LM555 and LM556 Timer Circuits

  This page presents general information and some useful tips for using the LM555 timer. If you would like to use any of these ideas please take the time to do some experimenting before using them in an actual circuit. All of the solutions on this page can be applied to the LM556 Dual timer chip as well.

  Some of these circuits were developed just to see if the concept would work and have no intended purpose.

  The menu below will take you to various sections of this page that relate to the items in the menu. New additions appear at the bottom of the list.

  1. Calculation Value Note
  2. RESET And CONTROL Input Terminal Notes
  3. LM555 - Monostable Oscillator Calculator
  4. LM555 - Astable Oscillator Calculator
  5. Basic Circuits For The LM555 Timer
  6. Triggering And Timing Helpers For Monostable Timers
  7. Controlling Circuits For LM555 Timers
  8. Advanced Circuits For The LM555 Timer
  9. LM556 Timers with Complimentary or Push-Pull Outputs
  10. Interlocked Monostable Timers
  11. Power-Up Reset For Monostable Timers
  12. Cross Canceling For Monostable Timers
  13. RS - Flip-Flop Made With A LM556 Timer
  14. Using The LM555 As A Voltage Comparator
  15. LM555 With A 50% Duty Cycle (Adjustable)
  16. Bipolar LED Driver
  17. Electronic Time Constant Control
  18. Voltage Controlled Pulse Width Oscillator
  19. Sweeping Output Frequency Siren
  20. D - Flip-Flop Made With A LM556 Timer
  21. Various LM555 Time delay Circuits
  22. Variable Period Oscillator (CD4017)
  23. Missing Pulse Detectors Circuits
  24. Fixed 50% Output Duty Cycle Using Logic Devices
  25. Three Stage Cycling Timer Circuit
  26. Fixed 50% Output Duty Cycle Using 556 D-Flip_Flop
  27. RESET Terminal - Currents And Voltages
  28. 555 Timer Current Draws
  • Special Function LM555 Circuits (External Page)
  • Various LM555 - LED Flasher Circuits (External Page)

    LM555 Data sheet - National Semiconductor (.pdf)

    LM556 Data sheet - National Semiconductor (.pdf)

    LM555 Timer tutorial - By Tony van Roon

    LM555 Timer Internal Circuit Block Diagram



    Calculation Value Note

      Data sheets for the 555 Timer use the value 1.44 and 0.693 as constants in the timing calculations depending on the way in which the equation was written. The value 0.693 may not be quite accurate as it is not an accurate reciprocal of 1.44. The case may be just the opposite however which would mean that 0.693 is correct and this would make its reciprocal 1.443.

      This page will continue to use the values given in the data sheets until a determination as to what is the correct value of this number, 0.6930 as given or 0.6944 as calculated.


    "RESET" And "CONTROL" Input Terminal Notes

      Most of the circuits at this web site that use the LM555 and LM556 timer chips do not show any connections for the "RESET" and "CONTROL" inputs for these devices. This was done in order to keep the schematics as simple as possible.

      When the "RESET" terminal is not going to be used it is normal practice to connect this input to the supply voltage. This is especially true when the CMOS version of these timers is used as the inputs of these devices are very sensitive.

      In many cases the "CONTROL" input does not require a bypass capacitor when a well regulated power supply is used. It is good practice however to place a 0.1 microfarad or larger capacitor at this terminal.


    TIMING CALCULATORS FOR THE LM555

    With Schematic diagrams
    LM555 - MONOSTABLE OSCILLATOR CALCULATOR

    Value Of
    R1

    Ohms
    Value Of
    C1

    Microfarads
    Output
    Pulse

    Seconds
    Resistor values are in Ohms - Capacitor values are in Microfarads
    A browser capable of running Javascript is required for these calculators.

