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555 IC
Introduction : Oscillators are broadly classified into two categories namely sinusoidal and non-sinusoidal oscillators. Sinusoidal oscillators produce a sine wave output and the non-sinusoidal oscillators produce a square or pulsed output. A multivibrator circuit is basically a non-sinusoidal oscillator with a regenerative feedback. It is a two state circuit that has zero, one or two stable output states. Depending on the number of stable output states there are three basic types of multivibrator circuits namely Bistable multivibrator having two stable states, Monostable multivibrator having one stable state and Astable multivibrator having zero stable states. In the case of Monostable and Bistable multivibrator an external trigger pulse is required for their operation whereas in the case of Astable multivibrator it has automatic built in triggering which switches it continuously between its two unstable states both SET and RESET. There are two main components in every multivibrator namely the bistable circuit and the two passive networks that are connected in a basic feedback loop. The networks in the circuit can be Monostable (resistive), Astable (resistive-capacitive) or bistable. Astable multivibrator is built of two amplifying stages that are connected in a positive feedback loop using two resistive-capacitive coupling networks. The elements used for amplification may be junction field effect transistors, bipolar junction transistors, operational amplifiers or any other type of amplifier. IC 555 TIMER: The 555 timer is widely used as IC timer circuit and it is the most commonly used general purpose linear integrated circuit. It can run in either one of the two modes: Monostable (one stable state) or Astable (no stable state). In the
Monostable mode it can produce accurate time delays from microseconds to hours. In the Astable mode it can produce rectangular waveforms with a variable Duty cycle. The simplicity and ease with which both the multivibrator circuits can be configured around this IC is one of the main reasons for its wide use. Design of an Astable multivibrator using 555 timer IC, generating non-sinusoidal waveform in the form of Rectangular waveform as well as capacitor voltage waveform in the form of ramp waveform. The 555 timer IC was first introduced around 1970 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. It provided circuit designers with a relatively cheap, stable, and user-friendly integrated circuit for timer and multivibrator applications. The IC comprises of 23 transistors, 2 diodes and 16 resistors with built-in compensation for component tolerance and temperature drift. There are three resistors in the voltage divider net work section in it and all have the same value 5 KΩ in the bipolar version of this IC and hence the name 555.
flip-flop and its output is high_. A_ high output from the flip-flop when given to the base of the discharge transistor saturates it and thus discharges the transistor that is connected externally to the discharge pin 7. The complementary signal out of the flip-flop goes to pin 3, the output. The output available at pin 3 is low_._ These conditions will prevail until lower comparator triggers the flip-flop. Even if the voltage at the threshold input falls below (2/3) VCC that is upper comparator cannot cause the flip-flop to change again. It means that the upper comparator can only force the flip-flop’s output high. To change the output of flip-flop to low , the voltage at the trigger input must fall below + (1/3) Vcc. When this occurs, lower comparator triggers the flip-flop, forcing its output low_._ The low output from the flip-flop turns the discharge transistor off and forces the power amplifier to output a high. These conditions will continue independent of the voltage on the trigger input. Lower comparator can only cause the flip-flop to output low. From the above discussion it is concluded that for the having low output from the timer 555, the voltage on the threshold input must exceed the control voltage or + (2/3) VCC. This also turns the discharge transistor on. To force the output from the timer high, the voltage on the trigger input must drop below +(1/3) VCC. This turns the discharge transistor off. A voltage may be applied to the control input to change the levels at which the switching occurs. When not in use, a 0.01 nF capacitor should be connected between pin 5 and ground to prevent noise coupled onto this pin from causing false triggering. Connecting the reset (pin 4) to a logic low will place a high on the output of flip-flop. The discharge transistor will
