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Rebuilding IC555 - Astable Multivibrator

This Astable multivibrator is used to generate the PWM signal. Basically a rectangular signal of variable duty cycle and frequency.

555 timer is one of the most advanced and most popular IC of it’s time. This IC has a lot of configurations and circuit arrangements. We can design some small sections of a circuit using this IC without using any microcontroller. This IC has some applications in generating square wave up to 500khz with an adjustable duty cycle, generating precise time delays under monostable mode, bistable mode to make a two-latch switch configuration and Schmitt trigger. Other some applications with this IC are single button latch, 50% duty cycle free running oscillator, PPM, PWM, Ramp generator and touch switch. But here we are only discussing the astable operation of this IC.

Astable mode is auto triggered configuration of this IC which is used to generate a specific duty, frequency square wave. But before discussing more about the configurations let’s have a look on internal circuit and the pin diagram. Which will help to clear the 555-operation theory and working later on.

Internal circuit:
This IC has 3 resistor of 5K which together makes a voltage divider section. These 5K resistors divides the input voltage into 3 parts, first is VCC, then 2/3VCC and VCC/3. Two comparators are attached to those 5k resistor terminals AT 2/3VCC and VCC/3. The inverting terminal of comparator one is connected to 2/3VCC and non-inverting to the threshold pin. The non-inverting terminal of comparator two is connected with VCC/3 and inverting terminal to the trigger pin.

Then you will find a SR latch, and the output of comparators is the input of this latch. There is a PNP and NPN transistor configuration also one for the reset of this latch and other one is connected to the Discharge pin of the IC. The special thing about this IC is that it can generate very accurate time delays.

For more package info download the original datasheet from here. Some more features are:

- Output Can Source or Sink 200 mA
- Output and Supply TTL Compatible
- Direct Replacement for SE555/NE555
- Normally On and Normally Off Output
- Temperature Stability Better than 0.005% per °C”

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