The 555 IC Timer

 

INTRODUCTION

One of the most versatile linear ICs is the 555 timer which was introduced in early 1970 by Signetic Corporation. The 555 is a monolithic timing circuit. It can produce accurate and highly stable time delays or osscilations. It got its name because of its three 5kΩ resistors which is used in its voltage divider network. It is very much reliable , cheaper in cost and easy to use.

APPLICATION

• Monostable and Astable Multivibrator
• Waveform generator
• Temperature measurement
• Digital logic probes
• Analog frequency meter and tachnometer 
• Dc-Dc converter

Signetic Corporation gives the name SE/NE 555 timer. SE 555  is designed so such that for the operating temprature ranging from -55℃ to 125℃ whereas NE 555 ranges from 0℃ to 70℃. The important feature of 555 timer is that it operates from wide range of power supplies which is +5V to 18V supply voltage.

Mono stable Mode: It can produce accurate time delays from microseconds to hours.

A stable Mode: It produces reactangular waves with variable cycles.

555 Timer Pin Confriguation

 

 8-Pin DIP (Dual-In-Package) consist of 23 transistors, 2 diodes,  and 16 resistors.

Pin 1 (Grounded Terminal): All Voltages are measured with respect this terminal.

Pin 2 (Trigger Terminal): Gives Input to comparator which is responsible for transistion of flip flop that is set to reset. Whereas Output depends on Amplitude of external trigger plus.

Pin 3 (Output Terminal): For Output there are two ways that is load is connected to output terminal which is between pin 3 and supply pin that is called the Normally On Load and the output terminal which is between  pin 3 and pin 1 is called  the Normally Off  Load.

Pin 4(Rest Terminal):  To reset the timer in which negative impulse is applied due to which it gets reset. when the pin is not used for reset purpose it is connected to +Vcc which avoids possibility of false triggering.

Pin 5(Control Voltage Terminal): This pin controls the threshold and trigger pin.It determines the Pulse width of output waveform.

Pin 6(Threshold Terminal):This is non inverting input terminal of comparator which compares the voltage applied with reference voltage of+2/3 Vcc. This is responsible for set state of flip-flop.

Pin 7(Discharge Terminal): This pin is connected internally to collector of transistor and capacitor is connected between terminal and ground.It is called Discharge terminal because when transistor saturates, capacitor discharge through the transistor.

Pin 8(Supply Terminal): A supply Voltage of +5V to 18V with respect to ground.

Basic Timer Concept

Lets consider output Q is high, the transistor is saturated and the capacitor voltage is clamped at ground which means capacitor is shorted and cannot charge.
Non Inverting voltage referred to Threshold voltage while Inverting input voltage referred to as control voltage.  R-S flip-flop set which is saturated transisitor holds the threshold voltage at zero.
Control Voltage is fixed to 2/3Vcc.

R-S Flip-Flop

Suppose high voltage is applied to R input. This resets the RS flip-flop. Output Q goes to low and transistor is cutoff. Capacitor will charge as an freely . When capacitor will charge, threshold voltage will rise. Gradually Threshold Voltage will be greater than 10V. 

Block Diagram For the 555 Timer   

 

A 555 timer consist of two comparators (op-amps), RS flip-flop, two transistors and resistive network.
It consist of three equal resistors which acts as a voltage regulators. Output of both comparators (2/3Vcc and 1/3Vcc) is supplied to flip-flop. Flip-flop assumes its state according to output of the two comparators.The transistor saturates or cut-off according to output state of flip-flop.

The 555 Timer As An A stable Multivibrator

An a stable multivibrator is also known as free-running multivibrator , is a reactangular wave generating circuit. This circuit doesnot need any external trigger to change state like wise to Stable multivibrator. It can created by adding two transistor and capacitor to IC. 

 

 Timing between the output is either high or low is determined by externally connected two resistor and two capacitors.

Pin 1 is grounded.

Pin 4 and Pin 8 are shorted to supply +Vcc.

Pin 3 output is taken.

Pin 2 and Pin 6 are shorted and connected to ground by capacitor. 

Pin 7 is connected to supply +Vcc through a resistor Ra whereas between pin 6 and 7 resistor Rb is connected.

At pin 5  bypass capacitor is applied of 0.01 micro farad is connected.