go on and the power amplifier will output a low. This condition will continue until reset is taken high. This allows synchronization or resetting of the circuit’s operation. When not in use, reset should be tied to +VCC. You can see the output wave forms in the side diagrams. The two important parameters that we should understand from the output of timer is ON Time ( THIGH=T1 ) and OFF Time ( TL0W=T2 ). ON Time – is the time for which the timer output Vout has remained in HIGH State. We notate this with THIGH. OFF Time – is the time for which the timer output Vout has remained in LOW state. We notate this with TL0W. (ON Time and OFF Time are dependent on the values of RA, RB and C in Astable Multivibrator. So we can obtain the desired ON Time and OFF Time at the timer output with the proper calculation of RA, RB and C values.) Although the timer 555 has been used in a wide variety of often unique applications it is very hard on its power supply lines, requiring quite a bit of current, and injecting many noise transients. This noise will often be coupled into adjacent ICs falsely triggering them. The 7555 is a CMOS version of the 555. Its quiescent current requirements are considerably lower than that of 555, and the 7555 does not contaminate the power supply lines. It is pin compatible with the 555. So this
The Timer IC 555’s pin details are as follows:
PIN 1: Ground Pin: Connects to the 0V power supply , All voltages are measured with respect to this terminal. PIN 2: Trigger input : The output of the timer depends on the amplitude of the external trigger pulse applied to this pin. When a negative going pulse of amplitude greater than 1/3 VCC is applied to this pin the output of the timer high. The output remains high as long as the trigger terminal is held at a low voltage. If Pin 2 is low and pin 6 LOW, output goes and stays HIGH, If pin 6 high and pin 2 goes low, output goes low while pin 2 low. This pin has a very high impedance about 10 MΩ and will trigger with about 1 μA. PIN 3: Output: The output of the timer is measured here with respect to ground. There are two ways by which a load can be
connected to the output terminal: either between pin 3 and ground or between pin3 and supply voltage +VCC. When the output is low the load current flows through the load connected between pin3 and +VCC into the output terminal and is called sink current. The current through the grounded load is zero when the output is low. For this reason the load connected between pin 3 and +VCC is called the normally on load and when connected between pin 3 and ground is called normally off-load. On the other hand, then the output is high the current through the load connected between pin 3 and +VCC is zero. This current is called source current. The maximum value of sink or source current is up to 200mA. (Pin 3 and 7 are in phase) goes HIGH (about 2 V less than rail) and LOW (about 0.5 V less than 0 V) PIN 4: Reset input: The timer can be reset (disabled) by applying a negative pulse to this pin. When the reset function is not in use, the reset terminal should be connected to +VCC to avoid any possibility of false triggering. Reset pin is internally connected HIGH via 100 KΩ, must be taken below 0.8 V to reset the IC chip.
Astable Multivibrator : These circuits are not stable in any state and switch outputs after predetermined time periods. The result of this is that the output is a continuous square/rectangular wave with the properties depending on values of external resistors and capacitors. Thus, while designing these circuits following parameters need to be determined:
- Frequency (or the time period) of the wave.
- The duty cycle of the wave. We are explaining an astable multivibrator, often called a free-running multivibrator. An astable multivibrator can be designed using different types of components, say using transistors (and associated components) alone or by using op amps (and associated components) or can be design using 555 Timer IC. If you are wondering what all are the practical applications of an astable multivibrator, let’s begin by seeing a few. One of the most common application for an astable multivibrator is to generate time delays. Let’s say you want to turn an LED “ ON ” for 1 second and keep it “ OFF ” for 0.5 seconds, and then an astable multivibrator is the best circuit you could use to build this application. This circuit does not require any external trigger to change the state of the output, hence the name free-running. Before going to make the circuit, make sure your 555 IC is working.
555 Timer Astable Multivibrator Circuit Diagram An Astable Multivibrator can be designed by adding two resistors a capacitor (RA, RB and C in circuit diagram) to the 555 Timer IC. These two resistors and the capacitor (values) are selected appropriately so as to obtain the desired ‘ON’ and ‘OFF’ timings at the output terminal (pin 3). So basically, the ON and OFF time at the output (i.e. the ‘HIGH’ and ‘LOW’ state at the output terminal) is dependent on the values chosen for RA, RB and C. Note: - The capacitor C2 (0.01uF) is connected to pin number 5 (Control Voltage Terminal) in all 555 IC based circuits in which that particular pin (pin 5 – control voltage terminal) is not used. This capacitor is used to avoid noise problems that could arise in the circuit if that pin is left open.
