Working of IC 555


The working of IC 555 was very important to know since it was widely used in many applications.The IC 555 was widely used for producing delays,oscillators and much more.The working of IC 555 was based on the internal components as you can see in the diagram above.It consists of Comparator,Flip flop and transistors which make this IC work. Now lets see the working of this IC.


Here in this IC one Comparator on the right side has a threshold pin 6 and a control pin 5.The control voltage was internally wired which equals the control voltage as +2/3 Vcc.Output of this comparator is applied to set S input of the Flip flop.Whenever the threshold voltage exceeds the control voltage, comparator will set the flip flop and its output will be high.A high output from the flip flop saturates the discharge transistor and it leads to discharging of the capacitor connected externally to the pin 7.The complementary signal out of the flip flop goes into the pin 3 as output.At this stage the output obtained at the pin 3 of the IC will be low.The above conditions will exist until the comparator 2 on left top hand triggers the flip flop.Now even if the voltage at the threshold input falls below 2/3 Vcc, that is comparator 1 cannot cause the flip flop to change again.

To change the output of Flip flop to low the voltage at the trigger input must fall below +1/3 Vcc.When this occurs comparator 2 triggers the flip flop forcing its output low.The low output from the flip flop turns the discharge transistor off and forces the power amplifier to output a high signal.These conditions will continue independent of the voltage on the trigger input.

From the above we can understand basic two things.For getting low output from the timer 555 the voltage on the threshold must  reach +2/3 Vcc and it also turn the discharge transistor ON.To force the output from the timer high the voltage on the trigger must drop below +1/3 Vcc. This also turns the discharge transistor OFF.

A voltage may be applied to the control input to change levels at which the switching occurs.When not in use it is advised to connect a capacitor of 0.01nF between pin 5 and ground to prevent the noise coupled onto the pin to prevent false triggering.The pin 4 was used for the reset purpose.Connecting this pin to logic low will place a high on the output of the flip flop.The discharge transistor will go on and the power amplifier will output a low.This condition will continue until the reset is taken high.This is mainly used for synchronization or resetting of the circuits operation.When not in use it should be connected to the +Vcc.

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İsmail Sahillioğlu Recent comment authors
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İsmail Sahillioğlu

So cool! I do thank you so much both for this anf for the other posts. Really nice.