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555 Timer

Modes
o There are three main modes that timers use. Monostable, Astable and Bistable
o Monostable mode produces an output signal (a pulse) that lasts for a specific length
of time
o
Astable mode produces a
shiftingxxxx (Rising and
falling)output signal. The signal is a square wave, that jumps between high and
low. Use switches or changes,, not jumps
o
o

The length of the high signal is called the mark time, and the length of the low signal
is called the space time
Bistable mode does not use any timing. It goes high when turned on, and low when
turned off
Monostable Mode
o In monostable mode, the output (pin 3) produces a constant signal until the timer is
turned off
o
Figure 1 shows a 555 timer IC in monostable mode
o
Timer is activated when a low pulse is applied to pin 2. This is usually done by
connecting to ground
o
The output (pin 3) goes high for the duration of the RC time constant. The resistor
and capacitor combination controls pins 6 and 7. Initially the capacitor is uncharged,
as it is connected pin 7. Pin 7 is grounded while the output (pin 3) is low. When the
circuit is activated, the cap starts charging. When it the voltage reaches 2/3 of Vcc,
the threshold (pin 6) stops the timer. The output then goes low, and the discharge
pin is grounded, discharging the capacitor
o

T = 1.1RC

T = Time, R = Resistance in Ohms, C = capacitance in Farads
R1 should be between
10k and 14M, and C2 should be between 100pF
and 1000uF specify TTL or CMOS 555
(7555) type resistor ranges, the two types
are different
o
Pin 2 (trigger) is connected to +V to keep the voltage on the pin high (timer off) until
the switch is pressed. The resistor, R2, is to limit the current, and protect the pin. R2
would be about 10KΩ
o
Pin 2 is kept high until the trigger is grounded (by a transistor or a switch), turning
the timer on
o
Monostable mode is useful for switch debouncing. This is when you add a manual
switch to a digital circuit. One physical press may look like several rapid presses to a
digital circuit, where a 555 timer monostable circuit will filter that out by producing
a single pulse of the correct length in time
o

Grounding pin 4 (Reset) would stop the timer prematurely
Astable Mode
o
The output in astable mode constantly
jumpsxxxx between off and on
(ground and +Vcc). This output is a square wave. Figure 2a

The ‘trough’ of the square wave is a space period (ground), and the ‘peak’ is
called a mark period (+Vcc)
o
The mark period can be a different length to the space period. This is known as the
‘mark-to-space ratio’, or the ‘duty cycle’. It is measured as a ratio, such as 1:3 (1 is
the mark, 3 is the space) or as a percentage, such as 25% (which is the percentage of
the wavelength that is mark period).
o
Figure 2b shows a 555 timer in astable mode
o
The mark period (the time the output is on) is controlled by the RC combo of R1, R2
and C. This is the same principle that monostable uses
o

T = 0.69RC

T = 0.69(R1 + R2)C
The space period uses the RC combo of R2 and C. When the voltage on pin 2 goes
above 2/3, output goes low, and pin 7 connects to ground. The capacitor then begins
to discharge through R2. The time it takes to discharge through R2 controls the
space period

T = 0.69RC

T = 0.69 x R2 x C
o
The astable period is the time taken for the whole cycle. This is the mark period, plus
the space period
o
Formulas use fundamental units, which means that if a R1 10kΩ, the forumula will
use the number 10000, and if C1 is 1μF, the number in the formula is 0.000001
o
To make it easier, Figure 4 shows simple units you can use. Eg, 10kΩ and 1μF will
yeild a result in milliseconds (ms)
o
F = 1.44 / (R1 + 2R2)C


F = Frequency
Bistable Mode
o
o
o
o
o
o
Bistable mode does not use any timing at all.
Figure 3 shows the 555 timer IC in bistable mode
When the trigger input is low (when the button is pushed), the output goes high (the
555 IC turns on)
When the reset input goes low (the reset button is pushed), the output goes low
(the IC turns off)
As timing is not used, pin 6 and 7 are not used
The resistors are to keep the voltage high on pins 2 and 4, to prevent unwanted
resets and triggers

Pin 1 is the ground, or the 0 volt connection to the power supply

Pin 8 is the power supply. The typical range is from 4.5 to 15v. Also called Vcc

Pin 2 is the trigger. This is where the timer is started. This will turn the chip on if the voltage
is less than 1/3 of Vcc


Pin 6 is the threshold. The output goes low when the voltage on this pin reaches 2/3 of Vcc

Pin 7 is discharge. Connects to ground when output goes low. Pins 2, 6 and 7 control the
timing. 2 and 7 are often connected to a resistor and capacitor combination for this purpose

Pin 4 is the reset input. The output is reset to low if pin 4 is grounded. This is not always
used, and in these cases it is wired directly to Vcc, to prevent unwanted grounding (which is
an unwanted reset)

Pin 3 is the output. This goes high (has a voltage) while the chip is ‘on’. If the chip is in
astable mode, the output will
jumpxxx between high and low. The output level stays
at 0V (low) or +Vcc (high). Can deliver up to 200mA Specify cmos/ttl !!!!

CMOS and TTL are different

Pin 5 is control, or offset. Used to control the trigger and threshold voltages for pins 2 and 6.
o
If this is not used, it should be connected to ground with a 0.01uF capacitor to
eliminate RF noise
o
Pin 5 can be controled by an external input, such as a potentiometer, to change the
thresholds. Combined with another timer, it can be used to create an AC signal,
whats this mean.???
Fig. 1 – Monostable Mode
Fig. 2a – Square Wave
Fig. 2b – Astable Mode
Fig. 3 – Bistable Mode
R
Ω
MΩ
kΩ
C
F
μF
μF
T
s
s
ms
F
Hz
Hz
kHz
WHAT IS THIS TABLE SHOWING?????
I think it is wrong
Fig. 4 – Time and Frequency Units