09-08-2012, 10:34 AM
555 TIMER
555 TIMER.pdf (Size: 468.21 KB / Downloads: 67)
Introduction
The 555 timer is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. It was produced by Signetics Corporation in early 1970. The original name was the SE555/NE555 and was called "The IC Time Machine". The 555 gets its name from the three 5-KΩ resistors used in typical early implementations. It is widely used because of its ease to use, low price and reliability.
It is one of the most popular and versatile integrated circuits which can be used to build lots of different circuits. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8)(Refer to Figure 1).
The 555 Timer is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillations. The timer basically operates in one of the two modes—monostable (one-shot) multivibrator or as an astable (free-running) multivibrator. In the monostable mode, it can produce accurate time delays from microseconds to hours. In the astable mode, it can produce rectangular waves with a variable duty cycle. Frequently, the 555 is used in astable mode to generate a continuous series of pulses, but you can also use the 555 to make a one-shot or monostable circuit.
The 555 can source or sink 200 mA of output current, and is capable of driving wide range of output devices. The output can drive TTL (Transistor-Transistor Logic) and has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/°C.
Applications of 555 timer in monostable mode include timers, missing pulse detection, bounce free switches, touch switches, frequency divider, capacitance measurement, pulse width modulation (PWM) etc.
RS Flip-Flop
Figure 3 shows one way to build an RS flip-flop. In a circuit like this, one of the transistors is saturated, and the other is cut off. For instance, if the right transistor is saturated, its collector voltage will be approximately zero. This means that there is no base current in the left transistor. As a result, the left transistor is cut off, producing high collector voltage. This high collector voltage produces a large base current that keeps the right transistor in saturation.
The RS flip-flop has two outputs, Q and Q (the output of the left and the right transistor respectively). These are two state outputs, either low or high voltages. Further, the two outputs are always in opposite states. When Q is low, Q is high. When Q is high, Q is low. For this reason Q is called the complement of Q.
The output states can be controlled with the S and R inputs. If we apply a large positive voltage to the S input, we can drive the left transistor into saturation. This will cut off the right transistor. In this case, Q will be high and Q will be low. The high S input can then be removed because the saturated left transistor will keep the right transistor in cutoff.
ASTABLE MULTIVIBRATOR
We now take up the application of 555 timer as an astable multivibrator. An astable multivibrator is a wave-generating circuit in which neither of the output levels is stable. The output keeps on switching between the two unstable states and is a periodic, rectangular waveform. The circuit is therefore known as an ‘astable multivibrator’. Also, no external trigger is required to change the state of the output, hence it is also called ‘free-running multivibrator’. The time for which the output remains in one particular state is determined by the two resistors and a capacitor externally connected to the 555 timer.
THEORY
Figure 4 shows 555 timer connected as an astable multivibrator. Pin 5 is bypassed to ground through a 0.01 μF capacitor. The power supply (+VCC) is connected to common of pin 4 and pin 8 and pin 1 is grounded. If the output is high initially, capacitor C starts charging towards through RA and RB. As soon as the voltage across the capacitor becomes equal toCCVCCV32, the upper comparator triggers the flip-flop, and the output becomes low. The capacitor now starts discharging through RB and transistor Q1. When the voltage across the capacitor becomesCCV31, the output of the lower comparator triggers the flip-flop, and the output becomes high. The cycle then repeats. The output voltage and capacitor voltage waveforms are shown in Figure 5.
MONOSTABLE MULTIVIBRATOR
We now discuss another important application of 555 timer, that is, 555 timer as a monostable multivibrator. A monostable multivibrator is a pulse-generating circuit having one stable and one quasi-stable state. Since there is only one stable state, the circuit is known as ‘monostable multivibrator’. The duration of the output pulse is determined by the RC network connected externally to the 555 timer. The stable state output is approximately zero or at logic-low level. An external trigger pulse forces the output to become high or approximately. After a predetermined length of time, the output automatically switches back to the stable state and remains low until a trigger pulse is again applied. The cycle then repeats. That is, each time a trigger pulse is applied, the circuit produces a single pulse. Hence, it is also called ‘one-shot multivibrator’.
