1. 555 Timer

2. Pin configuration The 555 timer consists of; Two voltage comparators
A bi-stable flip flop
A discharge transistor
and a resistor divider network.

3. Internal Circuit Of 555 Timer

4. + C1 C2 Q RD SD 5K T 2 4 5 6 7 8 3 1 VCC Reset VA Output VB RS-FF
Control Voltage VA
Threshold
Output
Trigger VB
Discharge
DischargeTransistor
GND
RS-FF
Voltage Divider
Comparator

5. The result for comparing
2/3 VCC + C1 C2 . 5K VA VB
VCC RD SD 5 6 2 RD SD V6 V2
<2/3 VCC
<1/3 VCC 1
>2/3 VCC
>1/3 VCC 1
<2/3 VCC
>1/3 VCC 1
>2/3 VCC
<1/3 VCC
Not permitted
1/3 VCC

6. Q RD SD T output RD SD Q T 1 1 off 1 on 1 1 V6 V2 <2/3 UCC1
off 1 on 1 1
remain
remain V6 V2
<2/3 UCC
<1/3 UCC
>2/3 UCC
>1/3 UCC Q T 1 保持 on
off
remain
remain

7. Monostable Multivibrator & Waveforms

8. uO Monostable Multivibrator (One-shot) Operation +VCC 4 8 R1 5 VA + 6uC R1
+VCC ui
VCC + C1 C2 Q RD SD 5K VA VB T 1 3 4 8
(Re) uO 6 5 2 7 1
Steady state 1 1 1
Q=0
>2/3 VCC on
(grand)

9. Monostable Multivibrator (One-shot) OperationuC R1
+VCC ui
VCC + C1 C2 Q RD SD 5K VA VB T 1 3 4 8
(reset) uO 6 5 2 7
Transiet state 1 1 1
Q=1
< 1/3 UCC
off
(grand)

10. Monostable Multivibrator(One-shot) Operationuc R1
+VCC ui
VCC + C1 C2 Q RD SD 5K VA VB T 1 3 4 8
(reset) uO 6 5 2 7 1
Steady state 1 1 1 1
Q=1
Q=0
>2/3 VCC
(grand)

11. ui t uC t uO t What is a monostable multivibrator?
With one stable state uC t
Normally in its stable state,to change to its unstable state only when triggered
2/3UCC uO t
Once triggered,to remain in its unstable state for a predetermined length of time and then return to its stable state automatically tp
=RC ln3=1.1RC

12. Astable Multivibrator & Waveforms

13. 2) Astable Multivibrator(Oscillator)
UCC + C1 C2 Q RD SD . 5K VA VB T 1 3 2 4 5 6 7 8
(grand) uO
charge . uC R1 R2 + – 1 1 1
>2/3 UCC
<1/3 UCC
first
uC=0 1
RD=1
SD=0
discharge

14. . uC uO uO 4 8 5 6 2 7 1 3 +VCC uC C R1 R2 tp1 tp2 t tp1
=(R1+R2)C ln2=0.7(R1+R2)C
tp2
=R2C ln2=0.7R2C
T=tp1+tp2
=0.7(R1+2R2)C
What is an astable multivibrator?
Duty Cycle = tp1/T
With one stable state

15. Applications using 555 timer
A light dimmer ;
A car tachometer
Traffic Lights ;
Infra Red (IR) remote control ;