliminate any noise of source voltage to the input of a comparator. Normally a 10nF capacitor is to this terminal. Or you can leave it open.
In this image, it is the oscilloscope view of output waveform (at pin 3) when simulating the above Astable Multivibrator in Proteus (ISIS). To download this Proteus design, if you want, click the Download button.
- This configuration generates Rectangular waveform.
- Maximum output (sourcing/sinking) current can be 200mA.
- Output voltage is about 1.5V less than Source Voltage.
- There is a limit up to which frequency you can use 555 timer IC. At high frequency waveform become distorted.
- It can be used as a clock pulse generator or to drive a speaker in calling bell.