LM555 and LM556 Timer Circuits

  This page presents general information and tips for using the LM555 timer and devices with other letter prefixes. There will be minor internal circuitry differences between 555 timer IC's from the various manufacturers but they all should be useable for the circuits on this page.

  If you would like to use any of these ideas, take the time to do some testing before using the LM555 timer in an actual circuit. All of the solutions on this page can also be applied to the LM556 - Dual timer.

  Some of the circuits on this page were developed just to see if they would work and have no intended use.

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

- -   TIMER PAGE SECTIONS   - -

  1. RESET And CONTROL Input Terminal Notes

  2. LM555 - Monostable Oscillator Calculator

  3. LM555 - Astable Oscillator Calculator + Capacitor Calculator

  4. Basic Circuits For The LM555 Timer

  5. Triggering And Timing Helpers For Monostable Timers

  6. Controlling Circuits For LM555 Timers

  7. Advanced Circuits For The LM555 Timer

  8. LM556 Timers with Complimentary or Push-Pull Outputs

  9. Interlocked Monostable Timers

  10. Power-Up Reset For Monostable Timers

  11. Cross Canceling For Monostable Timers

  12. Reset/Set - Flip-Flop Made With A LM556 Timer

  13. Using The LM555 As A Voltage Comparator Or Schmitt Trigger

  14. 50% Output Duty Cycle (Variable)

  15. Bipolar LED Driver

  16. Electronic Time Constant Control

  17. Voltage Controlled Pulse Width Oscillator

  18. Sweeping Output Siren

  19. D Type Flip-Flop Made With A LM556 Timer

  20. Time Delay Circuits

  21. Variable Period Oscillator (CD4017)

  22. Missing Pulse Detectors / Negative Recovery Circuits

  23. 50% Output Duty Cycle (Fixed) Using Logic Devices

  24. Three Stage Cycling Timer Circuit (Traffic Light Circuit)

  25. RESET Terminal - Currents And Voltages

  26. 555 Timer Current Draws

  27. Delayed Re-Triggering

  28. 555 Timer Output Section

  29. Power ON Delay Circuits

  30. Power OFF Delay Circuits

  31. Average 51.5 % Output Duty Cycle Using A 555 Timer

  32. Driving Loads Of Greater Than 15 Volts Or 200 Milliamps

  33. 'N' Steps And Stop Circuit (CD4017)

  34. Multiple - Monostable Trigger Inputs

  35. Timer Output Voltage Losses

  36. RC Delayed Timer Triggering

  37. Threshold Terminal Current Limiting

  38. Fixed Frequency / Variable Duty Cycle Oscillator


- Other Related Pages -

  • Special Function LM555 Circuits
  • Various LM555 - LED Flasher Circuits
  • Astable Multivibrator Applet (External Page - Java Script)
  • 555 Timer IC (External Page - Wikipedia.org)
  • LM555 Data sheet - National Semiconductor (.pdf)
  • CMOS LM555 Data sheet - National Semiconductor (.pdf)
  • LM556 Data sheet - National Semiconductor (.pdf)
  • LM555 Timer tutorial - By Tony van Roon
  • The Electronics Club - 555 and 556 Timer Circuits

    CMOS Versions Of The 555 Timer

      All of the information on this page can be applied to the low current, CMOS versions of the 555 timer as well.

      However, the CMOS versions have a lower output current rating and may not be able to drive some loads. Also, the outputs of some CMOS timers can source more current than they can sink.

      For single sided loads, an NPN or PNP driver transistor can be added to the output of the timer to increase the current capacity of the timer. ( See section 31 of this page for more information. )


    This Page Is Not Applicable To The LM558

      This page does not apply the LM558 - Quad Timer IC which is significantly different when compared to the 555 and 556 timers.

      The differences include: (1) The output of each 558 timer is an open collector transistor with a 100 milliamp current capacity while the 555 and 556 timers have bipolar outputs with a 200 milliamp capacity. (2) The TRIGGER input of the 558 is EDGE Triggered while the TRIGGER input of the 555 and 556 timers are LEVEL Triggered.

