The document discusses the 555 timer integrated circuit. It describes the 555 timer's basic components, including two voltage comparators, a bistable flip flop, discharge transistor, and resistor divider network. It also discusses the 555 timer's pin configurations and two common modes of operation: astable and monostable. Formulas are provided for calculating the timing parameters in each mode. Sample calculations are shown for an astable oscillator with a frequency of 200Hz and 78% duty cycle.

1. Chapter 7
Timer555

2. 55
5.2
555 timer;
- What is the 555 timer?
- Pin configurations of the 555 timer ;
- 555 timer in astable operation ;
- 555 timer in monostable operation ;
- What are the 555 timer applications?

3. 55
5.3
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.

4. 55
5.4
The 555 timer consists of;
two voltage comparators
a bi-stable flip flop
a discharge transistor
and a resistor divider network

5. 555 Layout
555.5
Also available:
•556 (two-555’s in one DIP package)
•555 in a “metal can” configuration
1
2
3
4
8
7
6
5
Ground
Trigger
Output
Reset
Vcc
Discharge
Threshold
Control
1
2
3
4
5
6
7
8
Ground
Trigger
Output
Reset
Vcc
Discharge
Threshold
Control

6. 55
5.6
Functional Diagram of 555 Timer

7. Reference and Comparators
55
5.7
The Comparators
provide digital logic
to an SR Latch
Comparators compare voltage
levels.
•If “+” is higher, output = 1
•If “-” is higher, output = 0
A 3-resistor
voltage divider
provides
reference
voltages

8. Reference and Comparators
55
5.8
Pin 8: Vcc
connection
Pin 5: “Control”
connection,
used with filter
cap.
Pin 1: Gnd
connection
Pin 6: “Threshold”
connection (high
Comparator)
Pin 2: “Trigger”
connection (low
Comparator)

9. Latch and Output
55
5.9
The Q’ output controls
the transistor (“on” or
“off”).
The SR Latch
holds its output
states
The transistor
acts like a switch.
The reset input can be
used to enable or
disable the timer.
The buffer has important
electrical functions.

10. Latch and Output
55
5.1
0
Pin 4: Reset
connection (active
low reset)
Pin 7:
“Discharge”
connection
Pin 3: Digital
Output

11. 555 Timer Configurations
In this course we will use the 555 timer in 2
modes:
◦Astable
With some calculations, we can determine the values of the
capacitor and the 2 resistors (R1 and R2) for astable
operations.
◦Monostable
We can determine the value of the resistor and the
capacitor with a simple formula for one-shot operations.
There are many other configurations and
applications for this device. 555
.11

12. Calculations: Astable
C)RR(693.0T
CR693.0T
BAH
BL


555
.12
TH TL
Notes:
•The value 0.693 is a factor associated with the
charge/discharge cycle of the 555 timer.
•Duty Cycle must be > 50%
Time High, Time Low Set

13. 55
5.1
3
Astable Output Voltage

14. 55
5.1
4
Monostable Output Voltage

15. 55
5.1
5
Monostable Characteristics
Generates a single pulse of a fixed time
duration each time it receives an input
trigger pulse
Length of pulse is given by:
T= 1,1 RC

16. 555.16
Sample Calculation
Design an oscillator with a frequency of
200Hz with a duty cycle of 78%.
.1Determine Period (T):
.2Determine TH and TL:
s005.0
Hz200
1
F
1
T 
ms1.1s0011.0s005.0%22T
ms9.3s0039.0s005.0%78T
L
H


Time High, Time Low Set

17. © 55
5.1
7
END