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What is 555 Timer and Its Applications

Warm hints: The word in this article is about 2025 and  reading time is about 20 minutes.  
Guidance: This article introduces 555 timer basics in detail and its applications.

What is 555 Timer

555 clock is a medium-sized coordinated circuit that consolidates simple capacities and rationale capacities. For there are three resistors of 5 kω in the coordinated circuit, it is named as 555 clock. 555 clock has minimal effort and dependable execution. It just needs a few outer resistors and capacitors to frame beat age and transformation circuits, for example, multivibrator, monostable trigger and Schmitt trigger. It is additionally broadly utilized as a clock in instrument, mechanical assembly, family machines, electronic estimations and programmed control.

555 Timer Circuit

555 Timer Circuit Diagram

The circuit of 555 clock comprises of two voltage comparators, three proportionate arrangement resistors, a RS flip-flop, a release tube. It has two reference voltages- – 1/3Vcc and 2/3Vcc.

555 Internal Circuit Diagram
555 Internal Circuit Diagram
  • (1)Comparator – Voltage comparators C1 and C2 are two perfect operational intensifiers of indistinguishable structure. The comparators has two info terminals, U+ and U-individually shows the voltage connected to the relating input terminal, and u c signifies the examination consequence of the comparator. At the point when U+>U-, u c= u h; when U+<U-, Uc=u l.
  • (2)Voltage Divider – Three resistors with an obstruction of 5kω associated in arrangement structure a voltage divider, which gives a reference voltage to comparators C1 and C2. At the point when the terminal CO isn’t utilized in activity, it is for the most part grounded through a capacitor of 0.01μF to sidestep high-recurrence obstruction.
  • (3)Basic RS Flip-Flop – It comprises of two NAND entryways, which is an immediate reset terminal that can be set to 0 from outside. Whenever R=0, Q= 0; when S=1, Q=1.
  • (4)Transistor Switch (release tube) – The transistor TD comprises a switch whose state is constrained by the terminals. Whenever Q=1, the transistor is killed; and when Q = 0, the transistor is turned on.
  • (5)Output Buffer – The yield support is the inverter G3 associated with the yield terminal, which is utilized to improve the heap conveying limit of the clock and the impacts of the disconnected burden on the clock.

Functions of 555 timer Pins

  • Pin 1 ground end. Connect the negative end of the external power supply VSS or ground, generally connected.
  • Pin 2 Trigger (TR)
  • Pin 3 Output terminal Vo (OUT)
  • Pin 4 reset ends. When the terminal is connected to a low level, the time base circuit does not work. At this time, regardless of the TH level, the output of the time-base circuit is “0”. Terminals should be connected to high levels when not in use.
  • Pin 5 voltage control terminal. If the external voltage is connected to the terminal, the reference voltage of the two internal comparators can be changed. When the terminal is not used, it should be connected in series with the capacitor of 0.01 uF to prevent interference.
  • Pin 6 Threshold (TH).
  • Pin 7 discharge terminal (DIS). The terminal is connected to the electrode of the discharge tube group and is used as discharge of capacitors when the timer is used.
  • Pin 8 connects to external power Vcc. The voltage range of Vcc in bipolar time-base circuit is 4.5-16V, and that of Vcc in CMOS time-base circuit is 3-18V. Usually 5V is used.

555 timer working

The function of 555 timer is mainly determined by two comparators. The output voltage of the two comparators controls the states of RS flip-flops and discharge tubes. When the voltage is applied between the power supply and the ground and the pin 5 floats, the voltage at the same phase input end of the voltage comparator C1 is 2/3VCC, and the voltage at the reverse input end of the comparator C2 is 1/3VCC. If the voltage of trigger input Tr is less than 1/3 Vcc, the output of comparator C2 is 0. In this case, the RS trigger can be set to 1 so that the output is 1. If the threshold input th voltage is greater than 2/3vcc and the TR voltage is greater than 1/3vcc, the output of C 1 is 0 and the output of C 2 is 1. In this case, the RS trigger can be set to 0, so that the output is 0 level.

