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In 1957 a battery was discovered in Bagdad. It was made by the Parthians, who ruled Bagdad from 250 B.C.E. to 224 C.E., and was used to electroplate silver.

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ne555logoThe 555 Timer is a very cheap, popular and useful precision timing device that can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing stabilized waveforms of varying duty cycles from 50 to 100%. The 555 timer chip is extremely robust and stable 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of applications such as one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies and converters etc, in fact any circuit that requires some form of time control as the list is endless.


The single 555 Timer chip in its basic form is a Bipolar 8-pin mini Dual-in-line Package (DIP) device consisting of some 25 transistors, 2 diodes and about 16 resistors arranged to form two comparators, a flip-flop and a high current output stage as shown below. As well as the 555 Timer there is also available the NE556 Timer Oscillator which combines TWO individual 555's within a single 14-pin DIP package and low power CMOS versions of the single 555 timer such as the 7555 and LMC555 which use MOSFET transistors instead.

A simplified "block diagram" representing the internal circuitry of the 555 timer is given below with a brief explanation of each of its connecting pins to help provide a clearer understanding of how it works.


555 Timer Block Diagram 

•  Pin 1. −     Ground, The ground pin connects the 555 timer to the negative (0v) supply rail. 
•  Pin 2. −     Trigger, The negative input to comparator No 1. A negative pulse on this pin "sets" the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a "LOW" to a "HIGH" state. 
•  Pin 3. −     Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc - 1.5V so small speakers, LEDs or motors can be connected directly to the output.
•  Pin 4. −     Reset, This pin is used to "reset" the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic "1" level when not used to prevent any unwanted resetting of the output.
•  Pin 5. −     Control Voltage, This pin controls the timing of the by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise. 
•  Pin 6. −     Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from "HIGH" to "LOW" state. This pin connects directly to the RC timing circuit. 
•  Pin 7. −     Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to "discharge" the timing capacitor to ground when the output at pin 3 switches "LOW".
•  Pin 8. −     Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.

The 555 Timers name comes from the fact that there are three 5kΩ resistors connected together internally producing a voltage divider network between the supply voltage at pin 8 and ground at pin 1. The voltage across this resistive network holds the positive input of comparator two at 2/3Vcc and the positive input to comparator one at 1/3Vcc. The two comparators produce an output voltage dependant upon the voltage difference at their inputs which is determined by the charging and discharging action of the externally connected RC network. The outputs from both comparators are connected to the two inputs of the flip-flop which inturn produces either a "HIGH" or "LOW" level output at Q based on the states of its inputs. The output from the flip-flop is used to control a high current output switching stage to drive the connected load producing either a "HIGH" or "LOW" voltage level at the output pin. 

The most common use of the 555 timer oscillator is as a simple astable oscillator by connecting two resistors and a capacitor across its terminals to generate a fixed pulse train with a time period determined by the time constant of the RC network. But the 555 timer oscillator chip can also be connected in a variety of different ways to produce Monostable or Bistable multivibrators as well as the more common Astable Multivibrator.

The Monostable 555 Timer

The operation and output of the 555 Monostable is exactly the same as that for the transistorised one we look at previously in the Monostable Multivibrators tutorial. The difference this time is that the two transistors have been replaced by the 555 timer device. Consider the 555 Monostable circuit below.

Monostable 555 Timer



When a negative (0V) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer oscillator, the internal comparator, (comparator No1) detects this input and "sets" the state of the flip-flop, changing the output from a "LOW" state to a "HIGH" state. This action inturn turns "OFF" the discharge transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1. This allows the timing capacitor to start to charge up through resistor, R1 until the voltage across the capacitor reaches the threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage divider network. At this point the comparators output goes "HIGH" and "resets" the flip-flop back to its original state which inturn turns "ON" the transistor and discharges the capacitor to ground through pin 7. This action also causes the output to change its state back to the original stable "LOW" value awaiting another trigger pulse to start the timing process over again. Then as before, the Monostable Multivibrator has only ONE stable state.


The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully. Once triggered, the 555 Monostable will remain in this "HIGH" unstable output state until the time period set up by the R1.C1 network has elapsed. The amount of time that the output voltage remains "HIGH" or at a logic "1" level, is given by the following time constant equation.


Where, t is in seconds, R is in Ω's and C in Farads.

Example No1

A Monostable 555 Timer is required to produce a time delay within a circuit. If a 10uF timing capacitor is used calculate the value of the resistor required to produce an output time delay of 500ms. 

500ms is the same as saying 0.5s so by rearranging the formula above, we get the calculated value for the resistor, R as: 


The calculated value for the timing resistor required to produce the required time constant of 500ms is 45.5KΩ's which does not exist as a standard value resistor, so we would need to select the nearest preferred value resistor of 47kΩ's which is available in all the standard ranges of tolerance from the E12 (10%) to the E96 (1%), giving us a new recalculated time delay of 517ms. If this time difference of 17ms (500 - 517ms) is unacceptable a lower preferred value timing resistor can be selected and connected in series with a trimming resistor to adjust the pulse width to the desired value.


