555 Timer- Use In Electronic Circuits

By Author: Jason John
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There are a lot of circuits using 555 Timer IC. To know more about it, the block diagram must be explained in detail. A basic 555 timer consists of two operational amplifiers a set-reset flip-flop and two transistors. The operational amplifiers are designed to work as comparators. There will also be a voltage divider circuit which is built with the help of a resistive network. Each comparator is used to compare the threshold voltage with a reference voltage. The first comparator COMP 1 compares with a reference voltage of +2/3 Vcc. The second comparator COMP2 compares with a reference voltage of +1/3 Vcc. After comparing the voltages, their respective outputs are given to the input of the flip-flop.

From the pin configuration of a 555 timer you can see that a transistor is placed as a discharger whose collector terminal is connected to pin 7. The output of the flip flop is given to this transistor. According to the output, the transistor switches to its cut-off region or saturation region.

When the transistor is in the saturation stage, it provides a discharge path to an externally connected capacitor. Another ...
... separate connection is regarding the RESET terminal. A pulse given to this terminal will reset the whole 555 timer circuit. This pulse is provided with the help of another transistor.

The COMP 1 includes two inputs - A threshold input [pin 6] and a control input [pin 5]. Mostly, the pin 5 is not used and hence the control voltage will be equal to +2/3 Vcc. This pin is mostly used to change the switching levels. If not used, a capacitor of very small value is placed in between pin 5 and the ground. This will help in stopping the noise and thus stop false triggering. The capacitor value will be of 0.01 nano farad. The output of COMP1 is given to the SET [S] input of the S-R Flip-Flop. When comparing the voltages, if the reference voltage is less than the threshold voltage, the output will go to a high state. This high state causes the transistor to discharge through the external capacitor at pin 7. Pin 3 is the output, which receives the complimentary signal from the flip-flop. It will thus be in the low state. All these conditions will be in a standby as long as the COMP 2 triggers the flip-flop. Another point to note is that COMP 1 can only help in providing a high output for the flip-flop
. Even if the threshold voltage is lesser than the reference voltage, COMP 1 will not set the flip-flop to change again. For the flip-flop to reset, the threshold voltage must fall below +1/3 Vcc. As soon as this occurs, COMP 2 provides a trigger in the flip-flop and thus forces it to provide a low output. This in turn produces a HIGH output at the output pin because the reset of the flip-flop causes the discharge transistor to turn off. Like the COMP 1, the COMP 2 can only make the flip-flop to go to a low output. It cannot set the state again.

Thus 555 timer basics are that a low output is produced in the 555 timer when the threshold voltage is more than the reference voltage of +2/3 Vcc. This causes the discharge transistor to turn ON. To make the output high, the threshold voltage must fall below +1/3 Vcc. This will cause the discharge transistor to go in the OFF mode. You must also note that when the RESET pin is not used, it must be circuited to the Vcc.

A lot of electronic circuits using the application of 555 timer are being designed nowadays. Some of the applications include:
555 timer astable multivibrator
555 timer monostable multivibrator
555 timer oscillator circuit
555 timer ramp generator

Though they are used in other electronic circuits, they are mostly used to perform any of the operations mentioned above.

555 Timer Astable Multivibrator:
The astable multivibrator is used to produce a rectangular wave. As this circuit does not need an external trigger for switching the output state, it is also called a free running oscillator. The connection must be given exactly as the circuit diagram. That is, the pin 4 and pin 8 should be connected to the + Vcc supply. Pin 1 should be grounded. The output must be taken from pin 3. Also pin 2 and pin 6 should be connected to a capacitor and then given to the ground. Most important of all is the resistors and capacitor that is connected externally. Their values determine the switching time of the 555 timer.

555 Timer Monostable Multivibrator:
To obtain a monostable multivibrator, there should be a change of state from stable to quasi-stable. For this, a trigger input like a very small width negative pulse must be given with an amplitude that is greater than +2/3 Vcc to the 2nd pin of the 555 timer IC. But in quiescent condition of the output, the input must be kept at Vcc volts. Pin 1 should be in the grounded state. As usual do not forget to ground the 5th pin with a 0.01 micro farad capacitor in between. A resistor should be given between the pins 6 and 8. The 8th pin is connected to the positive supply.

555 Timer Oscillator Circuit:
This electronic circuits is generally known as a voltage to frequency converter. It is called so because the frequency obtained in the output can be easily changed by changing the voltage in the input. To make this possible the 5th pin of the 555 timer is given an external voltage with the help of a potentiometer. Thus a change in the pot value will change the control voltage of the circuit. When the control voltage increases, the discharge time of the capacitor increases and hence the output frequency decreases. When control voltage is reduced, the capacitor discharges soon and thus the frequency in the output increases.

555 Timer Ramp Generator:
As the name specifies, this circuit is used to generate a ramp circuit. The basic idea is to charge a capacitor with the help of a voltage source through a resistor. This will produce an exponential waveform. At the same time the capacitor is charged from a constant current source which helps in producing a ramp. Another important point to note is about the transistor that is used. Other than other electronic circuits using 555 timer, where a NPN transistor is used, here a PNP transistor is used. This is what helps in making a constant charging current.