  1. When RD = 0, Q = 0, output voltage Uo = UOL is low, TD saturates and turns on;
  2. When RD = 1, U TH > 2/3Vcc, UTR > 1/3Vcc, the voltage comparator C1 output effective low level, C2 output high level, the basic RS flip-flop terminal R input effective level, so Q = 0, Uo = UOL, TD saturation and open;
  3. When RD = 1, UTH < 2/3Vcc, UTR > 1/3Vcc, C1 and C2 output high levels (both invalid levels), the basic RS flip-flops remain unchanged, so the U and TD remain unchanged in their original state.
  4. When RD = 1, UTH < 2/3Vcc, UTR < 1/3Vcc, C1 output high level, C2 output effective low level, the terminal S of basic RS flip-flop input effective level, so Q = 1, Uo = UoL, TD is cut off;
  5. When RD = 1, U TH > 2/3 Vcc, UTR < 1/3 Vcc, C1 and C2 all output low level. For basic RS flip-flops, their input is limited. At this point Q = 1, Uo = Uoh, TD is cut off. However, when both UTR and UTTH become invalid at the same time, their status can not be determined.
××00Turn on
>2/3Vcc> 1/3Vcc10Turn on
< 2/3Vcc<1/3Vcc11Cut off
< 2/3Vcc> 1/3Vcc1unchangedunchanged

555 Timer Applications

555 timer astable multivibrator

555 timer astable multivibrator
555 timer astable multivibrator

Multivibrator is the fundamental circuit of 555-timer application. It potential that the circuit has no consistentnation (that is, rectangular wave generator) butsolely two transient steady states, whose characteristic is to generate a rectangular wave sign with a positive frequency and amplitude. Its output stateconstantlyswitch between “1” and “0”.

In the voltage-on state, because the voltage on the capacitor C cannot be changed abruptly, the 555 chip is in the set state,Uo=1, discharge tube TD is cutoff (Pin7 disconnected from ground), and Vcc charges capacitor C via R1, R2. When Uc rises, Uo=0 and TD is grew to become on, the voltage of capacitor’s terminal C passes R2 and the discharge tube TD to discharge the ground and then Uc drops. When Uc falls, Uo changes from zero to 1, TD is became off, and Vcc fees C once more through R1 and R2. After the above method is repeated for several times, a continuous rectangular pulse will be generated at the output terminal Uo. Among all the electric poweredcomponents, R1, R2 and C are timing elements which decide the charging and discharging time of the circuit. Tl ≈ 0.7 (Rl + R2) C, T2 ≈ 0.7R2C.

After the power is became on, assuming it is a high level, T will be becameoff and the capacitor C will be charged. The charging circuit is Vcc—R1—R2—C—ground, which rises exponentially. When it rises to 2/3Vcc (TH, the terminal degree is increased than), the output flips to a low level. If V0 is a low level, T is turned on, C is discharged, and the discharge circuit is C-R2—T—ground, which decreases exponentially. When it falls to 1/3Vcc (TH, the terminal stage is much less than), the output is grew to become to a excessive level, and the discharge tube T is grew to become off, the capacitor is recharged again. Therefore this system repeat once more and again and generates oscillation.

After analysis, the obligation cycle of the output high-level time, the output low-level time, and the out put rectangular of the oscillation length dutycycle are as follows. D=tPH/T=(R1+R2)/(R1+2R2). If R1>>R2, Uc is approximately sawtooth wave.

The multivibrator that constituted with the aid of the 555 is reliable in operation and handy to adjust. It has been broadly used in industrial control, timing, sound imitation and different related fields. however its oscillation frequency cannot be too high, normallynot exceeding severalhundred kilohertz. And its frequency balance is poor, and it’s prone to power-supply fluctuations and temperature variation.

555 Timer Monostable Trigger

The 555 clock frames a monostable trigger for timing postponement, molding and some time switches. At the point when the power is turned on, Vcc charges capacitor C through resistor R. At the point when the voltage on the capacitor ascends to VREF1, the comparator C1 inside the 555 clock yields an abnormal state. Since there is no trigger heartbeat right now, the comparator C2 yields low level, ie, RS=10. The fundamental RS flip-flop is reset, V0 yields a low level, and the release tube T28 is turned on, rapidly discharging the charge on the capacitor C , so that the comparator C1 yields a low level. Right now, RS =00, the essential RS flip-flop is in the stop mode, so the yield is never again changed and the circuit goes into an unfaltering state.