We now know that the time delay or output pulse width of a monostable 555 timer is determined by the time constant of the connected RC network. If long time delays are required in the 10's of seconds, it is not always advisable to use high value timing capacitors as they can be physically large, expensive and have large value tolerances, e.g. ±20%. One alternative solution is too use a small value timing capacitor and a much larger value resistor up to about 20MΩ's to produce the require time delay. Also by using one smaller value timing capacitor and different resistor values connected to it through a multi-position rotary switch, we can produce a Monostable 555 timer oscillator circuit that can produce different pulse widths at each switch rotation such as the switchable Monostable 555 timer circuit shown below.

Switchable 555 Timer



We can manually calculate the values of R and C for the individual components required as we did in the example above. However, the choice of components needed to obtain the desired time delay requires us to calculate with either kilohms, megaohms, microfarads or picafarads and it is very easy to end up with a time delay of frequency that is out by a factor of ten or even a hundred. We can make our life a little easier by using nomographs to show the monostable multivibrators expected frequency output for different combinations or values of both the R and C.


For example,

Monostable Nomograph



By selecting suitable values of C and R in the ranges of 0.001uF to 100uF and 1kΩ to 10MΩ's respectively, we can read the expected output frequency directly from the nomograph graph thereby eliminating any error in the calculations. In practice the value of the timing resistor for a monostable 555 timer should not be less than 1kΩ or greater than 20MΩ

Bistable 555 Timer


As well as the one shot 555 Monostable configuration above, we can also produce a Bistable (two stable states) device with the operation and output of the 555 Bistable being similar to the transistorised one we look at previously in the Bistable Multivibrators tutorial. The 555 Bistable is one of the simplest circuits we can build using the 555 timer oscillator chip. This bistable configuration does not use any RC timing network to produce an output waveform so no equations are required to calculate the time period of the circuit. Consider the Bistable 555 Timer circuit below.

Bistable 555 Timer (flip-flop)


The switching of the output waveform is achieved by controlling the trigger and reset inputs of the 555 timer which are held "HIGH" by the two pull-up resistors, R1 and R2. By taking the trigger input (pin 2) "LOW", switch in set position, changes the output state into the "HIGH" state and by taking the reset input (pin 4) "LOW", switch in reset position, changes the output into the "LOW" state. This 555 timer circuit will remain in either state indefinitely and is therefore bistable. Then the Bistable 555 timer is stable in both states, "HIGH" and "LOW". The threshold input (pin 6) is connected to ground to ensure that it cannot reset the bistable circuit as it would in a normal timing application.

555 Timer Output


We could not finish this 555 Timer tutorial without discussing something about the switching and drive capabilities of the 555 timer or indeed the dual 556 Timer IC. The output (pin 3) of the standard 555 timer or the 556 timer, has the ability to either "Sink" or "Source" a load current of up to a maximum of 200mA, which is sufficient to directly drive output transducers such as relays, filament lamps, LED's motors, or speakers etc with the aid of series resistors or diode protection. The ability to both "Sink" (absorb) and "Source" (supply) means that the output device can be connected between the output terminal of the 555 timer and the supply to sink the load current or between the output terminal and ground to source the load current. For example.

Sinking and Sourcing the 555 Timer





In the first circuit above, the LED is connected between the positive supply rail (+Vcc) and the output pin 3. This means that the current will "Sink" (absorb) or flow into the 555 timer output terminal and the LED will be "ON" when the output is "LOW". The second circuit above shows that the LED is connected between the output pin 3 and ground (0v). This means that the current will "Source" (supply) or flow out of the 555 timers output terminal and the LED will be "ON" when the output is "HIGH".


The ability of the 555 timer to both sink and source its output load current means that both LED's can be connected to the output terminal at the same time but only one will be switched "ON" depending whether the output state is "HIGH" or "LOW". The circuit to the left shows an example of this. the two LED's will be alternatively switched "ON" and "OFF" depending upon the output. Resistor, R is used to limit the LED current to below 20mA.


We said earlier that the maximum output current to either sink or source the load current via pin 3 is about 200mA and this value is more than enough to drive or switch other logic IC's, LED's or small lamps etc. But what if we wanted to switch or control higher power devices such as motors, electromagnets, relays or loudspeakers. Then we would need to use a Transistor to amplify the 555 timers output in order to provide a sufficiently high enough current to drive the load.

555 Timer Transistor Driver


The transistor in the two examples above, can be replaced with a Power MOSFET device or Darlington transistor if the load current is high. When using an inductive load such as a motor, relay or electromagnet, it is advisable to connect a "freewheel diode" directly across the load terminals to absorb any back emf voltages generated by the inductive device when it changes state.


