2. What is the 555 timer?
• The 555 timer is one of the most remarkable integrated
circuits ever developed. It comes in a single or dual
package and even low power cmos versions exist -
ICM7555.
• Common part numbers are LM555, NE555, LM556,
NE556. The 555 timer consists of two voltage
comparators, a bi-stable flip flop, a discharge transistor,
and a resistor divider network.
4. – The voltage divider (blue) has three equal 5K resistors.
It divides the input voltage (Vcc) into three equal
parts.
– The two comparators (red) are op-amps that compare
the voltages at their inputs and saturate depending
upon which is greater.
• The Threshold Comparator saturates when the voltage at the
Threshold pin (pin 6) is greater than (2/3)Vcc.
• The Trigger Comparator saturates when the voltage at the
Trigger pin (pin 2) is less than (1/3)Vcc
Inside the 555 Timer
5. – The flip-flop (green) is a bi-stable device. It generates two
values, a “high” value equal to Vcc and a “low” value equal to
0V.
• When the Threshold comparator saturates, the flip flop is Reset (R) and
it outputs a low signal at pin 3.
• When the Trigger comparator saturates, the flip flop is Set (S) and it
outputs a high signal at pin 3.
– The transistor (purple) is being used as a switch, it connects pin
7 (discharge) to ground when it is closed.
• When Q is low, Qbar is high. This closes the transistor switch and
attaches pin 7 to ground.
• When Q is high, Qbar is low. This open the switch and pin 7 is no
longer grounded
6. What are the 555 timer
applications?
• Applications include
– precision timing,
– pulse generation,
– sequential timing,
– time delay generation and pulse width modulation
(PWM).
8. • Pin Functions - 8 pin package
• Ground (Pin 1)
• Not surprising this pin is connected directly to ground.
• Trigger (Pin 2)
• 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.
• Output (Pin 3)
• Output high is about 1.7V less than supply. Output high is capable of Isource up to
200mA while output low is capable of Isink up to 200mA.
• Reset (Pin 4)
• This is used to reset the latch and return the output to a low state. The reset is an
overriding function. When not used connect to V+.
9. • Control (Pin 5)
• Allows access to the 2/3V+ voltage divider point when the 555 timer is
used in voltage control mode. When not used connect to ground through a
0.01 uF capacitor.
• Threshold (Pin 6)
• This is an input to the upper comparator.
• Discharge (Pin 7)
• This is the open collector to Q14 in figure 4 below.
• V+ (Pin 8)
• This connects to Vcc and the Philips databook states the ICM7555 cmos
version operates 3V - 16V DC while the NE555 version is 3V - 16V DC.
Note comments about effective supply filtering and bypassing this pin
below under "General considerations with using a 555 timer"
10. Types of 555-Timer Circuits
• Astable Multivibrator
puts out a continuous
sequence of pulses
5V
Ra
C
0.01uF
LED
NE555
2
5
3
7
6
4
81
TR
CV
Q
DIS
THR
R
VCCGND
Rb
5V
12
1K
0.01uF
C
R
LED
NE555
2
5
3
7
6
4
81
TR
CV
Q
DIS
THR
R
VCCGND
Monostable Multivibrator (or one-
shot) puts out one pulse each time
the switch is connected
11. • Monostable Multivibrator (One Shot)
+V
-V
-
+
+V
-V
-
+
R
S
Q
Q
3
4
1
7
2
6
8
R
R
R
Control Flip-FlopTrigger Comparator
Threshold Comparator
Output
ResetVcc
Trigger
Monstable Multivibrator
One-Shot
C
Ra
cc
2
V
3
cc
1
V
3
12. Behavior of the Monostable Multivibrator
• The monostable multivibrator is constructed by adding an
external capacitor and resistor to a 555 timer.
• The circuit generates a single pulse of desired duration
when it receives a trigger signal, hence it is also called a
one-shot.
• The time constant of the
resistor-capacitor
combination determines
the length of the pulse.
13. – Used to generate a clean pulse of the correct height
and duration for a digital system
– Used to turn circuits or external components on or off
for a specific length of time.
– Used to generate delays.
– Can be cascaded to create a variety of sequential
timing pulses. These pulses can allow you to time and
sequence a number of related operations.