555 TIMER.pdf (Size: 468.21 KB / Downloads: 67)
Introduction
The 555 timer is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. It was produced by Signetics Corporation in early 1970. The original name was the SE555/NE555 and was called "The IC Time Machine". The 555 gets its name from the three 5-KΩ resistors used in typical early implementations. It is widely used because of its ease to use, low price and reliability.
It is one of the most popular and versatile integrated circuits which can be used to build lots of different circuits. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8)(Refer to Figure 1).
The 555 Timer is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillations. The timer basically operates in one of the two modes—monostable (one-shot) multivibrator or as an astable (free-running) multivibrator. In the monostable mode, it can produce accurate time delays from microseconds to hours. In the astable mode, it can produce rectangular waves with a variable duty cycle. Frequently, the 555 is used in astable mode to generate a continuous series of pulses, but you can also use the 555 to make a one-shot or monostable circuit.
The 555 can source or sink 200 mA of output current, and is capable of driving wide range of output devices. The output can drive TTL (Transistor-Transistor Logic) and has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/°C.
Applications of 555 timer in monostable mode include timers, missing pulse detection, bounce free switches, touch switches, frequency divider, capacitance measurement, pulse width modulation (PWM) etc.
RS Flip-Flop
Figure 3 shows one way to build an RS flip-flop. In a circuit like this, one of the transistors is saturated, and the other is cut off. For instance, if the right transistor is saturated, its collector voltage will be approximately zero. This means that there is no base current in the left transistor. As a result, the left transistor is cut off, producing high collector voltage. This high collector voltage produces a large base current that keeps the right transistor in saturation.
The RS flip-flop has two outputs, Q and Q (the output of the left and the right transistor respectively). These are two state outputs, either low or high voltages. Further, the two outputs are always in opposite states. When Q is low, Q is high. When Q is high, Q is low. For this reason Q is called the complement of Q.
The output states can be controlled with the S and R inputs. If we apply a large positive voltage to the S input, we can drive the left transistor into saturation. This will cut off the right transistor. In this case, Q will be high and Q will be low. The high S input can then be removed because the saturated left transistor will keep the right transistor in cutoff.
ASTABLE MULTIVIBRATOR
We now take up the application of 555 timer as an astable multivibrator. An astable multivibrator is a wave-generating circuit in which neither of the output levels is stable. The output keeps on switching between the two unstable states and is a periodic, rectangular waveform. The circuit is therefore known as an ‘astable multivibrator’. Also, no external trigger is required to change the state of the output, hence it is also called ‘free-running multivibrator’. The time for which the output remains in one particular state is determined by the two resistors and a capacitor externally connected to the 555 timer.
THEORY
Figure 4 shows 555 timer connected as an astable multivibrator. Pin 5 is bypassed to ground through a 0.01 μF capacitor. The power supply (+VCC) is connected to common of pin 4 and pin 8 and pin 1 is grounded. If the output is high initially, capacitor C starts charging towards through RA and RB. As soon as the voltage across the capacitor becomes equal toCCVCCV32, the upper comparator triggers the flip-flop, and the output becomes low. The capacitor now starts discharging through RB and transistor Q1. When the voltage across the capacitor becomesCCV31, the output of the lower comparator triggers the flip-flop, and the output becomes high. The cycle then repeats. The output voltage and capacitor voltage waveforms are shown in Figure 5.
MONOSTABLE MULTIVIBRATOR
We now discuss another important application of 555 timer, that is, 555 timer as a monostable multivibrator. A monostable multivibrator is a pulse-generating circuit having one stable and one quasi-stable state. Since there is only one stable state, the circuit is known as ‘monostable multivibrator’. The duration of the output pulse is determined by the RC network connected externally to the 555 timer. The stable state output is approximately zero or at logic-low level. An external trigger pulse forces the output to become high or approximately. After a predetermined length of time, the output automatically switches back to the stable state and remains low until a trigger pulse is again applied. The cycle then repeats. That is, each time a trigger pulse is applied, the circuit produces a single pulse. Hence, it is also called ‘one-shot multivibrator’.