555 Timer and Its Application

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Pin configurations of the 555 timer ; 555 timer in astable operation ; 555 timer in monostable operation ; applications?

1 ANALOG ELECTRONICS (AE) 555 Timer and Its Application 1 Prepared by: BE-EE Amish J. Tankariya SEMESTER-III SUBJECT- ANALOG ELECTRONICS (AE) GTU Subject Code : OBJECTIVES 555 timer; What is the 555 timer? Pin configurations of the 555 timer ; 555 timer in astable operation ; 555 timer in monostable operation ; applications? 555 TIMER The 555 is a popular, versatile chip used in many timing applications. By itself, it s neither a one-shot nor an astable multivibrator, but it can be hooked up to perform as either of these. 4 1

WHAT IS A 555 TIMER? A 555 Timer is an 8 pin mini dual-in-line package IC. The 555 IC is capable of producing accurate time delays and/ or oscillations. Introduced in 1972 by SigNetics.

2 WHAT IS A 555 TIMER? A 555 Timer is an 8 pin mini dual-in-line package IC. The 555 IC is capable of producing accurate time delays and/ or oscillations. Introduced in 1972 by SigNetics. (Signal Networks Electronics) The 555 is still in widespread use, thanks to its ease of use, low price, and good stability. PIN CONFIGURATION OF IC 555 : 5 6 PIN CONFIGURATION OF IC 555 : BLOCK DIAGRAM OF IC 555 Pin 1: It is a GROUND pin. Pin 2: Trigger : As name indicates, it starts the timing cycle of IC. Pin : It is the OUTPUT pin. Pin 4: This is RESET pin(active low). Pin 5: This pin is known is CONTROL VOLTAGE pin. Pin 6: This is known as THRESHOLD pin. It finalizes timing cycle of IC Pin 7: This is known as DISCHARGE pin Pin 8: This pin is used to provide positive supply to the IC v-18v

3 PIN DETAIL: Pin 1. Ground, The ground pin connects 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/Vcc causing the output to switch from a LOW to a HIGH state. Pin. 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. 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/Vcc 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 Flip-flop when the voltage applied to it exceeds 2/Vcc 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 9 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. INSIDE THE 555 TIMER The 555 timer consists basically of two comparators, a flip-flop, a discharge transistor, and a resistive voltage divider, 10 INSIDE THE 555 TIMER The voltage divider (blue) has three equal 5K resistors. It divides the input voltage (Vcc) into three equal parts. The two comparators (red) are op-amps that compare the voltages at their inputs and saturate depending upon which is greater. The Threshold Comparator saturates when the voltage at the Threshold pin (pin 6) is greater than (2/)Vcc. The Trigger Comparator saturates when the voltage at the Trigger pin (pin 2) is less than (1/)Vcc + - If V+ > V-, the output is a digital 1 If V-> V+, the output is a digital 0 11 CONTINUE The flip-flop (green) is a bi-stable device. It generates two values, a high value equal to Vcc and a low value equal to 0V. When the Threshold comparator saturates, the flip flop is Reset (R) and it outputs a low signal at pin. When the Trigger comparator saturates, the flip flop is Set (S) and it outputs a high signal at pin. The transistor (purple) is being used as a switch, it connects pin 7 (discharge) to ground when it is closed. When Q is low, Qbar is high. This closes the transistor switch and attaches pin 7 to ground. When Q is high, Qbar is low. This open the switch and pin 7 is no longer grounded 12

4 555 TIMER CONFIGURATIONS In this course we will use the 555 timer in 2 modes: Monostable In this mode 555 functions as a one shot pulse generator. e.g. Timers, touch switches, pulsewidth- modulation(pwm) Astable The 555 can operate as an oscillator. Uses include LED & Lamp flashers, Logic Clocks, Security Alarms. There are many other configurations and applications for this device too. 1 MONOSTABLE MULTIVIBRATOR (ONE SHOT) The monostable multivibrator is constructed by adding an external capacitor and resistor to a 555 timer. The circuit generates a single pulse of desired duration when it receives a trigger signal, hence it is also called a one-shot. The time constant of the resistor-capacitor combination determines the length of the 14 pulse. MONOSTABLE MULTIVIBRATOR (ONE SHOT) MONOSTABLE OPERATION: V cc R a 6 Trigger 2 2 V cc 1 Vcc 8 4 R R Threshold Comparator - +V + -V +V - + -V Trigger Comparator Reset R Q S Q Control Flip-Flop Output [when: trgr=10cmp11s=1q=1qn0trnstroffccharges When C>2/cmp21R=1 And after trigger pulse, input=1=vcccmp10s=0 Hence as R=1 and S=0out=0.} C 7 R 1 Monstable Multivibrator One-Shot

