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555 and 556 Timer Circuits
Inputs | Output | Astable | Duty Cycle |
Monostable |
Edge-trigger
| Bistable | Buffer

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Circuits
Also See: ICs (chips) | Capacitance | AC, DC and Electrical Signals

Introduction

Example circuit symbol (above)

Actual pin arrangements (below)

There is more information
about
555 timers and their circuits on the
Electronics in Meccano
website.

The 8-pin
555 timer must be one of the most useful ICs ever made and it is used in many
projects. With just a few external components it can be used to build many
circuits, not all of them involve timing!

A popular version is the NE555 and this is suitable in most cases where a
'555 timer' is specified. The 556 is a dual version of the 555 housed in a
14-pin package, the two timers (A and B) share the same power supply pins. The
circuit diagrams on this page show a 555, but they could all be adapted to use
one half of a 556.

Low power versions of the 555 are made, such as the ICM7555, but these should
only be used when specified (to increase battery life) because their maximum
output current of about 20mA (with a 9V supply) is too low for many standard 555
circuits. The ICM7555 has the same pin arrangement as a standard 555.

The circuit symbol for a 555 (and 556) is a box with the pins arranged to
suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply,
555 pin 3 output on the right. Usually just the pin numbers are used and they
are not labelled with their function.

The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to
15V (18V absolute maximum).

Standard 555 and 556 ICs create a significant 'glitch' on the supply when
their output changes state. This is rarely a problem in simple circuits with no
other ICs, but in more complex circuits a smoothing capacitor (eg 100µF)
should be connected across the +Vs and 0V supply near the 555 or 556.

The input and output pin functions are described briefly below and there are
fuller explanations covering the various circuits:

Astable -
producing a square wave

Monostable -
producing a single pulse when triggered

Bistable -
a simple memory which can be set and reset

Buffer - an
inverting buffer (Schmitt trigger)

Datasheets are available
from:

DatasheetArchive.com

Datasheets.org.uk

DatasheetCatalog.com

Inputs of 555/556
Trigger input: when
< 1/3 Vs ('active low') this makes the output
high (+Vs). It monitors the discharging of the timing capacitor in an astable
circuit. It has a high input impedance > 2M.

Threshold input: when
> 2/3 Vs ('active high') this makes the
output low (0V)*. It monitors the charging of the timing capacitor in astable
and monostable circuits. It has a high input impedance > 10M.
* providing the trigger input is
> 1/3 Vs, otherwise the trigger input will
override the threshold input and hold the output high (+Vs).

Reset input: when less than about 0.7V
('active low') this makes the output low (0V), overriding other inputs. When not
required it should be connected to +Vs. It has an input impedance of about
10k.

Control input: this can be used to adjust
the threshold voltage which is set internally to be
2/3 Vs. Usually this function is not required and the
control input is connected to 0V with a 0.01µF capacitor to eliminate electrical
noise. It can be left unconnected if noise is not a problem.

The discharge pin is not an input, but it
is listed here for convenience. It is connected to 0V when the timer output is
low and is used to discharge the timing capacitor in astable and monostable
circuits.

Output of 555/556
The output of a standard 555 or 556 can sink and
source up to 200mA. This is more than most ICs and it is sufficient to
supply many output transducers directly, including LEDs (with a resistor in
series), low current lamps, piezo transducers, loudspeakers (with a capacitor in
series), relay coils (with diode protection) and some motors (with diode
protection). The output voltage does not quite reach 0V and +Vs, especially if a
large current is flowing.

To switch larger currents you can connect a transistor.

The ability to both sink and source current means that two devices can be
connected to the output so that one is on when the output is low and the other
is on when the output is high. The top diagram shows two LEDs connected in this
way. This arrangement is used in the Level Crossing
project to make the red LEDs flash alternately.

Loudspeakers
A loudspeaker (minimum resistance 64) may be
connected to the output of a 555 or 556 astable circuit but a capacitor (about
100µF) must be connected in series. The output is equivalent to a steady DC of
about ½Vs combined with a square wave AC (audio) signal. The capacitor blocks
the DC, but allows the AC to pass as explained in capacitor coupling.

Piezo transducers may be connected directly to the output and do not require
a capacitor in series.

Relay coils and other inductive loads
Like all ICs, the 555 and 556 must
be protected from the brief high voltage 'spike' produced when an inductive load
such as a relay coil is switched off. The standard protection diode
must be connected 'backwards' across the the relay coil as shown in the diagram.

However, the 555 and 556 require an extra diode
connected in series with the coil to ensure that a small 'glitch' cannot be fed
back into the IC. Without this extra diode monostable circuits may re-trigger
themselves as the coil is switched off! The coil current passes through the
extra diode so it must be a 1N4001 or similar rectifier diode capable of passing
the current, a signal diode such as a 1N4148 is usually not suitable.

