555 Timer/Oscillator Tutorial


555 Timer/Oscillator Tutorial

© by Tony van Roon


The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was called “The IC Time Machine” and was also the very first and only commercial timer ic available.It provided circuit designers and hobby tinkerers with a relatively cheap, stable, and user-friendly integrated circuit for both monostable and astable applications. Since this device was first made commercially available, a myrad of novel and unique circuits have been developed and presented in several trade, professional, and hobby publications.The past ten years some manufacturers stopped making these timers because of competition or other reasons.Yet other companies, like NTE (a subdivision of Philips) picked up where some left off.
This primer is about this fantastic timer which is after 30 years still very popular and used in many schematics.Although these days the CMOS version of this IC, like the Motorola MC1455, is mostly used, the regular type is still available, however there have been many improvements and variations in the circuitry. But all types are pin-for-pin plug compatible. Myself, every time I see this 555 timer used in advanced and high-tech electronic circuits, I’m amazed.It is just incredible.
In this tutorial I will show you what exactly the 555 timer is and how to properly use it by itself or in combination with other solid state devices without the requirement of an engineering degree.This timer uses a maze of transistors, diodes and resistors and for this complex reason I will use a more simplified (but accurate) block diagram to explain the internal organizations of the 555. So, lets start slowly and build it up from there.

Table 1, Manufacturers

The first type-number, in Table 1 on the left, represents the type which was/is preferred for military applications which have somewhat improved electrical and thermal characteristics over their commercial counterparts, but also a bit more expensive, and usually metal-can or ceramic casing. This is analogous to the 5400/7400 series convention for TTL integrated circuits.

Fig 1/2, 555 pin out

The 555, in fig. 1 and fig. 2 above, come in two packages, either the round metal-can called the ’T’ package or the more familiar 8-pin DIP ’V’ package.About 20-years ago the metal-can type was pretty much the standard (SE/NE types).The 556 timer is a dual 555 version and comes in a 14-pin DIP package, the 558 is a quad version with four 555’s also in a 14 pin DIP case.

Fig 3, 555 Block

Inside the 555 timer, at fig. 3,are the equivalent of over 20 transistors, 15 resistors, and 2 diodes, depending of the manufacturer. The equivalent circuit, in block diagram, providing the functions of control, triggering, level sensing or comparison, discharge, and power output.Some of the more attractive features of the 555 timer are: Supply voltage between 4.5 and 18 volt, supply current 3 to 6 mA, and a Rise/Fall time of 100 nSec.It can also withstand quite a bit of abuse.

The Threshold current determine the maximum value of Ra + Rb.For 15 volt operation the maximum total resistance for R (Ra +Rb) is 20 Mega-ohm.

The supply current, when the output is ’high’, is typically 1 milli-amp (mA) or less.The initial monostable timing accuracy is typically within 1% of its calculated value, and exhibits negligible (0.1%/V) drift with supply voltage.Thus long-term supply variationscan be ignored, and the temperature variation is only 50ppm/°C (0.005%/°C).

All IC timers rely upon an external capacitor to determine the off-on time intervals of the output pulses.As you recall from your study of basic electronics, it takes a finite period of time for a capacitor (C) to charge or discharge through a resistor (R).Those times are clearly defined and can be calculated given the values of resistance and capacitance.
The basic RC charging circuit is shown in fig. 4.Assume that the capacitor is initially discharged.When the switch is closed, the capacitor begins to charge through the resistor.The voltage across the capacitor rises from zero up to the value of the appliedDC voltage.The charge curve for the circuit is shown in fig. 6.The time that it takes for the capacitor to charge to 63.7% of the applied voltage is known as the time constant (t).That time can be calculated with the simple expression:

Fig 4, 555 Timing

t = R X C

Assume a resistor value of 1 MegaOhm and a capacitor value of 1uF (micro-Farad).The time constant in that case is:

t = 1,000,000 X 0.000001 = 1 second

Assume further that the applied voltage is 6 volts.That means that it will take one time constant for the voltage across the capacitor to reach 63.2% of the applied voltage.Therefore, the capacitor charges to approximately 3.8 volts in one second.

Fig 4 1, Pulse change

Fig. 4-1, Change in the input pulse frequency allows completion of the timing cycle.As a general rule, the monostable ’ON’ time is set approximately 1/3 longer than the expected time between triggering pulses.Such a circuit is also known as a ’Missing Pulse Detector’.