      Individual LM558 timers are not designed to operate in an astable mode. Two 558 timers must be connected in a loop to make an astable oscillator.

      The THRESHOLD input terminals for the 555, 556 and 558 timers are all LEVEL triggered.


    LM555 Timer Internal Circuit Block Diagram

    LM555 Timer Internal Circuit Block Diagram

      Print the diagram in the centre of a sheet of paper and then draw a circuit using the ICs pin locations.

    Example LM555 circuit.

    LM556 Timer Internal Circuit Block Diagram

      Print the diagram in the centre of a sheet of paper and then draw a circuit using the ICs pin locations.


    RESET And CONTROL Terminal Notes

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

      If the RESET terminal of a 555 or 556 timer is not going to be used, it is normal practice to connect this input to the supply voltage. If the RESET terminal is left unconnected the operation of the timer will not be affected, however, the RESET of CMOS version of these timers should not be left unconnected as the inputs of these devices are more sensitive and this may cause problems.

      In many cases the CONTROL input does not require a bypass capacitor if a well regulated power supply is used. However, it is good practice to place a 0.1 microfarad (C2) capacitor at this terminal to minimize voltage spikes during transitions of the timer's output transistors.

      It is also good practice to place a 0.1uF bypass capacitor (C1) across the power supply and located as close to the IC as possible. This will also reduce voltage spikes when the output transistors of the timer change states.

    Typical Pin 4 And 5 Connections

      Note - If the period of the power supply variations is short when compared to the period of the timer, the overall effect of C2 is reduced.

      For example; If the power supply - ripple voltage is 120 Hz and the oscillator frequency is 1000 Hz then C2 will have greater benefit than if the oscillator frequency is 10 Hz.

      Therefore, at low astable frequencies or long monostable times the effectiveness of a capacitor at the CONTROL input is less than at higher frequencies and short pulse times.


    Calculation Value Notes

      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. While these numbers are not exact reciprocals of one another they are close enough to be used without concern.

      For ease of use, the calculators on this page have capacitor values entered in microfarads. This value is multiplied by the calculator to produce the correct result. (1uF = 0.000,001F = 1-6F)


    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 (1K = 1000) - Capacitor values are in Microfarads (1uF = 1)

      NOTE: The leakage currents of electrolytic capacitors will affect the actual output results of the timers. To compensate for leakage it is often better to use a higher value capacitor and lower value resistors in the timer circuits.

    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 (1K = 1000) - Capacitor values are in Microfarads (1uF = 1)

      NOTE: The leakage currents of electrolytic capacitors will affect the actual output results of the timers. To compensate for leakage it is often better to use a higher value capacitor and lower value resistors in the timer circuits.

    LM555 Astable Oscillator Circuit Diagram


      The next calculator can find the capacitance needed for a particular output frequency if the values of R1 and R2 are known.

    LM555 - ASTABLE CAPACITOR CALCULATOR

    Value Of
    R1

    Ohms
    Value Of
    R2

    Ohms
    Frequency
    Desired

    Hertz
    Capacitance
    uF

    LM555 Astable Oscillator - Free Running Frequency Chart


    RESET And CONTROL Input Terminal Notes

    Basic Circuits For The LM555 Timer

      The following diagrams show some basic circuits and calculations for the LM555 timer.

    Circuit 1

    Circuit 2

    Circuit 3

    Circuit 4

    Circuit 5

      Circuit 5 also has a trigger input that can remain closed and still allow the timer to complete its cycle. This means that the trigger input pulse can be longer than the output pulse.


    RESET And CONTROL Input Terminal Notes

    Triggering And Timing Helpers For Monostable Timers

      The LM555 timer and its twin brothers the LM556 are 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 delay of approximately 1/20th of a second from the time the input goes to zero volts until the trigger threshold of 1/3Vcc is reached. This short delay can eliminate false triggering in most cases and if the problem persists the value of the capacitor or resistor can be increased as needed.

