How timer IC 555 works?

In this topic we will learn about very basic and multipurpose timer IC 555 and how it can be used for implementing different timer operations. This is a cheap and easy to handle chip which most of the hobbyist use during their initial phase of learning electronics. Lot of different projects can be made using this IC just by connecting few components around it. It comes in 8-pin DIP (Dual in Line Pin) plastic package.  It is mostly used in timer applications hence commonly known as 555 timer IC.

Let us look at the internal block diagram of this timer IC. It comprises of a voltage divider circuit built around three 5KΩ resistors. It is then followed by two operational amplifiers working as voltage-comparator. The output of operational amplifiers are fed to SR Flip-Flop and the output is fed out through an inverting circuit and a discharge transistor is also connected to flip flop output to discharge the timing capacitor connected to IC pin for timer applications. As we have already learned about all these basic building blocks in previous topics, hence it should be easy to understand the functionality of this IC at this stage. This is the reason I have arranged the topics in this sequence.

Operation:

Positive supply voltage is provided to Pin-8 and negative (Gnd) to Pin-1. Recommended voltage as per data sheet of LM555 is minimum 4.5V to a maximum of 16V. Let us assume we connect it to 12V DC supply.  Looking at the voltage divider circuit, whatever supply we give between Pin-8(Vcc) and Pin-1(Gnd) is divided in three equal parts as all the resistance are equal i.e. 5KΩ. Pin-4 is connected to Vcc as it is active low reset pin for SR Flip-Flop, hence if left open or low, it will always keep the flip-flop in reset state and hence no change will happen to its output pins.

• Hence at the junction of middle and bottom resistance the voltage is 1/3^rd of supplied voltage (Vcc). This is fed to inverting input of bottom op-amp hence work as reference voltage for it. This comes out to be 4V (for 12V supply).
• And similarly the voltage at junction of top and middle resistor the voltage comes out to be 2/3^rd of supplied voltage (Vcc). This is fed to non-inverting input of top op-amp hence act as reference voltage for it. This comes out to be 8V (for 12V supply). This junction is also given out as Pin-5 (control voltage) which can be used to apply external voltage to control the width of pulse generated by this timer. This we will see going further.
• If any voltage lower than 1/3Vcc is applied on Pin-2 (Trigger), output of lower op-amp is ZERO (Low) which is connected to ‘R’ input of flip-flop, hence the output of Flip-Flop Q=1 and Q’=0. Since the output Q’ is inverted and then taken as output at Pin-3, hence the state of Pin-3 is HIGH (Reverse of Q’). This state remains till ‘S’ input is made ZERO (Low). Since voltage at this Pin has triggered the output pin as HIGH, hence is called Trigger pin.
• When a voltage higher than 2/3^rd of Vcc is applied at Pin-6 (Threshold) the ‘S’ input goes low hence the state of Q’ changes to ‘1’ and hence its inverted state at Pin-3 is ‘0’. Since voltage above 2/3^rd Vcc changed the output state to Low hence this pin is also called as threshold pin.
• Now if we want to change the threshold and trigger voltage levels for a fixed supply at Pin-8, we can give additional supply at Pin-5 (Control Voltage). This additional voltage will get added to the reference voltages hence the will change the limits for trigger and threshold pin at which the output state changes. That’s the reason this pin is called control voltage. This can be used to modulate the pulse width at output pin or for modulating the pulse at output based on voltage variation at Pin-5. During normal operation this pin is bypassed with small capacitor (0.01µf) to avoid any interference with external voltages.
• Pin-7 is called discharge pin and is used to discharge the timing capacitor connected to this IC when used in timer mode. This we will understand better when we will see its application.

The pin out diagram of this timer IC is as shown here. Ground, Trigger, Output and Reset pins are on one side and Vcc, Discharge, Threshold and Control Voltage pins are on the other side.

Astable Multivibrator using LM555 IC: 

Let us learn about the basic application of this IC as astable multivibrator. It’s called multivibrator as the output changes its state from high to low and low to high. And Astable means both these states are not stable, they keep changing. There is another mode called Monostable where one (Mono) state is stable and the other is temporary. This we will see in next topic.

In this circuit we have connected resistor R1 between Vcc and pin-7(discharge) and R2 between pin-7 and pin-6(threshold). Trigger(2) and threshold(6) pins are connected together which is connected to a capacitor through negative supply.

Assume diode D1 is not connected now, later we will see the impact of connecting this diode across R2. That is the reason this is connected with dotted line. Also C2 is optional as it will not impact much in test environment but during practical implementation this is recommended to bypass control voltage(5) to ground or negative of supply. Usually we use a 0.01µf capacitor.

The complete wave generation cycle is explained in below graph

Step-1: Initially when we power this circuit, the capacitor C1 is fully discharged hence the voltage across it is zero hence trigger and threshold pins are at zero voltage (tagged as VC1 in the circuit). This pushes output at pin-3 as high and the hence the discharge transistor inside IC is OFF. Capacitor C1 start charging through resistor R1 and R2 as mentioned in graph as “First Charging Cycle”. The output remains high till the voltage at Vc1 just crosses 2/3^rd Vcc.

Step-2: As the voltage at Vc1 reaches just more than 2/3^rd Vcc, the output state of IC changes to low and the discharge transistor inside IC turns ON. This forces the charge on capacitor to start discharging through resistor R2 and through transistor to ground and this continue till the voltage across capacitor (Vc1) reaches just below 1/3^rd Vcc. This triggers the output again to high and the discharge transistor is turns OFF hence again the voltage across capacitor reaches just above 2/3^rd Vcc. And this cycle repeats till the power is supplied to the circuit.

Here we can see that while charging there are two resistors (R1 & R2) coming in charging path and while discharging only R2 comes in discharge path.

So the time taken for charging can be calculated as t1 = 0.693 X (R1+R2) X C1 sec.

And the time taken for discharge can be calculated t2 = 0.693 X R2 X C1 sec.

That is the reason the time period for which the output remains HIGH is more and the time period for which the output remains LOW is less.

If we want both the times to be equal, we need to any way make the resistance in charging path and in discharging path equal. To do this we use same value of R1 and R2 i.e. R1 = R2 = R and we can connect the diode D1 in parallel to R2 as shown in circuit diagram with dotted line.

Although practically the forward biased resistance of a diode is note ZERO, but considering theoretically ZERO, the resistance R2 act as short during charging cycle as D1 is forward biased. Hence charging time t1 is nearly equal to 0.693 X R X C1 (as we have taken R1 = R2 = R).

While discharging, the diode D1 is reversed biased hence the discharging current flows through resistance R2 which is equal to R.

So the discharging time t2 is also equal to 0.963 X R X C1. In this condition t1 = t2 hence the pulse width of for ON time = pulse width for OFF time. Hence theoretically we will get a square wave of 50% duty cycle. Though there are other ways to create square wave with perfect 50% duty cycle, but this is very basic method hence we covered here.

Checkout this video to see the simulation of astable multivibrator using IC555. We have connected a Yellow LED at the output pin to see the HIGH and LOW status of the output. LED turns ON when output is HIGH and goes OFF when output is LOW. A voltage tag is placed at timing capacitor 5µf, you can see at what voltage level the state of output changes. This shows the charging and discharging cycle. You can also see the direction of flow of current through capacitor during charging and discharging cycle.

We will see the other modes of timers in upcoming topics. Keep visiting…

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