On the off chance that a trigger heartbeat is connected to the info terminal V1 of the monostable trigger, when the falling edge of the trigger heartbeat comes, since the electric capability of the 2 stick is lower than VREF2, the comparator C2 yields an abnormal state. Right now, RS=01, the essential RS flip-flop is Set. Q yields a low level, the circuit starts to enter the transient unfaltering state, yielding an abnormal state V0 , and the release tube T28 is cut off. Vcc begins to charge capacitor C through resistor R. The voltage Vc on the capacitor rises exponentially. At the point when Vc ascends to VREF1, comparator C1 yields an abnormal state. Since the outside trigger heartbeat has been fixed, comparator C2 yields a low level,that is, RS=10. The essential RS flip-flop is reset, finishing the transient enduring state, the circuit naturally comes back to the underlying consistent state. V0 turns into a low level, and the release tube T28 is turned on. This circuit produces beat with widths from a couple of microseconds to minutes, and its exactness is 1℅. The information terminal of the control voltage (Pin 5) is grounded through a capacitor of 0.01μF to forestall beat impedance.

555 timer Schmitt Trigger

555 timer  Schmitt Trigger
555 timer Schmitt Trigger

Schmidt flip-flop is one of the common circuits in digital system. They can convert slowly changing pulse waveforms into rectangular pulses required by digital circuits. Schmidt circuit has two kinds of stable states, but unlike ordinary flip-flops, the transformation and maintenance of the two kinds of stable states require external trigger signals, so its triggering method is horizontal trigger.

The circuit and waveform of Schmidt flip-flop are shown in Fig. 4. The hysteresis voltage is 1/3 VCC. If the adjustable voltage VCO (1.5-5V) is externally connected to the voltage control terminal_, the side-gap voltage can be changed, and the Schmidt flip-flop can easily convert triangular wave into square wave.

When the input signal UI < 1/3vcc, the basic RS trigger is set to 1, i.e. image = 0, q = 1, and the output Uo is high; when the input signal UI < 1/3vcc < UI < 2/3vcc, the circuit remains unchanged and the output Uo is still high; when the input signal UI < 2/3vcc, the RS trigger is set to 0, i.e. q = 0, image = 1, and the output Uo is low; if the input signal UI < 1 < UI < 2/3vcc, the output Uo is low again. With Ui, as long as Ui is more than 2/3 VCC, the circuit remains in state. If the UI decreases, as long as 1/3vcc < UI < 2/3vcc, the circuit state remains unchanged; when UI = 1/3vcc, the trigger will be set again, and the circuit output will return to high level. Schmitt Trigger usually uses in Waveform transformation, Pulse wave shaping or Pulse discrimination .


11 thoughts on “What is 555 Timer and Its Applications

  1. Very comprehensive information about 555 timer

  2. I found a 555 timer FM modulator circuit on the Internet. But in order to make it an effective FM transmitter, what I have to do is connect the antenna to the output modulation signal? Or do I have to tune a series of LC resonant circuits to carrier frequencies? Are all 555 timers operated the same?

  3. Good day, sir
    I am drawing a 555 time circuit~

  4. What do you think of 555 timer?

  5. I’ve used PartSim for discrete circuits, but when I wanted to add a 555 timer chip, there was nothing available in the component list. I read about creating my own project model from spice text, and I tried, but it didn’t make sense to me. Can we use 555 chip to make circuit?


  6. Good day, sir
    I am just making a 555 timer to a circuit with a 16×2 LCD display successfully.
    This is a valuaful article, I will suggest it to my students.

    1. Congratulations!

  7. Hello~ sir
    I just want to ask that: Why is it that op-amps used as ICs, such as LM741 require a +Vcc and -Vcc for biasing whereas there is no -Vcc for a 555 timer?

    1. The 555 uses two comparators with a string of three 5k+5K+5k resistors to create precise ratios of 1/3 *, 2/3 * Vcc, the supply voltage, Vcc ( c=collector ) with 0V as Vee for ease of single supply operation.

  8. I am trying to connect the 555 timer to a pulse modulator. Notice in the figure below that the input of – Comp1 is internally connected to 2/3 VCC. The control voltage is applied to pin 5. The problem is how to apply the varying voltage to a point that has been set to a specific voltage. Wouldn’t that lead to a short time?

  9. Oh~
    I just make a troubles about 555 Timer PWM generator….

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