    LM555 Monostable Oscillator Circuit Diagram

    LM555 Monostable Oscillator Output Time Chart

    RESET And CONTROL Input Terminal Notes

    LM555 - ASTABLE OSCILLATOR CALCULATOR

    Value Of
    R1

    Ohms
    Value Of
    R2

    Ohms
    Value Of
    C1

    Microfarads
    Output Time
    HIGH

    SECONDS
    Output Time
    LOW

    SECONDS
    Output Period
    HIGH + LOW

    SECONDS
    Output
    Frequency

    HERTZ
    Output
    Duty Cycle

    PERCENT
    Resistor values are in Ohms - Capacitor values are in Microfarads
    A browser capable of running Javascript is required for these calculators.

    LM555 Astable Oscillator Circuit Diagram

    LM555 Astable Oscillator Free Running Frequency Chart

    RESET And CONTROL Input Terminal Notes

    Basic Circuits For The LM555 Timer

      The following diagram shows some basic circuits and calculations for the LM555 timer.

    Basic LM555 Timer Schematics

    RESET And CONTROL Input Terminal Notes

    Triggering And Timing Helpers For Monostable Timers

      The venerable LM555 timer chip and its twin brothers the LM556 have been a cornerstones of model railroad electronics but the sensitivity of the trigger input gives rise to many false triggering problems. The addition of a 470K ohm resistor and a 0.1uF capacitor at the trigger input (Pin 2) will provide a time delay of 1/20th of a second from the time the input goes to zero volts until the trigger threshold is reached (1/3Vcc). This can eliminate false triggering in most cases and if the problem persists the size of the capacitor can be increased.

      The following schematic shows two additions to the basic 555 timer circuit. One reduces the trigger sensitivity and the other will double the output pulse duration without increasing the R1 and C1 values.

    555 Timer Helpers Schematic

      The addition of a capacitor to the trigger will not work for short output pulses as there is also a short delay in the recovery of the trigger terminal voltage.

      The second 555 timer helper will extend the timers output duration without having to use large values of R1 and/or C1. By connecting a 1.8K ohm resistor between the supply voltage and pin 5 of the 555 timer chip the output pulse duration will approximately be doubled.

      To achieve long output times electrolytic capacitors are used for C1 and the value of R1 may be as high as 1 Meg. However with high resistance values for R1 the leakage current of the timing capacitor (C1) becomes a significant factor in the operation of the timer.

      The circuit will run much longer than expected and may never time out if the leakage current is equal to the current through the resistor at some voltage. Tantalum capacitors could be used as they have very low leakage currents but these are expensive and not available in large capacitance values.

      The boxed in area of the drawing shows the internal circuit at pin 5 of the timer with the 1.8K resistor added. The voltage at pin 5 will be increased from 0.66Vcc to 0.84Vcc which is equal to the voltage across the capacitor after two time constants. This allows the same output time to be achieved with a smaller resistance or capacitance value thus reducing the error caused by the capacitor leakage current. Conversely, for a given value of R1 and C1 the output time will be doubled. (One time constant is equal to R1 times C1).

      The trigger voltage level of the timer will also be increased with the addition of the resistor to pin 5 but this should have no effect for most applications.

      This is not an ideal solution to solving long duration timing situations but will be OK for times of less than five minutes.

    RESET And CONTROL Input Terminal Notes

    Controlling Circuits For LM555 Timers

      The following diagrams show some methods of using one timer to control a second timer. Some of these are unusual but still practical and can provide ideas for other control schemes.

      In the following diagrams a ONESHOT oscillator controls an ASTABLE oscillator. Three methods are shown.

    LM555 Control methods #1 schematic

    RESET And CONTROL Input Terminal Notes


    Advanced Circuits For The LM555 Timer

      The following diagrams show some advanced circuits for the LM555 timer. These circuits were developed to provide certain functions that are not usually associated with this device.

      The parts values in these circuits were selected for testing purposes and can be adjusted to suit the needs of a particular application as long as the normal operating parameters of the LM555 are maintained.

      These circuits should be viewed as experimental. Before using any of them for specific applications they should be tested to determine the best values for the components and the practicality of their use.

    RESET And CONTROL Input Terminal Notes


    LM556 Timers with Complimentary or Push-Pull Outputs

      In the next circuit an LM556 - dual timer IC is configured so that the output of the second timer is 180 degrees out of phase with the first.

