555 TimerCh. Srividhya
Asst Prof
ECE
JITS
Pin configuration
The 555 timer consists of; Two voltage comparators
A bi-stable flip flop
A discharge transistor
and a resistor divider network.
Internal Circuit Of 555 Timer
VA
VB
Output
Threshold
Control
Voltage
Trigger
Discharge
ResetVCC
Voltage Divider
Comparator
RS-FF
Discharge
TransistorGND
+ +C1
+ +C2
Q
QRD
SD
5KΩ
5KΩ
5KΩ
T
2
4
5
6
7
8
3
1
<2/3 VCC <1/3 VCC 1 0
>2/3 VCC >1/3 VCC 0 1
<2/3 VCC >1/3 VCC 1 1
>2/3 VCC <1/3 VCC 0 0
RD SDV6 V2
The result for
comparing
1/3 VCCNot permitted
2/3 VCC
+ +
C1
+ +C2
.
.
5KΩ
5KΩ
5KΩ
VA
VB
VCC
RD
SD
5
6
2
V6 V2
<2/3 UCC <1/3 UCC
>2/3 UCC >1/3 UCC
<2/3 UCC >1/3 UCC
Q T
1
0
保持
on
off
保持
Q
QRD
SD
Toutp
ut
RD SD
1 0
10
1 1
Q T
1
0
remain
on
off
remain
remain remain
Monostable Multivibrator & Waveforms
Monostable Multivibrator (One-shot) Operation
(grand)
uC
R1
+VCC
ui
VCC
+ +C1
+ +C2
Q
QRD
SD
5KΩ
5KΩ
5KΩ
VA
VB
T
1
3
48 (Re)
uO
6
5
2
7
>2/3 VCC
0
1 1 0
1
Q=0
on
1 Steady state
(grand)
uC
R1
+VCC
ui
VCC
+ +C1
+ +C2
Q
QRD
SD
5KΩ
5KΩ
5KΩ
VA
VB
T
1
3
48 (reset)
uO
6
5
2
7
1
1 0
1
Q=1
off
Transiet state
0
< 1/3 UCC
0
0
1
Monostable Multivibrator (One-shot) Operation
Monostable Multivibrator(One-shot) Operation
(grand)
uc
R1
+VCC
ui
VCC
+ +C1
+ +C2
Q
QRD
SD
5KΩ
5KΩ
5KΩ
VA
VB
T
1
3
48 (reset)
uO
6
5
2
7
>2/3 VCC
1
0 1
0
Q=1
0
1 01
1
0
Steady state
Q=0
ui
t
uC
tuO
t
tp =RC ln3=1.1RC
2/3UCC
What is a monostable
multivibrator?
•With one stable state
•Normally in its stable
state,to change to its
unstable state only
when triggered
•Once triggered,to remain
in its unstable state for a
predetermined length of
time and then return to its
stable state automatically
Astable Multivibrator & Waveforms
UCC
+ +C1
+ +C2
Q
QRD
SD
.
.
.
5KΩ
5KΩ
5KΩ
VA
VB
T
1
32
4
56
7
8
(grand)
uO
2) Astable Multivibrator(Oscillator)
first
uC=0
0
1
11
0
0
>2/3 UCC
RD=1
SD=0
.
.
uC
R1
R2
.+
–
charge
discharge
1
<1/3 UCC
4856
2
7 1
3
+VCC
uO
.
uC..
C
R1
R2
tp1=(R1+R2)C ln2=0.7(R1+R2)C
tp2 =R2C ln2=0.7R2C
T=tp1+tp2 =0.7(R1+2R2)C
tp1 tp2
2/3UCC
1/3UCC
uC
t
uO
t
What is an astable
multivibrator?Duty Cycle = tp1/T
Applications using 555 timer
A light dimmer ;
A car tachometer
Traffic Lights ;
Infra Red (IR) remote control ;
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