Uses of the Monostable Multivibrator
14. Astable Pulse-Train Generator (Multivibrator)
+V
-V
-
+
+V
-V
-
+
R
S
Q
Q
3
4
1
7
2
6
8
R
R
R
Control Flip-FlopTrigger Comparator
Threshold Comparator
Output
Vcc
Astable Pulse-Train Generator
C
R1
R2
15. Behavior of the Astable Multivibrator
• The astable multivibrator is simply an oscillator. The astable
multivibrator 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.
• The capacitor C charges through the series resistors R1 and R2
with a time constant
(R1 + R2)C.
• The capacitor discharges
through R2 with a time
constant of R2C
16. – Flashing LED’s
– Pulse Width Modulation
– Pulse Position Modulation
– Periodic Timers
Uses of the Astable Multivibrator
17. Flashing LED’s
• 40 LED bicycle light with 20 LEDs flashing
alternately at 4.7Hz
18. Understanding the Astable Mode Circuit
• 555-Timers, like op-amps can be configured in different ways to
create different circuits. We will now look into how this one
creates a train of equal pulses, as shown at the output.
19. First we must examine how capacitors charge
• Capacitor C1 is charged up by current flowing
through R1
• As the capacitor charges up, its voltage increases
and the current charging it decreases, resulting in
the charging rate shown
VV V
R1
1k
U2
TOPEN=0
12
C1
1uF
U1
TCLOSE =0
1 2
0
V1
10V
I
V V
R
V
k
C A P A C I T O R C A P A C I T O R
=
−
=
−1
1
1 0
1
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 ms
V( U2 : 1 ) V( R1 : 2 ) V( V1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
Ca p a c i t o r Vo l t a g e
20. Capacitor Charging Equations
• Capacitor Current
• Capacitor Voltage
• Where the time constant
Ti me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 ms
I ( R1 ) I ( C1 )
0 A
2 mA
4 mA
6 mA
8 mA
1 0 mA
Ca p a c i t o r a n d Re s i s t o r Cu r r e n t
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 ms
V( U2 : 1 ) V( R1 : 2 ) V( V1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
Ca p a c i t o r Vo l t a g e
I I eo
t
=
−
τ
V V eo
t
= −
−
1 τ
τ = = ⋅ =R C R C m s1 1 1
21. Understanding the equations
• Note that the voltage rises to a little above 6V
in 1ms.
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 ms
V( U2 : 1 ) V( R1 : 2 ) V( V1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
Ca p a c i t o r Vo l t a g e
( ) .1 6 3 21
− =−
e
22. Capacitor Charging and Discharging
• There is a good description of capacitor charging
and its use in 555 timer circuits at
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
23. 555 Timer
• At the beginning of the
cycle, C1 is charged through
resistors R1 and R2. The
charging time constant is
• The voltage reaches
(2/3)Vcc in a time
1)21(693.01arg CRRTt ech +==
1)21(arg CRRech +=τ
24. 555 Timer
• When the voltage on the
capacitor reaches (2/3)Vcc,
a switch (the transistor) is
closed (grounded) at pin 7.
• The capacitor is discharged
to (1/3)Vcc through R2 to
ground, at which time the
switch is opened and the
cycle starts over. 1)2(arg CRedisch =τ
1)2(693.02arg CRTt edisch ==
25. 555 Timer
• The frequency is then given by
f
R R C R R C
=
+ ⋅
=
+ ⋅
1
0 6 9 3 1 2 2 1
1 4 4
1 2 2 1. ( )
.
( )
26. Output voltage high
turns off upper LED
and turns on lower
LED
Capacitor is charging through Ra and Rb
Output is high for
0.693(Ra+Rb)C
555 Animation
http://www.williamson-labs.com/pu-aa-555-timer_slow.htm
27. Output is low
so the upper
LED is on and
the lower LED
is off
Capacitor is discharging
through Rb
Output is low for
0.693(Rb)C
555 Animation
28. PWM: Pulse Width Modulation
• Signal is compared to a sawtooth wave
producing a pulse width proportional to
amplitude
29. What Can Be Done With PWM?
• Question: What happens if voltages like the
ones above are connected to a light bulb?
Answer: The longer the duty cycle, the longer
the light bulb is on and the brighter the light.
Low
Duty Cycle
Medium
Duty Cycle
High
Duty Cycle
30. What Can Be Done With PWM?
• Average power can be controlled
• Average flows can also be controlled by fully opening and
closing a valve with some duty cycle