Top of page
| Inputs | Output | Astable | Duty Cycle |
Monostable |
Edge-trigger
| Bistable | Buffer

555/556 Astable

555 astable output, a square wave
(Tm and Ts
may be different)

555 astable
circuit

An astable circuit produces a
'square wave', this is a digital waveform with sharp transitions between low
(0V) and high (+Vs). Note that the durations of the low and high states may be
different. The circuit is called an astable because it is not stable in
any state: the output is continually changing between 'low' and 'high'.

The time period
(T) of the square wave is the time for one complete cycle, but it is usually
better to consider frequency (f) which is the
number of cycles per second.

T = 0.7 × (R1 + 2R2) × C1
and  f =

1.4

(R1 + 2R2) × C1

T
= time period in seconds (s)
f    = frequency in hertz (Hz)

R1 = resistance in ohms ()
R2 =
resistance in ohms ()
C1 =
capacitance in farads (F)

The time period can be split into two parts: T = Tm + Ts
Mark
time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time
(output low): Ts  = 0.7 × R2 × C1

Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is
much larger than R1.

For a standard astable circuit Tm cannot be less than Ts, but this is not too
restricting because the output can both sink and source current. For example an
LED can be made to flash briefly with long gaps by connecting it (with its
resistor) between +Vs and the output. This way the LED is on during Ts, so brief
flashes are achieved with R1 larger than R2, making Ts short and Tm long. If Tm
must be less than Ts a diode can be added to the circuit as explained under duty cycle
below.

Choosing R1, R2 and C1

555 astable frequencies

C1

R2 = 10k
R1 = 1k

R2 = 100k
R1 = 10k

R2 = 1M
R1 = 100k

0.001µF

68kHz

6.8kHz

680Hz

0.01µF

6.8kHz

680Hz

68Hz

0.1µF

680Hz

68Hz

6.8Hz

1µF

68Hz

6.8Hz

0.68Hz

10µF

6.8Hz

0.68Hz
(41 per min.)

0.068Hz
(4 per
min.)

R1 and R2 should be in the range 1k to 1M. It is best
to choose C1 first because capacitors are available in just a few values.

Choose C1 to suit the frequency range you require (use the table as
a guide).

Choose R2 to give the frequency (f) you require. Assume that R1 is
much smaller than R2 (so that Tm and Ts are almost equal), then you can use:

R2 =

0.7

f × C1

Choose R1 to be about a tenth of R2 (1k min.)
unless you want the mark time Tm to be significantly longer than the space
time Ts.

If you wish to use a variable resistor it is best to make it R2.

If R1 is variable it must have a fixed resistor of at least 1k in series
(this is not required for R2 if it is variable).

Astable operation
With the output high (+Vs) the capacitor C1 is charged by
current flowing through R1 and R2. The threshold and trigger inputs monitor the
capacitor voltage and when it reaches 2/3Vs (threshold
voltage) the output becomes low and the discharge pin is connected to 0V.

The capacitor now discharges with current flowing through R2 into the
discharge pin. When the voltage falls to 1/3Vs (trigger
voltage) the output becomes high again and the discharge pin is disconnected,
allowing the capacitor to start charging again.

This cycle repeats continuously unless the reset input is connected to 0V
which forces the output low while reset is 0V.

An astable can be used to provide the clock signal for circuits such
as counters.

A low frequency astable (< 10Hz) can be used to flash an LED
on and off, higher frequency flashes are too fast to be seen clearly. Driving a
loudspeaker or piezo transducer with a low frequency of less than 20Hz will
produce a series of 'clicks' (one for each low/high transition) and this can be
used to make a simple metronome.

An audio frequency astable (20Hz to 20kHz) can be used to produce a
sound from a loudspeaker or piezo transducer. The sound is suitable for buzzes
and beeps. The natural (resonant) frequency of most piezo transducers is about
3kHz and this will make them produce a particularly loud sound.

Duty cycle
The duty cycle of an astable circuit is the proportion of the
complete cycle for which the output is high (the mark time). It is usually given
as a percentage.

For a standard 555/556 astable circuit the mark time (Tm) must be greater
than the space time (Ts), so the duty cycle must be at least 50%:

Duty cycle  =

Tm

=

R1 + R2

Tm + Ts

R1 + 2R2

555 astable circuit with diode across
R2

To achieve a duty cycle
of less than 50% a diode can be added in parallel with R2 as shown in
the diagram. This bypasses R2 during the charging (mark) part of the cycle so
that Tm depends only on R1 and C1:

Tm = 0.7 × R1 × C1   (ignoring 0.7V across diode)

Ts  = 0.7 × R2 × C1   (unchanged)

Duty cycle with
diode  =

Tm

=

R1

Tm + Ts

R1 + R2

Use a signal
diode such as 1N4148.

Example projects using 555 astable: Flashing LED |
Dummy Alarm |
Heart-shaped Badge
| 'Random' Flasher

Top of page
| Inputs | Output | Astable | Duty Cycle |
Monostable |
Edge-trigger
| Bistable | Buffer

555/556 Monostable

555 monostable output, a single
pulse

555 monostable circuit with manual
trigger

A monostable circuit produces
a single output pulse when triggered. It is called a monostable because
it is stable in just one state: 'output low'. The 'output high' state is
temporary.