Looking at the curve in fig. 6. you can see that it takes approximately 5 complete time constants for the capacitor to charge to almost the applied voltage.It would take about 5 seconds for the voltage on the capacitor to rise to approximately the full 6-volts.

Fig 4 2, Equivalent Circuit

Definition of Pin Functions:

Refer to the internal 555 schematic of Fig. 4-2

Pin 1 (Ground):The ground (or common) pin is the most-negative supply potential of the device, which is normally connected to circuit common (ground) when operated from positive supply voltages.

Pin 2 (Trigger):This pin is the input to the lower comparator and is used to set the latch, which in turn causes the output to go high.This is the beginning of the timing sequence in monostable operation.Triggering is accomplished by taking the pin from above to below a voltage level of 1/3 V+ (or, in general, one-half the voltage appearing at pin 5).The action of the trigger input is level-sensitive, allowing slow rate-of-change waveforms, as well as pulses, to be used as trigger sources.The trigger pulse must be of shorter duration than the time interval determined by the external R and C.If this pin is held low longer than that, the output will remain high until the trigger input is driven high again.One precaution that should be observed with the trigger input signal is that it must not remain lower than 1/3 V+ for a period of time longer than the timing cycle.If this is allowed to happen, the timer will re-trigger itself upon termination of the first output pulse.Thus, when the timer is driven in the monostable mode with input pulses longer than the desired output pulse width, the input trigger should effectively be shortened by differentiation.The minimum-allowable pulse width for triggering is somewhat dependent upon pulse level, but in general if it is greater than the 1uS (micro-Second), triggering will be reliable.A second precaution with respect to the trigger input concerns storage time in the lower comparator.This portion of the circuit can exhibit normal turn-off delays of several microseconds after triggering; that is, the latch can still have a trigger input for this period of time after the trigger pulse.In practice, this means the minimum monostable output pulse width should be in the order of 10uS to prevent possible double triggering due to this effect.The voltage range that can safely be applied to the trigger pin is between V+ and ground.A dc current, termed the trigger current, must also flow from this terminal into the external circuit.This current is typically 500nA (nano-amp) and will define the upper limit of resistance allowable from pin 2 to ground.For an astable configuration operating at V+ = 5 volts, this resistance is 3 Mega-ohm; it can be greater for higher V+ levels.

Pin 3 (Output):The output of the 555 comes from a high-current totem-pole stage made up of transistors Q20 – Q24.Transistors Q21 and Q22 provide drive for source-type loads, and their Darlington connection provides a high-state output voltage about 1.7 volts less than the V+ supply level used.Transistor Q24 provides current-sinking capability for low-state loads referred to V+ (such as typical TTL inputs).Transistor Q24 has a low saturation voltage, which allows it to interface directly, with good noise margin, when driving current-sinking logic.Exact output saturation levels vary markedly with supply voltage, however, for both high and low states.At a V+ of 5 volts, for instance, the low state Vce(sat) is typically 0.25 volts at 5 mA.Operating at 15 volts, however, it can sink 200mA if an output-low voltage level of 2 volts is allowable (power dissipation should be considered in such a case, of course).High-state level is typically 3.3 volts at V+ = 5 volts; 13.3 volts at V+ = 15 volts.Both the rise and fall times of the output waveform are quite fast, typical switching times being 100nS.The state of the output pin will always reflect the inverse of the logic state of the latch, and this fact may be seen by examining Fig. 3.Since the latch itself is not directly accessible, this relationship may be best explained in terms of latch-input trigger conditions.To trigger the output to a high condition, the trigger input is momentarily taken from a higher to a lower level. [see “Pin 2 – Trigger”].This causes the latch to be set and the output to go high.Actuation of the lower comparator is the only manner in which the output can be placed in the high state.The output can be returned to a low state by causing the threshold to go from a lower to a higher level [see “Pin 6 – Threshold”], which resets the latch.The output can also be made to go low by taking the reset to a low state near ground [see “Pin 4 – Reset”].The output voltage available at this pin is approximately equal to the Vcc applied to pin 8 minus 1.7V.