Thus far we have look at using the 555 Timer to generate monostable and bistable output pulses. In the next tutorial about Waveform Generation we will look at connecting the 555 in an astable multivibrator configuration. When used in the astable mode both the frequency and duty cycle of the output waveform can be accurately controlled to produce a very versatile waveform generator.


The 555 Oscillator


In the previous tutorial we saw that the 555 Timer IC can be connected either in its Monostable mode thereby producing a precision timer of a fixed time duration, or in its Bistable mode to produce a flip-flop type switching action. But we can also connect the 555 timer IC in an Astable mode to produce a very stable 555 Oscillator circuit for generating highly accurate free running waveforms whose output frequency can be adjusted by means of an externally connected RC tank circuit consisting of just two resistors and a capacitor.


The 555 Oscillator is another type of relaxation oscillator for generating stabilized square wave output waveforms of either a fixed frequency of up to 500kHz or of varying duty cycles from 50 to 100%. In the previous 555 Timer tutorial we saw that the Monostable circuit produces a single output one-shot pulse when triggered on its pin 2 trigger input. In order to get the 555 Oscillator to operate as an astable multivibrator, it is necessary to continuously re-trigger the 555 IC after each and every timing cycle. This is basically achieved by connecting the trigger input (pin 2) and the threshold input (pin 6) together, thereby allowing the device to act as an astable oscillator. Then the 555 Oscillator has no stable states as it continuously switches from one state to the other. Also the single timing resistor of the previous monostable multivibrator circuit has been split into two separate resistors, R1 and R2 with their junction connected to the discharge input (pin 7) as shown below.

Astable 555 Oscillator




In the 555 Oscillator above, pin 2 and pin 6 are connected together allowing the circuit to re-trigger itself on each and every cycle allowing it to operate as a free running oscillator. During each cycle capacitor, C charges up through both timing resistors, R1 and R2 but discharges itself only through resistor, R2 as the other side of R2 is connected to the discharge terminal, pin 7. Then the capacitor charges up to 2/3Vcc (the upper comparator limit) which is determined by the 0.693(R1+R2)C combination and discharges itself down to 1/3Vcc (the lower comparator limit) determined by the 0.693(R2.C) combination. This results in an output waveform whose voltage level is approximately equal to Vcc - 1.5V and whose output "ON" and "OFF" time periods are determined by the capacitor and resistors combinations. The individual times required to complete one charge and discharge cycle of the output is therefore given as:

Astable 555 Oscillator Charge and Discharge Times




Where, R is in Ω's and C in Farads.


When connected as an astable multivibrator, the output from the 555 Oscillator will continue indefinitely charging and discharging between 2/3Vcc and 1/3Vcc until the power supply is removed. As with the monostable multivibrator these charge and discharge times and therefore the frequency are independent of the supply voltage. The duration of one full cycle is therefore equal to the sum of the two individual times that the capacitor charges and discharges added together and is given as:

555 Oscillator Cycle Time



The output frequency of oscillations can be found by inverting the equation above for the total cycle time giving a final equation for the output frequency of an Astable 555 Oscillator as:

555 Oscillator Frequency Equation




By altering the time constant of just one of the RC combinations, the Duty Cycle better known as the "Mark-to-Space" ratio of the output waveform can be accurately set and is given as the ratio of resistor R2 to resistor R1. The Duty Cycle for the 555 Oscillator, which is the ratio of the "ON" time divided by the "OFF" time is given by:

555 Oscillator Duty Cycle




The duty cycle has no units as it is a ratio but can be expressed as a percentage (%). If both timing resistors, R1 and R2 are equal the output duty cycle will be given as 2:1 or 33%.

Example No1


An Astable 555 Oscillator is constructed using the following components, R1 = 1kΩ, R2 = 2kΩ and capacitor C = 10uF. Calculate the output frequency from the 555 oscillator and the duty cycle of the output waveform.


  t1 - Charge "ON" time is calculated as:




  t2 - Discharge "OFF" time is calculated as:




  Total periodic time is calculated as:




  The output frequency, ƒ is therefore given as:




  Giving a duty cycle value of:




As the timing capacitor, C charges through resistors R1 and R2 but only discharges through resistor R2 the output duty cycle can be varied between 50 and 100% by changing the value of resistor R2. By decreasing the value of R2 the duty cycle increases towards 100% and by increasing R2 the duty cycle reduces towards 50%. If resistor, R2 is very large relative to resistor R1 the output frequency of the 555 astable circuit will determined by R2.C only. The problem with this basic astable 555 oscillator configuration is that the duty cycle, the "mark-to-space" ratio will never go below 50% as the presence of resistor R2 prevents this. In other words we cannot make the "ON" time shorter than the "OFF" time as (R1 + R2)C will always be greater than R1.C. One way to overcome this problem is to connect a signal bypassing diode in parallel with resistor R2 as shown below.