      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 values of R1 and C1.

    555 Timer Helpers Schematic

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

      The value of the 0.1uF capacitor at the trigger terminal can be made larger to further delay the triggering of the timer when the input goes LOW. Other values can be used in place of the 470K resistor as well.

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

      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 0.88Vcc which is approximately 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 by the addition of the resistor at Pin 5.

      * - One time constant is equal to R (Ohms) times C (Farads) in seconds. In terms of voltage, one time constant is equal to a rise in voltage across the capacitor from 0 to 63.2 percent its maximum voltage. (1uF = 0.000,001F = 1 X 10-6F)

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

      To achieve long output times, electrolytic capacitors are often used for C1 and the value of R1 can be as high as 1 Megohm. 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.

      Adding a resistor to the CONTROL terminal is not an ideal solution to solving long duration timing situations but should work for pulse times of less than ten minutes.


    Reversed Trigger Input Control Of 555 Timers

      The following method allows the timer to be triggered by a normally closed switch. This would be useful in applications such as intrusion alarms where the protection circuit is broken if a window or door is opened

    Reversed Trigger Input


    RESET And CONTROL Input Terminal Notes

    Controlling Circuits For LM555 Timers

      The following diagrams show some methods of using one-shot timer to control an astable oscillator.

    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 typically 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.

      Before using any of these circuits for specific applications they should be tested to determine the best values for the components and the practicality of their use.



    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 timer A to the TRIGGER and THRESHOLD terminals of timer B. The 10K ohm resistor limits the current that can flow into the THRESHOLD terminal of timer B.

      Due to the ability of the timers to source or sink current, the current from one timers output can flow into the other timer's output depending on which output is HIGH or LOW. The typical output conditions 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 timer A.

      Normal triggering methods and period lengths are not affected.

      Both timer's RESET terminals are available and can be used individually or together.

      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 1000 Hz.


    RESET And CONTROL Input Terminal Notes

    Interlocked Monostable Timers

      In the following circuit the timers are interlocked so that while 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 through a diode and placing a resistor in the trigger circuit. The resistor limits the current 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 555 Timers

      Monostable 555 timer circuits will automatically trigger and start a timing cycle when power is applied to the circuit. The timer's internal circuitry is largely responsible for this triggering but it is also caused stray or installed capacitance at the TRIGGER terminal of the timer.

      Triggering at power-up can be a undesirable if the period is long and there is no way to stop the cycle once it has begun.

      The stray capacitance can be from a number of sources but a typical source is the wires that connect a push button used to trigger the timer.

      To prevent timer from starting, a simple RC and transistor circuit can be connected to the timer's RESET terminal so that when power is applied to the circuit, the timer is automatically held RESET by transistor Q1 until C1 is almost fully charged.

      The length of the resetting action can roughly be determined by R1 X C1 X 3 .

      The example circuit shows a monostable oscillator but the method could also temporarily hold an astable 555 oscillator in a reset condition at power-up.

    LM555 Power-Up Reset Method 1

      The following circuit is another method of stopping the timing cycle at power-up. In this case, a pulse is sent to the THRESHOLD terminal which stops the timing cycle when the power is applied.

    LM555 Power-Up Reset Method 2


    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 triggered, timer A will be reset.

      This means that only one timer can be running at a 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

    Reset/Set Flip-Flop Made With A LM556 Timer

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

      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 operate on 12 volts, or less, and draw approximately 15 milliamps when they are stalled.

      Due to the design of the LM556 timer chip there are multiple output options available in this circuit. 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 test version of the LM556 Flip-Flop circuit used to create a "Truth Table" that shows the OUTPUT states for a given INPUT state.

    Logic Function diagram

      Because there are two inputs for each half of the 556 timer, the input voltages must go above and below the TRIGGER voltage and above the THRESHOLD levels for the circuit to operate correctly. Therefore the ratios of R1/R3 and R2/R4 is important but their actual values are not.