Pin 4 (Reset):This pin is also used to reset the latch and return the output to a low state.The reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA from this pin is required to reset the device.These levels are relatively independent of operating V+ level; thus the reset input is TTL compatible for any supply voltage.The reset input is an overriding function; that is, it will force the output to a low state regardless of the state of either of the other inputs.It may thus be used to terminate an output pulse prematurely, to gate oscillations from “on” to “off”, etc.Delay time from reset to output is typically on the order of 0.5 µS, and the minimum reset pulse width is 0.5 µS.Neither of these figures is guaranteed, however, and may vary from one manufacturer to another.In short, the reset pin is used to reset the flip-flop that controls the state of output pin 3.The pin is activated when a voltage level anywhere between 0 and 0.4 volt is applied to the pin.The reset pin will force the output to go low no matter what state the other inputs to the flip-flop are in.When not used, it is recommended that the reset input be tied to V+ to avoid any possibility of false resetting.

Pin 5 (Control Voltage):This pin allows direct access to the 2/3 V+ voltage-divider point, the reference level for the upper comparator.It also allows indirect access to the lower comparator, as there is a 2:1 divider (R8 – R9) from this point to the lower-comparator reference input, Q13.Use of this terminal is the option of the user, but it does allow extreme flexibility by permitting modification of the timing period, resetting of the comparator, etc.When the 555 timer is used in a voltage-controlled mode, its voltage-controlled operation ranges from about 1 volt less than V+ down to within 2 volts of ground (although this is not guaranteed).Voltages can be safely applied outside these limits, but they should be confined within the limits of V+ and ground for reliability.By applying a voltage to this pin, it is possible to vary the timing of the device independently of the RC network. The control voltage may be varied from 45 to 90% of the Vcc in the monostable mode, making it possible to control the width of the output pulse independently of RC.When it is used in the astable mode, the control voltage can be varied from 1.7V to the full Vcc.Varying the voltage in the astable mode will produce a frequency modulated (FM) output.In the event the control-voltage pin is not used, it is recommended that it be bypassed, to ground, with a capacitor of about 0.01uF (10nF) for immunity to noise, since it is a comparator input.This fact is not obvious in many 555 circuits since I have seen many circuits with ’no-pin-5’ connected to anything, but this is the proper procedure.The small ceramic cap may eliminate false triggering.

Pin 6 (Threshold):Pin 6 is one input to the upper comparator (the other being pin 5) and is used to reset the latch, which causes the output to go low.Resetting via this terminal is accomplished by taking the terminal from below to above a voltage level of 2/3 V+ (the normal voltage on pin 5).The action of the threshold pin is level sensitive, allowing slow rate-of-change waveforms.The voltage range that can safely be applied to the threshold pin is between V+ and ground.A dc current, termed the threshold current, must also flow into this terminal from the external circuit.This current is typically 0.1µA, and will define the upper limit of total resistance allowable from pin 6 to V+.For either timing configuration operating at V+ = 5 volts, this resistance is 16 Mega-ohm.For 15 volt operation, the maximum value of resistance is 20 MegaOhms.

Pin 7 (Discharge):This pin is connected to the open collector of a npn transistor (Q14), the emitter of which goes to ground, so that when the transistor is turned “on”, pin 7 is effectively shorted to ground.Usually the timing capacitor is connected between pin 7 and ground and is discharged when the transistor turns “on”.The conduction state of this transistor is identical in timing to that of the output stage.It is “on” (low resistance to ground) when the output is low and “off” (high resistance to ground) when the output is high.In both the monostable and astable time modes, this transistor switch is used to clamp the appropriate nodes of the timingnetwork to ground.Saturation voltage is typically below 100mV (milli-Volt) for currents of 5 mA or less, and off-state leakage is about 20nA (these parameters are not specified by all manufacturers, however).Maximum collector current is internally limited by design, thereby removing restrictions on capacitor size due to peak pulse-current discharge.In certain applications, this open collector output can be used as an auxiliary output terminal, with current-sinking capability similar to the output (pin 3).

Pin 8 (V +):The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the 555 timer IC.Supply-voltage operating range for the 555 is +4.5 volts (minimum) to +16 volts (maximum), and it is specified for operation between +5 volts and +15 volts.The device will operate essentially the same over this range of voltages without change in timing period.Actually, the most significant operational difference is the output drive capability, which increases for both current and voltage range as the supply voltage is increased.Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt.There are special and military devices available that operate at voltages as high as 18 volts.

