555 And 556 Timer Circuits

26/ 11/ 12

555 and 556 Tim er C irc uit s

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555 and 556 Ti mer Ci rcui ts
Inputs | Output | As table | Duty Cy c le | Monos table | Edge-trigger | Bis table | Buffer

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

Intr oduction
The8-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 ICM7555has the
same pin arrangement as a standard 555.

Ex ample c irc uit s y mbol (above)
Ac tual pin arrangements (below)

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.
The555 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 or556.
www. k ps ec . f reeuk . c om / 555t im er. ht m #t opof page

There is more information about
555 timers and their c irc uits on the

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

555 timers and their c irc uits on the
Elec tronic s in Mec c ano webs ite.

Astable - producing a squarewave
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 timingcapacitor 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 , otherwis e the trigger input will
override the thres hold input and hold theoutput 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 isconnected 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...