      Also, the impeadance of the inputs must be low enough to allow for these voltage levels to be achieved.

      The next diagram has the TRIGGER and THRESHOLD terminals of the timers separated. The basic function is the same as the circuit above but the output can only change when the input treminals are made low.

    Separated Trigger And Threshold Terminals


    LM556 Flip-Flop Input Options

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

    Input Options schematic

      In circuit A the SET and RESET inputs is brought to 0 Volts to change the state of the Flip-Flop.

      In circuit B the SET input is switched between 0 Volts and Vcc, the supply voltage, to change the state of the Flip-Flop. The RESET terminal is unconnected.

      In both circuit A and B, when the push buttons are OPEN the Flip-Flop will remain in its last state until the opposite signal is applied to an input.

      Circuits A and B also show two methods of connecting the LED's at terminals 1 and 13. The input method in circuit B would not be practical to produce the STATE 3 condition shown in the Truth Table on the previous diagram.



    LM556 Flip-Flop Notes


    RESET And CONTROL Input Terminal Notes


    LM555 Timer Used As A Voltage Comparator Or Schmitt Trigger

      The next section shows how an LM555 timer can be used as a voltage comparator or a Scmitt Trigger with a large offset voltage. The 555 timer is not well suited for this application but it is 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 / Schmitt Trigger

      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


    50% Output Duty Cycle (Variable)

      The LM555 timer can achieve a 50 percent duty cycle as shown in the next diagram. The duty cycle adjustment range of the give components values 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 in this manner is that the output frequency changes as the duty cycle changes.

    50% Duty Cycle schematic

    For The Record

      The circuit shown in the next diagram is not an accurate method of producing a 50 percent duty cycle using 555 timers, either bipolar or CMOS types. The circuit can produce a duty cycle that is close to 50 percent but when a load is added to the output of the timer, the voltage drops across its output transistors will increase and the duty cycle will shift.

    Not Accurate 50% Duty Cycle schematic


    RESET And CONTROL Input Terminal Notes


    Bipolar LED Driver

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

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

    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 switch the timing capacitors 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 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 two frequencies rather than switching abruptly between two frequencies.

    Sweeping Output Siren

      NOTE: The Sweeping Output Siren circuit has a limited sweep range and the duty cycle shifts with the changing output frequency.

      A better 555 based circuit for a sweeping oscillator would be to adapt the Variable Pulse Width Oscillator in the section above.

      A still better choice for a sweeping oscillator is a Voltage Controlled Oscillator (VCO) IC. See this Wikipedia page for basic information on Voltage-controlled oscillators and this datasheet for the LM321.

      Other devices include the TTL 74124 Dual Voltage-Controlled Oscillator and the CMOS CD4046B Phase-Locked Loop.


    RESET And CONTROL Input Terminal Notes


    D Type Flip-Flop Made With A LM556 Timer

      This circuit is a hybrid - D type Flip-Flop that is constructed from an LM556 - Dual Timer integrated circuit. The circuit is essentially an expensive version of the classic - two transistor Flip-Flop but with a bipolar output and a current capacity of 200 milliamps.

      Two versions of the circuit are shown. The modernized version separates the TRIGGER and THRESHOLD terminals on the timers and is therefore less finiky about the ratios of the resistors and capacitors at the timer inputs.

      In both versions, 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 D flip-flop the circuit acts as a binary divider.

    Classic D - Flip-Flop

      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 will limit the maximum rate at which the circuit can be switched.

      Because there are two switching levels, 1/3rd and 2/3rds of the supply voltage, the 556 timer is not ideally suited for this type of circuit, therefore the capacitance range of the input capacitors for the classic version of the circuit that will work reliabley is quite small; between 0.1 and 1.0 microfarads.

    Modernized D - Flip-Flop

      The circuit has some output switching time lag due to the RC time constants at the inputs but is unaffected by the different Trigger and Threshold voltage levels of the timers themselves as these inputs are now separated.

      For the modernized circuit, the range of capacitors that can be used at the inputs is large and the capacitors can be of much different values with out affecting the basic operation of the circuit.