Improved 555 Oscillator Duty Cycle


By connecting this diode, D1 between the trigger input and the discharge input, the timing capacitor will now charge up directly through resistor R1 only, as resistor R2 is effectively shorted out by the diode. The capacitor discharges as normal through resistor, R2. Now the previous charging time of t1 = 0.693(R1 + R2)C is modified to take account of this new charging circuit and is given as: 0.693(R1.C). The duty cycle is therefore given as D = R1/(R1 + R2). Then to generate a duty cycle of less than 50%, resistor R1 needs to be less than resistor R2.

555 Oscillator Applications


We said previously that the maximum output to either sink or source the load current via pin 3 is about 200mA and this value is more than enough to drive or switch other logic IC's, a few LED's or a small lamp etc and that we would need to use a bipolar transistor or MOSFET to amplify the 555's output to drive larger current loads such as motor or relays. But the 555 Oscillator can be used in a wide range of waveform generator circuits and applications that require very little output current such as in electronic test equipment for producing a whole range of different output test frequencies from very accurate sine, square and pulse waveforms or as LED or lamp flashers and dimmers to simple noise making circuits such as metronomes, tone and sound effects generators and even musical toys for Christmas.


We could very easily build a simple 555 oscillator circuit to flash a few LED's "ON" and "OFF", but one very nice and simple to build project using an astable based 555 oscillator is that of an Electronic Metronome. Metronomes are devices used to mark time in pieces of music by producing a regular and recurring musical beat or click. A simple electronic metronome can be made using a 555 oscillator as the main timing device and by adjusting the output frequency of the oscillator the tempo or "Beats per Minute" can be set. A tempo of 60 beats per minute means that one beat will occur every second and in electronics terms that equates to 1Hz. So by using some very common musical definitions we can easily build a table of the different frequencies required for our metronome circuit as shown below.

Metronome Frequency Table


Rate Beats per
Time (T)
Larghetto Very Slow 60 1sec 1.0Hz
Andante Slow 90 666ms 1.5Hz
Moderato Medium 120 500ms 2.0Hz
Allegro Fast 150 400ms 2.5Hz
Presto Very Fast 180 333ms 3.0Hz


The output frequency range of the metronome was simply calculated as the reciprocal of 1 minute or 60 seconds divided by the number of beats per minute required, for example (1/(60 secs / 90 bpm) = 1.5Hz) and 120bpm is equivalent to 2Hz, and so on. So by using our now familiar equation above for calculating the output frequency of an astable 555 oscillator circuit the individual values of R1, R2 and C can be found.

The time period of the output waveform for an astable 555 Oscillator is given as:




For our electronic metronome circuit, the value of the timing resistor R1 can be found by rearranging the equation above to give.




Assuming a value for resistor R2 = 1kΩ and capacitor C = 10uF the value of the timing resistor R1 for our frequency range is given as 71k6Ω at 60 beats per minute to 23k5Ω at 180 beats per minute, so a variable resistor (potentiometer) of 100kΩ would be more than enough for the metronome circuit to produce the full range of beats required and some more. Then the final circuit for our electronic metronome example would be given as:

555 Electronic Metronome


This metronome circuit demonstrates just one simple way of using a 555 oscillator to produce an audible sound or note. It uses a 100kΩ potentiometer to control the full range of output pulses or beats, and as it has a 100kΩ value it can be easily calibrated to give an equivalent percentage value corresponding to the position of the potentiometer. For example, 60 beats per minute equals 71.6kΩ or 72% rotation. Likewise, 120 beats per minute equals 35.6kΩ or 35% rotation, etc. Additional resistors or trimmer's can be connected in series with the potentiometer to pre-set the outputs upper and lower limits to predefined values, but these additional components will need to be taken into account when calculating the output frequency or time period. 

While the above circuit is a very simple and amusing example of sound generation, it is possible to use the 555 Oscillator as a noise generator/synthesizer or to make musical sounds, tones and alarms by constructing a variable-frequency, variable-mark/space ratio waveform generator. In this tutorial we have used just a single 555 oscillator circuit to produce a sound but by cascading together two or more 555 oscillator chips, various circuits can be constructed to produce a whole range of musical effects such as the police car "Dee-Dah" siren given in the example below.

555 Oscillator Police "Dee-Dah" Siren


The circuit simulates a warble-tone alarm signal that simulates the sound of a police siren. IC1 is connected as a 2Hz non-symmetrical astable multivibrator which is used to frequency modulate IC2 via the 10kΩ resistor. The output of IC2 alternates symmetrically between 300Hz and 660Hz taking 0.5 seconds to complete each alternating cycle.

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