Astable operation

The high time interval of each pulse is given by:

${\displaystyle t_{high}=\ln(2)\cdot (R_{1}+R_{2})\cdot C}$

The low time interval of each pulse is given by:

${\displaystyle t_{low}=\ln(2)\cdot R_{2}\cdot C}$

Hence, the frequency ${\displaystyle f}$ of the pulse is given by:

${\displaystyle f={\frac {1}{t_{high}+t_{low}}}={\frac {1}{\ln(2)\cdot (R_{1}+2R_{2})\cdot C}}}$

and the duty cycle (%) is given by:

${\displaystyle duty={\frac {t_{high}}{t_{high}+t_{low}}}\cdot 100}$

where ${\displaystyle t}$t is in seconds (time), ${\displaystyle R}$R is in ohms (resistance), ${\displaystyle C}$C is in farads (capacitance), ${\displaystyle \ln(2)}$ln(2) is the natural log of 2 constant, which is 0.693147181 (rounded to 9 trailing digits) but commonly is rounded to fewer digits in 555 timer books and datasheets as 0.7 or 0.69 or 0.693.
Resistor ${\displaystyle R_{1}}$R1 requirements:

• ${\displaystyle W}$W power capability of ${\displaystyle R_{1}}$R1 must be greater than ${\displaystyle {\frac {V_{cc}\cdot V_{cc}}{R_{1}}}}$, per ohms law.

• Particularly with bipolar 555s, low values of ${\displaystyle R_{1}}$R1 must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above.

The first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0V to ​²⁄₃ of V_CC from power-up, but only from ​¹⁄₃ of V_CC to ​²⁄₃ of V_CC on subsequent cycles.
To have an output high time shorter than the low time (i.e., a duty cycle less than 50%) a fast diode  can be placed in parallel with R₂, with the cathode on the capacitor side. This bypasses R₂ during the high part of the cycle so that the high interval depends only on R₁ and C, with an adjustment based the voltage drop across the diode. The voltage drop across the diode slows charging on the capacitor so that the high time is a longer than the expected and often-cited ln(2)*R₁C = 0.693 R₁C. The low time will be the same as above, 0.693 R₂C. With the bypass diode, the high time is

${\displaystyle t_{high}=\ln \left({\frac {2V_{\textrm {cc}}-3V_{\textrm {diode}}}{V_{\textrm {cc}}-3V_{\textrm {diode}}}}\right)\cdot R_{1}\cdot C}$

where V_diode is when the diode's "on" current is ​¹⁄₂ of V_cc/R₁ which can be determined from its datasheet or by testing. As an extreme example, when V_cc= 5 and V_diode= 0.7, high time = 1.00 R₁C which is 45% longer than the "expected" 0.693 R₁C. At the other extreme, when V_cc= 15 and V_diode= 0.3, the high time = 0.725 R₁C which is closer to the expected 0.693 R₁C. The equation reduces to the expected 0.693 R₁C if V_diode= 0.
The operation of RESET in this mode is not well-defined. Some manufacturers' parts will hold the output state to what it was when RESET is taken low, others will send the output either high or low.

 The 555 Timer As A Monostable Multivibrator

It is oftenly called one-shot multivibrator , is a pulse generator circuit. 

Pin 1 is grounded.

Trigger input is applied to pin 2.

Output is taken from pin 3.

Pin 4 is connected to +Vcc to avoid accidental reset.

Pin 5 is grounded through 0.01 micro faraday capacitor to avoid noise problem.

Pin 6 is connected to pin 7.

Ra is connected between pin 6 and pin 8.

Pin 7 is a connected with discharge capacitor.

Pin 8 is connected to supply Vcc.

Monostable Operation 

When output is at pin 3 is low then the circuit is in stable state, the transistor is grounded and capacitor  is shorted to ground.When negative pulse is applied to pin 2, the trigger input falls below +1/3Vcc, the output of comparator goes high which resets flip-flop and mean while transistor turns off and output pin 3 goes high.

The voltage across capacitor is given by:

 vc = Vcc(1-e^(-t/RC)) at t=T. 

Since,

 vc =(2/3)Vcc 

therefore :

2/3Vcc = Vcc(1-e^(-T/RC)), thus reduced to T = RC ln(3) seconds.

The output pulse width of time t, which is the time it takes to charge C to ​²⁄₃ of the supply voltage, is given by

${\displaystyle t=\ln(3)\cdot R\cdot C}$

where ${\displaystyle t}$t is in seconds(time), ${\displaystyle R}$R is in ohms (resistance), ${\displaystyle C}$C is in farads (capacitance), ln(3) ${\displaystyle \ln(3$ is the natural log of 3 constant, which is 1.098612289 (rounded to 9 trailing digits) but commonly is rounded to fewer digits in 555 timer books and datasheets as 1.1 or 1.099.