    RESET And CONTROL Input Terminal Notes


    Time Delay Circuits

    Time Recovery Delay Circuits

    Two Stage Time Delay Circuit

    Cascaded Time Delay Circuits

    Example Circuit - 4 Stage Cascade Delay

    BiDirectional Time Delay Circuit

      In the BiDirectional Time Delay Circuit, the B timer acts more as a Schmitt trigger with a delay than a conventional timer. See section 13 of this page for more detail.


    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 IC's operation.

    CD4017 Data sheet - National Semiconductor (.pdf)

    Variable Period Oscillator (Experimental)


      The next schematic shows an alternate arrangement for the timing resistors. This would allow the subsequent output pulses 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 / Negative Recovery Circuits

    Basic - Negative Recovery Circuit

      The first circuit is a simple, push button controlled, Negative Recovery timer circuit. Each time that S1 is closed the time remaining in the cycle is reset to zero. If the time does run out, closing S1 will restart the cycle.

      The following circuits can detect when a train of pulses stops or become too far apart. They can also be use to keep the timer at its zero count if the input is held in a steady state. This is called 'Negative Recovery'.

      The diode across R1 in these circuits causes C1 to quickly discharge when the power to the circuit is switched off. This allows the circuit to be ready for the next cycle more quickly.

    Basic - Missing Pulse Detectors

    Steady Output - Missing Pulse Detectors - Two Comparators

    Steady Output - Missing Pulse Detectors - Two Timers

      The next two circuits in this section produce the same result: The timer must be reset manually if it has timed out.

    Latching Output - Missing Pulse Detector

    Manual Start - Missing Pulse Detector


    RESET And CONTROL Input Terminal Notes


    Fixed 50% Output Duty Cycle Using Logic Devices

      The only way to achieve a true - 50 percent duty cycle from a 555 timer is to divide the output by 2 with a binary divider such as the 7473 or 7474 TTL logic ICs.

    Fixed 50% Output Duty Cycle


    RESET And CONTROL Input Terminal Notes


    Three Stage - Cycling Timer Circuit

      NOTE All three timers in this circuit will start when power is applied, therefore all but the first timer (A) will need to be Reset for the proper cycle order to be started automatically. (See item 10 in the index of this page for a method of resetting the timers.)

    A Single - Traffic Light Driver Circuit - Based On The Cycling Timer Circuit


    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 consumption of 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 THRESHOLD terminal is not restricted.

    Timer Current Draws


    RESET And CONTROL Input Terminal Notes

    Delayed Re-Triggering

      The following is a method of preventing a timer from being re-triggered before a certain time period has elapsed.

    Delayed Re-Trigger


    RESET And CONTROL Input Terminal Notes

    Timer Output Section

      The next diagram shows the output section of a National Semiconductor LM555 timer. This type of output can either source or sink current and is typical of 555 and 556 timer IC's.

      When the output of the timer is HIGH, it can supply current to a load. When the output of the timer is LOW, it can receive current from a load.

      Transistor Q3 is actually connected as a diode with the collector not carrying current. Although a circuit common symbol is shown, the collector is not connected to the ground of the timer.

    Output Circuit


    RESET And CONTROL Input Terminal Notes

    Power ON Delay Circuits

      These circuits will delay the application of power to an external circuit by using mechanical relays or transistors. Other output control devices could also be used.

      These circuits are not ideal as the relays are closed when power is supplied to the circuit. This means that the power is supplied to the load for a very short period until the relay can open.

    Power ON Delay Circuits

    Delay Circuit With Indicator LED


    Delayed Lock Out Circuit


    PUJT & Voltage Comparator - Power On - Delay Circuits


    Wait For Pulses - Delay Circuit

      A variation on the Power On delay circuits above is a delay after pulses start arriving.

      A resistor could be placed across capacitor C1 so that the timer will be reset if the pulses stop arriving. This resistor should have a resistance of at least three times the value of R1.