      This is done by connecting the OUTPUT of the "A" timer to the TRIGGER and THRESHOLD terminals of the "B" timer. The 10K ohm resistor in this connection limits the current that can flow into the THRESHOLD terminal of the "B" timer.

      Due to the current source or sink capability of the timer outputs the current from one timer's output can flow in to the other timers output depending on which output is HIGH or LOW. The usual outputs that are referenced to ground or supply are also available and in fact all three could be used at the same time.

      Circuits for both Astable and Monostable versions of this method are shown on the diagram.

    LM555 Complimentary Outputs schematic

      Timer "B" in this method acts as a voltage comparator and has no timing function. It is a slave to the "A" timer.

      Normal triggering schemes and timing lengths are not affected by this method.

      The timer RESET terminals are available and can be used individually or together if desired.

      Due to the unusual nature of this type of circuit testing should be done to determine if it is suitable for the use intended. The circuit is usable at frequencies below 1000Hz.

    RESET And CONTROL Input Terminal Notes

    Interlocked Monostable Timers

      In the following circuit the timers are "Interlocked" so that when one timer is running the second timer cannot be triggered.

      This is done by connecting the OUTPUT of each timer to the TRIGGER of the other via a diode and placing a resistor in the trigger circuit. The resistor limits the current that can flow from the opposite timers output when the trigger is closed on the stopped timer.

    LM555 Interlocked Timers schematic

      Normal triggering and timing lengths are not affected by this method.

    RESET And CONTROL Input Terminal Notes

    Power-Up Reset For Monostable Timers

      If there is any stray capacitance at the TRIGGER terminal when the power is applied to an LM555 circuit the timer will immediately be triggered and start a cycle. This can be a undesirable if the time is long and there is no other way to stop the cycle.

      The stray capacitance can be from a number of sources but the most likely cause is wires that lead to a push button used to start the timer.

      To prevent timer from starting, a resistor and capacitor have been added to the THRESHOLD terminal. When the power is applied to the circuit the timer is automatically RESET by the charge flowing through the 0.1 microfarad capacitor.

      As long as the capacitance at the THRESHOLD terminal is much larger than the capacitance at the TRIGGER this method should work correctly.

      When power is applied to the circuit the output of the LM555 will stay LOW except for a very brief pulse.

    LM555 Power-Up Reset schematic

      Normal triggering and timing lengths should not be affected by this method unless a unregulated power supply is used.

    RESET And CONTROL Input Terminal Notes

    Cross Canceling For Monostable Timers

      The following diagram shows a method that allows one LM555 timer to RESET another timer so that, for example, if timer 'A' is running; When timer 'B' is activated the 'A' timer is reset.

      This means that only one timer can be running at any time.

      As with the 'Power-Up Reset For Monostable Timers' circuit above, when the power is applied to the circuit both timers are RESET.

    LM555 Cross Canceling Timers schematic

      Normal triggering and timing lengths should not be affected by this method.

      The trigger switch of the running timer must be OPEN for the RESET to occur.

    RESET And CONTROL Input Terminal Notes


    RS - Flip-Flop Made With A LM556 Timer

      The circuit on this page is for a hybrid - SET / RESET type of logic Flip-Flop that is constructed from an LM556 - Dual Timer integrated circuit.

      The design is crude but effective for very low speed applications. Its greatest asset is that the outputs of the LM556 are capable of driving current loads of up to 200 milliamps with a minimal voltage loss.

      This circuit was originally developed to drive "Stall Motor" type switch machines that are used on model railroads. These motors use low voltage DC and pass approximately 15 milliamps when they are in a stalled condition.

      Due to the design of the LM556 timer chip there are multiple output options available in this design. These include the normal timer outputs which are bipolar and the 'DISCHARGE' terminals, (PINS 1 and 13), that are open collector circuits.


    LM556 Flip-Flop Truth Table

      The following diagram is for a testing version of the circuit used to create a "Truth Table" that shows the OUTPUT states for a given INPUT state.

    Logic Function diagram


    LM556 Flip-Flop Input Options

      The next diagram shows the basic input options that can be used with the LM556 Flip-Flop circuit. In actual applications the push buttons would be replaced by or supplemented with electronic input devices.

