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. Low power versions, such as the ICM7555, are available with the
same pin arrangement but their maximum output current is much lower and
they should only be used when specified (to increase battery life).
The 555 can be used in several circuits:
Astable - producing a square wave for flashing LEDs,
making sounds, driving counters etc.
Monostable - producing a single pulse when triggered, this can used for timing.
The circuit symbol pins are arranged to suit the circuit: for example pin 8 at the top
for the +Vs supply, pin 3 output on the right. Usually just the pin numbers are used and they are
not labelled with their function.
The 555 timer can be used with a supply voltage (Vs) in the range
4.5V to 15V (18V is the absolute maximum).
Pin 1 connects to 0V. Pin 8 connects to the positive supply +Vs.
Beware that the 555 creates a significant 'glitch' on the supply when its output changes
state. This is rarely a problem in simple circuits with no other ICs but in more complex circuits a
smoothing capacitor may be required.
555 pin arrangement
555 trigger input (pin 2)
When less than 1/3 Vs ('active low') this makes the output high (+Vs).
It has a high input impedance of at least 2M.
It monitors the discharging of the timing capacitor in an astable circuit.
555 threshold input (pin 6)
When greater than 2/3 Vs ('active high') this makes the output low (0V)*.
It has a high input impedance of about 10M.
It monitors the charging of the timing capacitor in astable and monostable circuits.
* providing the trigger input is greather than 1/3 Vs,
otherwise the trigger input will override the threshold input and hold the output high (+Vs).
555 reset input (pin 4)
When less than about 0.7V ('active low') this makes the output low (0V),
overriding the other inputs. When not required it should be connected to +Vs.
It has an input impedance of about 10k.
555 control input (pin 5)
This can be used to adjust the threshold voltage (used by the threshold input, pin 6) which is set internally
to be 2/3 Vs. Usually this function is not required and the control
input is often left unconnected. If electrical noise is likely to be a problem a 0.01µF capacitor
can be connected between the control input and 0V to provide some protection.
555 discharge (pin 7)
When the 555 output (pin 3) is low the discharge pin is connected to 0V internally.
Its function is to discharge the timing capacitor in astable and monostable circuits.
The output of a standard 555 can sink and source current.
This 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 diagram shows two LEDs connected in this way.
The maximum output current is 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 small motors (with diode protection).
The output voltage does not quite reach 0V and +Vs, especially if a large
current is flowing.
The maximum output current of low power versions of the 555 (such the ICM7555) is much
lower: about 20mA with a 9V supply.
Connecting the 555 to a loudspeaker
A loudspeaker (minimum resistance 64)
may be connected to the output of a 555 astable circuit but a capacitor (about
100µF) must be connected in series. The astable 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.
Connecting the 555 to relay coils and other inductive loads
Like all ICs, the 555 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 requires 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 sufficient current, a signal diode such as a 1N4148
is usually not suitable.
556 Dual Timer IC
The 556 is a dual version of the 555 timer housed in a 14-pin package, the
two timers (A and B) share the same power supply pins.
The circuit diagrams on this website show a 555, but they could all be adapted to use one half of a 556.
The 556 is less popular and may cost more than two 555s so you may prefer to use two 555 timers.
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