Fig 5, 555 timer Tester

Try the simple 555 testing-circuit of Fig. 5. to get you going, and test all your 555 timer ic’s.I build several for friends and family.I bring my own tester to ham-fests and what not to instantly do a check and see if they are oscillating.Or use as a trouble shooter in 555 based circuits.This tester will quickly tell you if the timer is functional or not.Although not foolproof, it will tell if the 555 is shorted or oscillating.If both Led’s are flashing the timer is most likely in good working order.If one or both Led’s are either off or on solid the timer is defective.Simple huh?
Fig 6, capacitor charging

The capacitor slows down as it charges, and in actual fact never reaches the full supply voltage.That being the case, the maximum charge it receives in the timing circuit (66.6% of the supply voltage) is a little over the charge received after a time constant (63.2%).

Fig 7, capacitor discharging

The capacitor slows down as it discharges, and never quite reaches the ground potential.That means the minimum voltage it operates at must be greater than zero.Timing circuit is 63.2% of the supply voltage.

Fig 8, discharge of a capacitor

The discharge of a capacitor also takes time and we can shorten the amount of time by decreasing resistance (R) to the flow of current.

Operating Modes: The 555 timer has two basic operational modes: one shot and astable.In the one-shot mode, the 555 acts like a monostable multivibrator.A monostable is said to have a single stable state–that is the off state.Whenever it is triggered by an input pulse, the monostable switches to its temporary state.It remains in that state for a period of time determined by an RC network.It then returns to its stable state.In other words, the monostable circuit generates a single pulse of a fixed time duration each time it receives and input trigger pulse.Thus the name one-shot.One-shot multivibrators are used for turning some circuit or external component on or off for a specific length of time.It is also used to generate delays.When multiple one-shots are cascaded, a variety of sequential timing pulses can be generated.Those pulses will allow you to time and sequence a number of related operations.

The other basic operational mode of the 555 is as and astable multivibrator.An astable multivibrator is simply and oscillator.The astablemultivibrator generates a continuous stream of rectangular off-on pulses that switch between two voltage levels.The frequency of the pulses and their duty cycle are dependent upon the RC network values.

One-Shot Operation: Fig. 4 shows the basic circuit of the 555 connected as a monostable multivibrator.An external RC network is connected between the supply voltage and ground.The junction of the resistor and capacitor is connected to the threshold input which is the input to the upper comparator.The internal discharge transistor is also connected to the junction of the resistor and the capacitor.An input trigger pulse is applied to the trigger input, which is the input to the lower comparator.

With that circuit configuration, the control flip-flop is initially reset.Therefore, the output voltage is near zero volts.The signal from the control flip-flop causes T1 to conduct and act as a short circuit across the external capacitor.For that reason, the capacitor cannot charge.During that time, the input to the upper comparator is near zero volts causing the comparator output to keep the control flip-flop reset.

Fig 9a, monostable

Notice how the monostable continues to output its pulse regardless of the inputs swing back up.That is because the output is only triggered by the input pulse, the output actually depends on the capacitor charge.

Monostable Mode:
The 555 in fig. 9a is shown here in it’s utmost basic mode of operation; as a triggered monostable.One immediate observation is the extreme simplicity of this circuit.Only two components to make up a timer, a capacitor and a resistor.And for noise immunity maybe a capacitor on pin 5.Due to the internal latching mechanism of the 555, the timer will always time-out once triggered, regardless of any subsequent noise (such as bounce) on the input trigger (pin 2).This is a great asset in interfacing the 555 with noisy sources. Just in case you don’t know what ’bounce’ is: bounce is a type of fast, short term noise caused by a switch, relay, etc. and then picked up by the input pin.
The trigger input is initially high (about 1/3 of +V).When a negative-going trigger pulse is applied to the trigger input (see fig. 9a), the threshold on the lower comparator is exceeded.The lower comparator, therefore, sets the flip-flop.That causes T1 to cut off, acting as an open circuit.The setting of the flip-flop also causes a positive-going output level which is the beginning of the output timing pulse.

The capacitor now begins to charge through the external resistor.As soon as the charge on the capacitor equal 2/3 of the supply voltage, the upper comparator triggers and resets the control flip-flop.That terminates the output pulse which switches


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