    Input Options schematic

      In CIRCUIT 'A' the SET and RESET inputs would be brought to '0' Volts to change the state of the Flip-Flop.

      In CIRCUIT 'B' the SET input would be switched between '0' Volts and the supply voltage to change the state of the Flip-Flop. The RESET terminal is unconnected.

      In both CIRCUITS 'A' and 'B' when the push buttons are 'OPEN' the Flip-Flop will remain in its last state until another INPUT signal is applied.

      Circuits 'A' and 'B' also show two methods of connecting the LED's at terminals 1 and 3. The method in circuit 'B' would not be practical for the STATE '3' condition shown in the "Truth Table".



    LM556 Flip-Flop Notes

    RESET And CONTROL Input Terminal Notes


    LM555 Timer Used As A Voltage Comparator

      The next section shows how the LM555 timer could be used as a voltage comparator. An application for which it is not particularly well suited but one that is in wide use with model railroaders.

      Shown on the schematic is a secondary output that uses the open collector at the DISCHARGE terminal (Pin 7) of the timer. This output can sink up to 200 milliamps and would be ideal for driving relays.

      The main disadvantage to using this circuit is the the large dead-band (1/3Vcc) between upper and lower threshold voltages. An optional resistor, R5, can be added to the circuit to lower and compress the detection voltage range but this only partially alleviates the problem.

    LM555 Voltage Comparator schematic

      The two graphs at the bottom of the diagram show the input voltages at which the OUTPUT of the LM555 will change states. The effect that resistor R5 has on the circuit can be seen in the right hand graph.

    RESET And CONTROL Input Terminal Notes


    LM555 With A 50% Duty Cycle (Adjustable)

      The LM555 timer can achieve a 50 percent duty cycle as shown in the next diagram. The duty cycle adjustment range of this circuit is from 42 to 55 percent.

      Resistors R1 and R2 were selected first and then resistor R3 was selected to give the best control range based on measurements at the output of the timer.

      The major disadvantage of using the LM555 for this application is that the output frequency changes as the duty cycle changes.

    50% Duty Cycle schematic

    RESET And CONTROL Input Terminal Notes


    Bipolar LED Driver

      This circuit uses two timers to drive Bipolar LEDs and give all of the possible output states.

      Two SPDT switches are used to set the input conditions but these could be replaced by electronic controls if desired.

    Bipolar LED Driver schematic

    RESET And CONTROL Input Terminal Notes


    Electronic Time Constant Control

      These circuits show methods of changing the operating frequency of astable LM555 timers electronically. Any source that can drive the base of transistor Q1 can control these circuits.

      The advantage of using this type of frequency control is that the duty cycle of the timer is not affected when the frequency is changed.

    Electronic Time Constant Control

    RESET And CONTROL Input Terminal Notes


    Voltage Controlled Pulse Width Oscillator

      The basic circuit operates at a frequency determined by R1, R2 and C1 and has a pulse width range of 0 to 100 percent.

      The following diagram shows a basic circuit with an open collector output that would require a pull up resistor at its output. The parts values are the nominal values of the components used.

      Note: This circuit is not suitable for high frequency operation, especially when using a second timer as the output stage.

    Variable Pulse Width Oscillator

      The following is a graph of the output pulse width of the basic circuit for a given control voltage input. All measurements were made with a good quality multimeter.

      The PLUS and MINUS inputs of IC 2 can be reversed to produce a decreasing pulse width for an increasing control voltage.

    Variable Pulse Width Oscillator Output Graph

      The next diagram uses a second LM555 timer as a power output stage for the basic oscillator. The output stage also has an open collector output at the Discharge terminal, PIN 7, that could be used.

    Variable Pulse Width Oscillator With LM555 Output

    RESET And CONTROL Input Terminal Notes


    Sweeping Output Frequency Siren

      This circuit is a variation of the Two Tone siren that is a standard for the LM555 timer. The circuit allows the output frequency of the 'B' timer to sweep between to frequencies rather than switching between two fixed frequencies.