    RESET And CONTROL Input Terminal Notes

    Power OFF Delay Circuits

      These circuits will delay the removal of power to an external circuit by using relays or transistors. Other output control devices such as opto isolators could also be used to control the load.

    Power OFF Delay Circuits

     

    RESET And CONTROL Input Terminal Notes

    Average 51.5 % Output Duty Cycle Using A 555 Timer

      The next circuit produces an average duty cycle of 51.5% over the entire resistance range of R2 at a supply voltage of 10 volts.

      At a supply voltage of 5 volts the average duty cycle increased to 52.7%. The span of the duty cycle also increased.

    51.5% Duty Cycle Oscillator


    RESET And CONTROL Input Terminal Notes

    Driving Loads Of Greater Than 15 Volts Or 200 Milliamps

      The next two circuits allow the 555 timer to drive loads that have a supply voltage that is greater than the 15 volt maximum of 555 timers.

      Higher current loads can be driven by transistors with a suitable current capacity and adjusting the base current as needed. Darlington and MOSFET transistors can drive loads of many amps.

      The 24 volt supply can be full wave DC and does not need to be filtered. The load's supply voltage could also be lower than the timer's supply voltage.

    High Voltage And Current Load Drivers


    RESET And CONTROL Input Terminal Notes

    'N' Steps And Stop Circuit (CD4017)

      The next circuit uses the outputs of a CD4017 - Decade Counter to stop a 555 timer at a given step and then wait until the counter is reset.

    'N' Steps And Stop Circuit


    RESET And CONTROL Input Terminal Notes

    Multiple - Monostable Trigger Inputs

      The next circuit allows a monostable oscillator to have multiple trigger inputs. The timer can be triggered and time out even if one, or more, of the input switches is held closed.

      Input signals that arrive before the current output pulse has ended will not be indicated (The timer cannot be retriggered part way through its cycle.)

    Multiple Trigger Inputs Circuit

      The schematic shows push buttons as the input but almost any device that causes the voltage to fall quickly can be used, including a bipolar output from another timer.


    Timer Output Voltage Losses

      The next two diagrams show the voltage losses/drops at the outputs of 555 timers when the output is HIGH and LOW. The data can only be used as a relative indication of the losses as other timers and circuits may have smaller or larger losses.

    Voltage Losses For A HIGH Output

    Voltage Losses For A LOW Output


    RESET And CONTROL Input Terminal Notes

    RC Delayed Timer Triggering

      The next circuit uses a Resistor and Capacitor at the TRIGGER input to delay the triggering of the timer. This is largely an extension of the Triggering And Timing Helpers For Monostable Timers in section 5 of this page.

      Two recovery schemes are also shown on the diagram.

    RC Delayed Timer Triggering


    RESET And CONTROL Input Terminal Notes

    Threshold Terminal Current Limiting

      The THRESHOLD terminal of 555 and 556 timers do not have internal current limiting. The following diagram shows how the THRESHOLD terminal's current can be restricted.

      If the current to the THRESHOLD terminal is not restricted the timer may malfunction. The current limiting resistor should be at least 1,000 ohms but can any higher value that is practical for the circuit. Using a higher value resistor will reduce the overall current usage of the entire circuit.

      The important thing is for the THRESHOLD terminal not to be exposed to the supply voltage or other low impedance source such as the output of another 555 timer without limiting its current.

    Threshold Terminal Current Limiting

      NOTE: Some timers may need the value of the THRESHOLD's current limiting resistor to be greater than 1K for the device to function correctly.


    RESET And CONTROL Input Terminal Notes

    Fixed Frequency / Variable Duty Cycle Oscillator

      This circuit is designed to produce an output at a fixed frequency and a wide range duty cycle.

      The circuit was breadboarded and functioned as shown in the tables on the diagram. The results were good except for a dip in the output frequency near the low end of the duty cycle range.

    Fixed Frequency / Variable Duty Cycle


    RESET And CONTROL Input Terminal Notes

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    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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    10 April, 2014