For the sake of explanation, let’s assume the circuit is powered up and right now the status at non inverting flip flop output
- Q is LOW. When Q is LOW, Vout will be HIGH (which we call as Timer Output).You see that Q is directly connected to base of transistor (at the discharge terminal). So when Q is LOW, transistor will be in its cut off state (OFF state). In this state, capacitor C is directly connected to power supply Vcc through resistors RA and RB. So the capacitor will start charging
towards the supply voltage Vcc and the charging time constant will be defined by RA and RB values as (RA+RB)C. The capacitor will charge towards Vcc and this will increase the threshold voltage (voltage across pin 6) of 555 IC. When the capacitor charges up to 2/3Vcc and beyond, the threshold voltage will also surpass 2/3Vcc level and this will force the op amp output (comparator 1) to go HIGH (take note that the reference voltage at – terminal of comparator 1 is 2/3Vcc). Since the op amp output of comparator 1 is connected ‘S’ (SET input) of SR flip flop, the flip flop will be triggered and the Q output (non inverting output) of flip flop will turn HIGH. You got up to this? You may recall that we have begun this explanation by assuming Q is LOW initially. Now as a result of capacitor charging, Q has turned HIGH automatically from LOW. When Q goes HIGH, Vout will automatically go to LOW as Vout is nothing but a compliment of Q. When Q is HIGH, the transistor at pin 7 (the discharge terminal) will be turned ON and the transistor will get saturated. When the transistor gets saturated, the pin 7 (discharge terminal) will act as a ground for the capacitor. As a result, a new path is available for the capacitor to discharge from 2/3Vcc level to zero volts. The capacitor will start discharging through the new path (through Rb) and this will result in decrease in voltage across the trigger terminal (pin 2) of 555 IC. The discharging time constant is defined by RbC. Once the capacitor discharges to a level below 1/3Vcc, resulting in the same voltage (voltage of capacitor) across the trigger terminal (notice that the reference input voltage at + terminal of comparator 2 is 1/3Vcc), the op amp output of comparator 2 will go HIGH. Since the output of comparator 2 is connected to ‘R’ – the RESET input terminal of SR flip flop, the Q output of flip flop will go from HIGH to LOW. When Q goes to LOW, Vout will automatically go to HIGH. Thus an automatic transition from HIGH to LOW and then from LOW to HIGH is achieved in an Astable Multivibrator. The cycle repeats.
or t2 = RC loge 3^ = 1.0986 RC So the time taken by the capacitor to charge from +1/3 VCC to +2/3 VCC tc = (t2 – t1) = (1.0986 – 0.405) RC = 0.693 RC Substituting R = (RA + RB) in above equation we have THIGH = tc = 0.693 (RA + RB) C Where RA and RB are in ohms and C is in farads. The time during which the capacitor discharges from +2/3 VCC to +1/3 VCC is equal to the time the output is low and is given as td or TL0W = 0.693 RB C where RB is in ohms and C is in farads The above equation is worked out as follows: Voltage across the capacitor at any instant during discharging period is given as vc = 2/3 VCC e– td/ RBC Substituting vc = 1/3 VCC and t = td in above equation we have +1/3 VCC = +2/3 VCC e–^ td/ RBC Or td = 0.693 RBC Overall period of oscillations, T = THIGH + TLOW = 0.693 (RA+ 2RB) C, The frequency of oscillations being the reciprocal of the overall period of oscillations T is given as f = 1/T = 1.414/ (RA+ 2RB)C Equation indicates that the frequency of oscillation / is independent of the collector supply voltage +VCC. Often the term duty cycle is used in conjunction with the astable multivibrator.
The duty cycle, the ratio of the time thigh during which the output is high to the total time period T is given as % duty cycle, D = thigh / T * 100 = (RA + RB) / (RA + 2RB) * 100 From the above equation it is obvious that square wave (50 % duty cycle) output cannot be obtained unless RA is made zero. However, there is a danger in shorting resistance RA to zero. With RA = 0 ohm, terminal 7 is directly connected to + VCC. During the discharging of capacitor through RB and transistor, an extra current will be supplied to the transistor from VCC through a short between pin 7 and +VCC. It may damage the transistor and hence the timer. There is a possibility to construct a symmetrical square wave can be obtained if a diode is connected across resistor RB, as illustrated in dotted lines in figure below. The capacitor C charges through RA and diode D to approximately + 2/3VCC and discharges through resistor RB and terminal 7 (transistor) until the capacitor voltage drops to 1/3 VCC. Then the cycle is repeated. To obtain a square wave output, RA must be a combination of a fixed resistor R and a pot, so that the pot can be adjusted to give the exact square wave.
are low cost to produce, and simple to design These multivibrators do not transfer the whole o/p signal to the i/p. This is due to resistance in the circuit, nonexistence of a totally closed loop at the o/p terminals, and the leaning of one transistor/capacitor to absorb energy at a somewhat different rate than the other. Although the amplifier restores the lost energy when it amplifies the signal, the signal will finally be too small to be of any use.
To design a 555 astable with a frequency of 1 kHz and a mark to space ratio (Duty cycle ratio**) of 2:
Manufacturers specify the maximum total resistance usually around 10 to 20MΩ that may be used with their particular variant of the 555, however using such high values can increase the error between calculated and actual frequencies, so for many uses a 1MΩ maximum can be recommended. The minimum total resistance value for the combination of R1 and R2 depends largely on the value of R1. The junction of R1/R2 is connected to pin 6 and to the trigger input pin 2. If the value of R1 is less than about 1KΩ, there is a danger that the trigger input may not be able reach a low enough voltage to trigger comparator 1, and so oscillations cannot take place. From this it can be assumed that if R1 must be 1KΩ or above and R1 + R2 should be kept below 1MΩ; a 10nF capacitor would allow a suitable total resistance of around 100KΩ to be calculated.