5 CKT OPERATION The resistive voltage divider is used to set the voltage for comparators. All three resistors are of equal value; therefore, the upper comparator has a reference of 2 V CC, and the lower comparator has a reference of 1 V CC. The comparators outputs control the state of the flip-flop. When the trigger input voltage (Input 2) is goes from high to low trigger (below 1 V CC ), comparator gives logic 1 (+Vcc) as an output and hence the flip-flop sets and the output jumps to its high level. The threshold input (Input 6) is normally connected to an external RC timing circuit. When the external capacitor voltage exceeds 2 V CC, the upper comparator resets the flip-flop, which in turn switches the output back to its low level. When the device output is low, the discharge transistor is turned on and provides a path for rapid discharge of the external timing capacitor. This basic operation allows the timer to be configured with external 17 components as an oscillator, a one-shot, or a timedelay element. DURATION OF ONE SHOT/PULSE The R and C are the external components whose values determine the time T (in seconds) for which the circuit is on we are interested in the time it takes for the voltage across the capacitor to be 2/Vcc, Time taken by capacitor to charge when it is in series with a resistor R, can be calculated from below equ. (Voltage across Capacitor) = Vcc * (1- e -t / (R*C) ) which can be rearranged to: 2/*Vcc = Vcc * (1- e -t / (R*C) ) 2/ = 1- e-t / (R*C) e -t / (R*C) = 1/ -t/ (R*C) = ln(1/) t = 1.1*R*C seconds 18 EXAMPLE: The monostable multivibrator like the one in Fig has the values of R = 1.2 kω and C = 0.1 µf. Determine the time T for which the circuit is on. Solution. The time T for which the circuit is on is given by ; ASTABLE MULTIVIBRATOR A 555 timer connected to operate in the astable mode as a free-running relaxation oscillator (astable multivibrator) is shown in Figure. Notice that the threshold input (THRESH) is now connected to the trigger input (TRIG). The external components R1, R2 and C form the timing circuit that sets the frequency of oscillation. 19 The capacitor connected to the control input is strictly for decoupling and has no effect on the operation. Circuit does not have an input from any other circuit. 20 5

6 ASTABLE MULTIVIBRATOR CIRCUIT OPERATION 21 Initially, when the power is turned on, the capacitor is uncharged and thus the trigger voltage (pin 2) is at 0 V. This causes the output of the lower comparator to be high(s=1) and the output of the upper comparator to be low(r=0), forcing the output of the flip-flop (Q=1), and thus the base of Transistor (Qn=0) low and keeping the transistor off. Now, Capacitor begins charging through R1 and R2 as indicated in Figure. When the capacitor voltage reaches 1 V CC, the lower comparator switches to its low output state(s=0 as well R=0 too so no change in output), Now when the capacitor voltage reaches 2 V CC, the upper comparator switches to its high output state (R=1 and S=0low output). This resets the flip-flop giving low output(q=0), which causes the base of Transistor (Qn=1) to go high, and turns on the transistor. This sequence creates a discharge path for the capacitor through R2 and the transistor, as indicated. The capacitor now begins to discharge, causing the upper comparator to go low(r=0 and S=0). At the point where the capacitor discharges down to 1 V CC, the lower comparator switches high (S=1 and R=0), setting the flip-flop, which makes the base of Transistor to be low and turns off the transistor. 22 Another charging cycle begins, and the entire process repeats. The result is a rectangular wave output whose duty cycle depends on the values of R1 and R2. CONTINUE The Astable Multivibrator is simply an oscillator. The Astable Multivibrator generates a continuous stream of rectangular off-on pulses that switch between two voltage levels. The frequency of the pulses and their duty cycle are dependent upon the RC network values. On Time duration T 1 can be obtain from charging of capacitor C through the series resistors R 1 and R 2 with a time constant (R 1 + R 2 )C. Off Time duration T 2 can be obtain from dicharging of 2 capacitor C through R 2 with a time constant of R 2 C FREQUENCY OF ASTABLE MULTIVIBRATOR The time T 1 for which the output is high is given by ; T 1 = (R 1 + R 2 ) C The time T 2 for which the output is low is given by; T 2 = R 2 C Total period T for the oscillation is; T = T 1 + T 2 = (R R 2 ) C The frequency f of the astable multivibrator is given by ; Note that f will be in Hz if resistance is in ohms and capacitance in farads. 24 6

7 EXAMPLE Determine the frequency of the circuit shown in Fig Given that R1 = kω ; R2 = 2.7 kω and C = 0.0 µf. Solution. The frequency of the circuit is given by ; BISTABLE 555 TIMER CONTINUE As well as the one shot 555 Monostable configuration above, we can also produce a Bistable (two stable states) device with the operation and output of the 555 Bistable being similar to the transistorised one we look at previously in the Bistable Multivibrators tutorial. The 555 Bistable is one of the simplest circuits we can build using the 555 timer oscillator chip. This bistable configuration does not use any RC timing network to produce an output waveform so no equations are required to calculate the time period of the circuit. 27 CONTINUE The switching of the output waveform is achieved by controlling the trigger and reset inputs of the 555 timer which are held HIGH by the two pull-up resistors, R1 and R2. By taking the trigger input (pin 2) LOW, switch in set position, changes the output state into the HIGH state and by taking the reset input (pin 4) LOW, switch in reset position, changes the output into the LOW state. This 555 timer circuit will remain in either state indefinitely and is therefore bistable. Then the Bistable 555 timer is stable in both states, HIGH and LOW. The threshold input (pin 6) is connected to ground to ensure that it cannot reset the bistable circuit as it would in a normal timing application 28 7

8 QUESTIONS 29 8

PRESENTATION ON 555 TIMER A Practical Approach

PRESENTATION ON 555 TIMER A Practical Approach By Nagaraj Vannal Assistant Professor School of Electronics Engineering, K.L.E Technological University, Hubballi-31 nagaraj_vannal@bvb.edu 555 Timer The