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 555 Timer is one of the most popular and versatile integrated circuits ever produced! It is 30 years old and still being used! It is a combination of digital and analog circuits. It is known as the time machine as it performs a wide variety of timing tasks. Applications for the 555 Timer include: Bounce-free switches and Cascaded timers Frequency dividers Voltage-controlled oscillators Pulse generators and LED flashers
555 Timer Digital Circuits(15EECC203) 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 1/3Vcc 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 Flip-flop 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.
Inside the 555 Timer Digital Circuits(15EECC203)
Inside the 555 Timer Digital Circuits(15EECC203) You will learn more about these components later in the course, for now just understand the following: The voltage divider has three equal 5K resistors. It divides the input voltage (Vcc) into three equal parts. The two comparators are op-amps which compare the voltages at their inputs and saturate depending upon which is greater. The flip-flop is a bi-stable device. It generates two values, a high value equal to Vcc and a low value equal to 0V. The transistor is being used as a switch, it connects pin 7 (discharge) to ground when it is closed.
Periodic Pulse Train from a 555 Timer 555-Timers, like op-amps can be configured in different ways to create different circuits. We will now look into how this one creates a train of equal pulses, as shown at the output.
First we must examine how capacitors charge 10V TCLOSE = 0 1 2 U1 V V R1 1k V 8V 1 10V V1 2 U2 TOPEN = 0 C1 1uF 6V 4V Capacitor Voltage 2V 0 Capacitor C1 is charged up by current flowing through R1 I V1 V R1 0V 0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms V(U2:1) V(R1:2) V(V1:+) CAPACITOR 10 V 1k CAPACITOR As the capacitor charges up, its voltage increases and the current charging it decreases, resulting in the charging rate shown Time
Capacitor Charging Equations 10mA 10V 8mA 8V 6mA Capacitor and Resistor Current 6V Capacitor Voltage 4mA 4V 2mA 2V 0A 0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms I(R1) I(C1) Capacitor Current Capacitor Voltage Time I 0V 0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms V(U2:1) V(R1:2) V(V1:+) I e o t V V e o t 1 Time Where the time constant RC R1 C1 1ms
Understanding the equations 10V 8V 6V Capacitor Voltage 4V 2V 0V 0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms V(U2:1) V(R1:2) V(V1:+) Time Note that the voltage rises to a little above 6V in 1ms. 1 ( 1 e ). 632
Capacitor Charging and Discharging There is a good description of capacitor charging and its use in 555 timer circuits at http://www.uoguelph.ca/~antoon/gadgets/555/555.html
555 Timer At the beginning of the cycle, C1 is charged through resistors R1 and R2. The charging time constant is ( R1 R2) C1 The voltage reaches (2/3)Vcc in a time 0. 693( R1 R2) C1
555 Timer When the voltage on the capacitor reaches (2/3)Vcc, a switch (the transistor) is closed at pin 7 and the capacitor is discharged to (1/3)Vcc, at which time the switch is opened and the cycle starts over
555 Timer The capacitor voltage cycles back and forth between (2/3)Vcc and (1/3)Vcc at times and 1 0. 693( R1 R2) C1 2 0. 693( R2) C1 13
555 Timer The frequency is then given by f 1 144. 0. 693( R1 2 R2) C1 ( R1 2 R2) C1
Types of 555-Timer Circuits 5V 5V Ra 4 8 R 4 8 C Rb 7 6 2 5 0.01uF DIS TH R TR R CV NE555 1 GND VCC Q 3 LED 1 2 1K C 7 6 2 5 0.01uF DIS THR TR R CV NE555 1 GND VCC Q 3 LED Astable Multivibrator puts out a continuous sequence of pulses Monostable Multivibrator (or one-shot) puts out one pulse each time the switch is connected 15
Monostable Multivibrator (One Shot) V cc 8 4 Reset R a 6 Trigger 2 2 V cc 3 1 Vcc 3 R R Threshold Comparator +V - + -V +V - + -V Trigger Comparator R Q S Q Control Flip-Flop Output 3 C 7 R 1 Monstable Multivibrator One-Shot
Behavior of the Monostable Multivibrator 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 pulse.
Uses of the Monostable Multivibrator Used to generate a clean pulse of the correct height and duration for a digital system Used to turn circuits or external components on or off for a specific length of time. Used to generate delays. Can be cascaded to create a variety of sequential timing pulses. These pulses can allow you to time and sequence a number of related operations.
Astable Pulse-Train Generator (Multivibrator) V cc 8 4 R Threshold Comparator R 1 R 2 6 R - + +V -V R Q Output 3 2 - + +V -V S Q Trigger Comparator Control Flip-Flop C 7 R 1 Astable Pulse-Train Generator
Behavior of the Astable Multivibrator 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. The capacitor C charges through the series resistors R 1 and R 2 with a time constant (R 1 + R 2 )C. The capacitor discharges through R 2 with a time constant of R 2 C
ton=0.693(r1+r2)c1 toff=0693*r2*c1 T=ton+toff ASTABLE CALCULATIONS FOR LAB Let c1=5uf and For given frequency of 21hz, duty cycle of 64% F=1/T=1/21=48msec Duty cycle=ton/t*100 0.64=ton/48msec ton=31msec Using toff =17msec => r2=5kω r1=4kω Final Values: R1=4K, R2=5K, C1=5uF Digital Circuits(15EECC203)
ASTABLE CALCULATIONS FOR LAB Let c1=1uf and For given frequency of 1hz, duty cycle of 64% ton=0.693(r1+r2)c1 toff=0.693*r2*c1 T=ton+toff F=1/T=1/1=1sec Duty cycle=ton/t*100 0.64=ton/1sec ton=0.64sec Using toff =0.36sec Final Values: => r2=0.5mω R1=0.5M, R2=0.1M, r1=0.14mω C1=1uF
Input Outputs of 555
Design for 1sec delay with 50% duty Cycle
Design for 1sec delay with 50% duty Cycle Use diode(in4001) across R2 ton=toff=0.693(r1)c1 Choose the c1 value Calculate the r1 with ton=toff=0.5sec R1=R2=calculated value C1=assumed value
Design for 1sec delay with 50% duty Cycle Use diode(in4001) across R2 ton=toff=0.693(r1)c1 Choose the c1 Calculate the r1 with ton=toff=0.5sec R1=R2=calculated value = 7.2K C1=assumed value=100uf