The duration of the pulse is called the time period (T) and this is
determined by resistor R1 and capacitor C1:

time period,
T = 1.1 × R1 × C1

T   = time period in seconds (s)
R1 = resistance in ohms ()
C1 = capacitance in farads (F)
The maximum reliable time
period is about 10 minutes.

Why 1.1? The capacitor charges to 2/3 = 67% so
it is a bit longer than the time constant
(R1 × C1) which is the time taken to charge to 63%.

Choose C1 first (there are relatively few values
available).

Choose R1 to give the time period you need. R1 should be in the
range 1k to 1M, so use a
fixed resistor of at least 1k in series
if R1 is variable.

Beware that electrolytic capacitor values are not accurate, errors
of at least 20% are common.

Beware that electrolytic capacitors leak charge which substantially
increases the time period if you are using a high value resistor - use the
formula as only a very rough guide!
For example the Timer Project
should have a maximum time period of 266s (about 4½ minutes), but many
electrolytic capacitors extend this to about 10 minutes!

Monostable operation
The
timing period is triggered (started) when the trigger input (555 pin 2)
is less than 1/3 Vs, this makes the output
high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the
time period has started further trigger pulses are ignored.

The threshold input (555 pin 6) monitors the voltage across C1 and
when this reaches 2/3 Vs the time period is over and
the output becomes low. At the same time discharge (555 pin 7) is
connected to 0V, discharging the capacitor ready for the next trigger.

The reset input (555 pin 4) overrides all other inputs and the timing
may be cancelled at any time by connecting reset to 0V, this instantly makes the
output low and discharges the capacitor. If the reset function is not required
the reset pin should be connected to +Vs.

Power-on reset or
trigger
circuit

Power-on reset or trigger
It may be useful to ensure that a monostable
circuit is reset or triggered automatically when the power supply is connected
or switched on. This is achieved by using a capacitor instead of (or in addition
to) a push switch as shown in the diagram.

The capacitor takes a short time to charge, briefly holding the input close
to 0V when the circuit is switched on. A switch may be connected in parallel
with the capacitor if manual operation is also required.

This arrangement is used for the trigger in the Timer Project.

Edge-triggering

edge-triggering
circuit

If the trigger input is still less than
1/3 Vs at the end of the time period the output will
remain high until the trigger is greater than 1/3 Vs.
This situation can occur if the input signal is from an on-off switch or sensor.

The monostable can be made edge triggered, responding only to
changes of an input signal, by connecting the trigger signal through a
capacitor to the trigger input. The capacitor passes sudden changes (AC) but
blocks a constant (DC) signal. For further information please see the page on capacitance. The
circuit is 'negative edge triggered' because it responds to a sudden fall in the
input signal.

The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger
is normally high (+Vs).

Example projects using 555 monostable: Adjustable Timer
| Electronic 'Lock'
| Light-sensitive Alarm

Top of page
| Inputs | Output | Astable | Duty Cycle |
Monostable |
Edge-trigger
| Bistable | Buffer

555/556 Bistable (flip-flop) - a memory circuit

555 bistable
circuit

The circuit is called a bistable
because it is stable in two states: output high and output low. It is
also known as a 'flip-flop'.

It has two inputs:

Trigger (555 pin 2) makes the output high.
Trigger is
'active low', it functions when < 1/3 Vs.

Reset (555 pin 4) makes the output low.
Reset is 'active
low', it resets when < 0.7V.

The power-on reset, power-on
trigger and edge-triggering circuits can all be used as described above for the
monostable.

Example projects using 555 bistable: Quiz | Model Railway Signal

Top of page
| Inputs | Output | Astable | Duty Cycle |
Monostable |
Edge-trigger
| Bistable | Buffer

555/556 Inverting Buffer (Schmitt trigger) or NOT gate

555 inverting buffer circuit
(a NOT
gate)

NOT gate symbol

The
buffer circuit's input has a very high impedance (about 1M) so it
requires only a few µA, but the output can sink or source up to 200mA. This
enables a high impedance signal source (such as an LDR) to switch a low
impedance output transducer (such as a lamp).

It is an inverting buffer or NOT gate because the output
logic state (low/high) is the inverse of the input state:

Input low (< 1/3 Vs) makes
output high, +Vs

Input high (> 2/3 Vs) makes
output low, 0V

When the input voltage is between
1/3 and 2/3 Vs the output remains in
its present state. This intermediate input region is a deadspace where there is
no response, a property called hysteresis, it is like backlash in a
mechanical linkage. This type of circuit is called a Schmitt trigger.

If high sensitivity is required the hysteresis is a problem, but in many
circuits it is a helpful property. It gives the input a high immunity to noise
because once the circuit output has switched high or low the input must change
back by at least 1/3 Vs to make the output switch
back.

Top of page
| Inputs | Output | Astable | Duty Cycle |
Monostable |
Edge-trigger
| Bistable | Buffer

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