555 Timer - WordPress.com

555 Timer - WordPress.com

Unit IV 555 Timer Timer? An automatic mechanism for activating a device at a preset time. Used to indicate how many times someone has done something. A Timer is used for producing precise time delay. Secondly, it can be used to repeat or initiate an action after/at a known period of time. This feature is very commonly used in several applications. By function timers can be categorized to two main types. A timer which counts upwards from zero for measuring elapsed time is

often called a stopwatch; a device which counts down from a specified time interval is more usually called a timer or a countdown timer Block Diagram of a basic Timer The charging unit consists of RC circuit When the Trigger input is activated the capacitor can be discharged completely so that voltages across the capacitor can be made zero. Immediately after the switch is released, the capacitor starts

charging through resistance R, with the time constant RC. IC 555 The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was called "The IC Time Machine" and was also the very first and only commercial timer IC available. 555 Pinout

Block Diagram of IC 555 Timer AC1 AC2 The 555 timer IC consists of Two analog voltage Comparators C1 and C2

RS-Flip Flop A discharge Transistor & Three 5K resistors. The comparator drives the F/F, which in turn drives the transistor. The output of the timer is generally taken at Q' output terminal. 555 Timer Pinout Pin 1. Ground, The the 555 timer to the negative (0v) supply rail.

Pin 2. Trigger, The negative input to comparator No 1. A negative pulse on this pin sets the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a LOW to a HIGH state. Pin 3. Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc 1.5V so small speakers, LEDs or motors can be connected directly to the output. Pin 4. Reset, This pin is used to reset the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic 1 level when not used to prevent any unwanted resetting of the output. Pin 5. Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output

signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise. Pin 6. Threshold, The positive input to comparator No 2. This pin is used to reset the Flipflop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from HIGH to LOW state. This pin connects directly to the RC timing circuit. Pin 7. Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to discharge the timing capacitor to ground when the output at pin 3 switches LOW. Pin 8. Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V. MULTIVIBRATOR A Multivibrator is an electronic circuit that generates square, rectangular,

pulse waveforms, also called nonlinear oscillators or function generators. Multivibrator is basically a two amplifier circuits arranged with regenerative feedback (+VE feedback). There are three types of Multivibrator: Astable Multivibrator: Circuit is not stable in either stateit continuously oscillates from one state to the other. (Application in Oscillators) Monostable Multivibrator: One of the state is stable but the other is not. (Application in Timer) Bistable Multivibrator: Circuit is stable in both the state and will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. (Application in Flip flop)

02/21/2020 555 as Astable Multivibrator Vc(t) Assume the Capacitor C is discharged when power is on. (ie) Vc(t=0)= 0. Therefore, Threshold = Trigger = 0. Hence R=0, S=1 which Set the F/F; Q=1,Q=0, 555 output is HIGH and the discharge transistor will be in OFF state.

Therefore Capacitor will charge through (Ra+Rb) to Vcc. When the voltage across C Vc(t) become greater than Vcc/3 (Trigger>Vcc/3) the comparator 2 o/p goes low and the S input =0, which keep the F/F in previous state and the capacitor continue charging. When Vc(t) > 2/3 Vcc, comparator 1 output goes high, (ie) R=1, and it reset the F/F : Q=0,Q=1. The transistor is turned on, and the capacitor begins discharging through Rb and Ton. When the Vc(t) becomes less than

the 1/3Vcc the lower comparator o/p is high and S input to FF is High , which set the F/F. Therefore, Q=1 and Q=0 and the transistor is turned off. The capacitor starts charging through the resistors Ra and Rb .This process repeats. The output is a square wave. The output wave form is shown in figure.

Charging and Discharging Times The voltage across the capacitor oscillates between Vcc/3 and 2Vcc/3. The capacitor charges through Ra+Rb and discharges through Rb only. Therefore, the discharging time (T1) and charging time (T2) are different. Hence the output is not a symmetric square wave. TH!=TL

Calculation of T1 and T2 During the time period T1, C is discharging. (ie) from initial voltage Vc(t)= Vcc to final voltage Thus Vc(t) can be written as, Vc(t) = Vcc ; discharging through Resistor Rb (ie) at t =0, Vc(t) = Vcc and at t=T1 Vc(t) = At t =T1, Vc(T1) = = (ie) 0.5 = Taking log on both sides, ln 0.5 = -T1/(RbC)

-0.693 = -T1/RbC , (TL) T1 = 0.693* Rb*C 2VCC/3 VCC/3 The general expression for the voltage across the capacitor during charging from initial voltage Vi to final voltage Vf is given by Vc(t) = Vf + (Vi-Vf) During time period T2, (TH) Capacitor charges from initial voltage Vi= to final voltage Vf = Vcc through Ra + Rb but when it reaches Vcc discharge takes place.