    Sweeping Output Frequency Siren

    RESET And CONTROL Input Terminal Notes


    D - Flip-Flop Made With A LM556 Timer

      The circuit on this page is for a hybrid - D type Flip-Flop that is constructed from an LM556 - Dual Timer integrated circuit. The circuit is essentially a High-Tech version of the classic transistor flip-flop.

      Each time the push button switch (S1) is closed the outputs of the timers will reverse so that one is HIGH and the other is LOW and vice versa.

      The circuit has some output switching time lag due to the RC time constants at the inputs and the different Trigger and Threshold voltage levels of the timers themselves.

      This circuit is not very useful but would make a good Push On / Push Off switch circuit and has a reasonably high sink or source output current level.

    D - Flip-Flop

    RESET And CONTROL Input Terminal Notes


    Various LM555 Time Delay Circuits

    Timer Recovery Delay Circuits

    Two Stage Time Delay Circuit

    RESET And CONTROL Input Terminal Notes


    Variable period Oscillator (CD4017)

      The following CD4017 circuits have not been tested and is presented here as a possibility only. If you experiment with this circuit please send me any problems found so that the circuit can be updated.

      The following circuits are designed to change the duration of each positive output pulse from the astable timer. The circuits use a CD4017 Decade Counter / Decoder to provide nine or ten steps in the cycle.

      The first circuit operates with a repeating ten step cycle. Each output pulse is longer than the previous until a count of ten is reached at which time the cycle will repeat.

      The second circuit has a nine step cycle that stops at the end of the cycle. The cycle is restarted or reset when the RESET input is briefly made high.

      The CD4017 can be configured to give count lengths between 1 and 10. Refer to the timing diagram in the CD4017 data sheet for a better understanding of the circuit's operation.

    CD4017 Data sheet - National Semiconductor (.pdf)

    Variable Period Oscillator (Experimental)


      The next schematic shows an alternate arrangement of the timing resistors. This would allow the output pulse to be of longer and shorter lengths during the cycle.

    Alternate Resistor Arrangement

      The next circuit provides nine counts of a normal timing length with the tenth count being longer and then repeating the cycle.

    Ten Step / Two Period Oscillator

    RESET And CONTROL Input Terminal Notes


    Missing Pulse Detector Circuits

    Basic - Missing Pulse Detectors

    Steady Output - Missing Pulse Detectors - Two Comparators

    Steady Output - Missing Pulse Detectors - Two Timers

    Latching Output - Missing Pulse Detector

    RESET And CONTROL Input Terminal Notes


    Fixed 50% Output Duty Cycle Using Logic Devices

    Fixed 50% Output Duty Cycle

    RESET And CONTROL Input Terminal Notes


    Three Stage Cycling Timer Circuit

    Three Stage Cycling Timer Circuit

    RESET And CONTROL Input Terminal Notes


    Fixed 50% Output Duty Cycle Using 556 D-Flip_Flop

    Fixed 50% Output Duty Cycle Using 556 D-Flip_Flop

    RESET And CONTROL Input Terminal Notes


    Devices Used For The Following Tests


    RESET Terminal - Currents And Voltages

      The next diagram gives the current from and the voltage at the RESET terminals of five 555 timer chips from different manufacturers.

      The only conclusion to be drawn here is that the RESET terminal should be held below 0.3 Volts to ensure that any of the devices is fully reset.

      In the transition voltage range of the RESET terminal mentioned on the diagram, the timers output is neither fully ON or OFF. This can cause high current flows in the timer itself. The voltage at the RESET terminal should pass through this range as quickly as possible to avoid problems.

    RESET Terminal - Currents And Voltages

    RESET And CONTROL Input Terminal Notes

    555 Timer Current Draws

      The next diagram shows the basic current usages of five 555 timer chips from different manufacturers.

      The RESET terminal current draw illustrates the need for a current limiting resistor as shown in some of the preceding circuits. Some devices will not function properly if the current to the RESET terminal is not limited.

    Timer Current Draws

    RESET And CONTROL Input Terminal Notes

    Return to the Main Circuit Index


    Please Read Before Using These Circuit Ideas

      The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

      If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

      Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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