Substituting relevant values, Vc(T2) = Vcc + (Vcc/3 Vcc) exp(-T2/((Ra+Rb)*C)) 2Vcc/3 =Vcc + (Vcc/3 Vcc) exp(-T2/ ((Ra+Rb)*C)) 2/3 = 1 + (1/3 -1) exp( -T2/ ((Ra+Rb)*C)) 1/3 = 2/3 exp (-T2/ ((Ra+Rb)*C)) (TH) => T2 = 0.693 (Ra+Rb)*C Total Time period T = T1 + T2 T= 0.693(Ra +2Rb)C Therefore f = 1.443 / [( Ra + 2Rb).C] Duty Cycle The Duty cycle of a square wave is defined as the ratio of the time

the wave is high (TH), to the time period T. TH Ra + Rb DC = -------------------- = -------------TL + TH Ra+2Rb Calculations: Astable TL 0. 693 R B C TH 0. 693(R A R B )C TH

TL Note that Duty Cycle D must always be > 0.5. To get 50% duty cycle, RA = 0, which would short out VCC 555.16 Monostable Multivibrator When the o/p of RS f-f is high the circuit o/p is high, the transistor is off and the capacitor charges through R to VCC. When this voltage crosses the 2/3VCC the

upper comparator o/p is high and RS f-f o/p is low and the circuit o/p is low, the transistor is on and the capacitor discharges to ground level. When the triggering input is applied at pin 2 and it crosses 1/3 VCC then the lower comparator o/p is high and RS f-f o/p is high the and the transistor is off then the capacitor charges through R to Vcc. This process will repeat. 02/21/2020

If vi =0, Vc = vf + (0 -vf) exp (-t/RC) Vc = Vf (1-exp (-t/RC) Applications of Monostable MV Frequency Divider The application of trigger pulse to a monostable multivibrator based on 555 gives a positive going pulse on the output. The monostable circuit can be used as a frequency divider if the timing interval is adjusted to be longer than the period of input signal at the trigger input.

02/21/2020 Pulse width Modulator 02/21/2020 It is basically a monostable multivibrator with a modulating input signal applied at the control voltage input (pin no.5). Internally, the control voltage is adjusted to 2/3 Vcc. Externally applied modulating signal changes the control voltage and hence the threshold voltage of the upper comparator.

As a result the time period required to charge the capacitor upto threshold voltage level changes and hence width of the output pulse varies depending upon the input signal called pulse width modulation. 02/21/2020 02/21/2020 02/21/2020 Sample Calculation

Design an oscillator with a frequency of 200Hz with a duty cycle of 78%. 1. Determine Period (T): 1 1 T 0. 005 s F 200Hz 2. Determine TH and TL:

TH 78 % 0. 005 s 0. 0039 s 3. 9ms TL 22 % 0. 005 s 0. 0011 s 1. 1ms 555.26 Sample Calculation 3. Since there are 2 variables in the TL equation, select C: C=10FF 4. Determine RB by using the TL equation: TL 0. 693R BC

1. 1ms 0. 693 R B 10F R B 158 . 7 555.27 Sample Calculation 5. Determine the value for RA: TH 0. 693(R A R B )C 3. 9ms 0. 693(R A 158 . 7)10F 562 . 8 R A 158 . 7

R A 404 . 1 555.28 Calculations: Astable (R A R B ) DC (R A 2R B ) F 1

0. 693 (R A 2R B ) C Notes: The value 0.693 is a factor associated with the charge/discharge cycle of the 555 timer. Duty Cycle must be > 50% 555.29 Sample Design Frequency, Duty Cycle Set

Build an oscillator using a 555 timer with a frequency of 72kHz at 75% D.C. Use a 100F capacitor. 555.30 Design Solution Frequency, Duty Cycle Set 1- Determine the ratio of the resistors Ra and Rb: Ra Rb

0. 75 Ra 2Rb Ra Rb 0. 75(Ra 2Rb) 0. 75Ra 1. 5Rb Ra 0. 75Ra 1. 5Rb Rb Ra 2Rb DC 2- Use the ratio in the frequency equation (substitution): f 1. 44 1. 44

1. 44 (Ra 2Rb)C (2Rb 2Rb)C 4RbC 555.31 Design Solution 3- Solve for Rb: 72kHz 1. 44 1. 44

50k 4RbC 4 72k 100 4-Solve for Ra: Ra 2Rb 100k 5-Use standard values (optional step): Ra=100k Rb=47k

555.32 Design Solution 6- Calculate actual frequency and DC: f 1. 44 74 . 2kHz (100k (2 47k)) 100 F Error 3. 1%

(100k 47k) DC 0. 757 (100k (2 47k)) Error 1% 555.33

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