EE 503, SP 555 Timer IC
EE 503, SP
555 Timer IC
EE 503, SP
Widely used IC
It can run in:
Monostable
mode
(one stable state) -
it can
produce accurate time delays from micro-secs
to hours
Astable
mode
( no stable states)
it can produce
Rectangular waves with variable duty cycle
Bistable
Mode
(two stable states)
And lots of other circuits
EE 503, SP
Pin Configuration
EE 503, SP
Pin Configuration
EE 503, SP
Functional Block Diagram
Upper comparator
Lower comparatorDischarge Transistor
Reset Transistor
_Q
Q
EE 503, SP
555 IC comprises of :
Two op-amp comparators
A flip-flop
A discharge transistor
A reset transistor
Three identical resistors (e.g
5 k)
Output stage
The resistors set the reference voltage
levels at the non-inverting I/P of the lower comparator & inverting I/P of the upper comparator at +Vcc/3
& +2Vcc/3
EE 503, SP
when output Q bar is high, the discharge transistor is off (as Q low)
When output low, discharge transistor On
Pin 6 Voltage (threshold) > 2Vcc/3 : O/P of upper comparator high, S=1, Q=1, discharge transistor On
Pin 2 voltage (trigger) < Vcc/3 : O/P of the lower comparator high, R =1, Q=0, discharge transistor off
EE 503, SP
Main Features1.
Timing from micro-secs
to hours
2.
Monostable
and astable
operation3.
Adjustable duty cycle
4.
Ability to operate from a wide range of supple voltages
5.
O/P compatible with CMOS, DTL & TTL (when used with a 5V supply)
6.
High current. O/P can sink or source 200 mA7.
Trigger and reset inputs are logic compatible
8.
O/P can be operated normal on and normal off9.
High temperature stability
EE 503, SP
Descriptions of the Pins
Pin 1:
GroundPin 2: Trigger: Pin 2 voltage (trigger) < Vcc/3 : O/P of the lower
comparator high
Pin 3: O/P :
Two ways of connecting a loadi) between pin 3 & ground (pin 1)Or ii) Between pin 3 & +Vcc
(pin 8)
EE 503, SP
When O/P is low:
(i)
If the load is connected between pin 3 & 1(gnd):
The current through the load is zero
normally off load
(ii) If the load is connected between pin 3 & 8(+Vcc):
The current through the load flows into the O/P terminal & is called sink current. The load connected between pin 3 & +Vcc
is called
normally on load
EE 503, SP
When O/P is high:
(i)
If the load is connected between pin 3 & 1(gnd):
The output terminal supplies
the current to the normally off load. This current is called source current
(ii) If the load is connected between pin 3 & 8(+Vcc):
The current through the load is zero
EE 503, SP
Pin 4: Reset: The timer can be reset or disabled (o/p voltage zero) by applying a ve
pulse to this pin
When not is use, this terminal should be connected to +Vcc
to avoid any possibility of false triggering
Pin 5: Control voltage: If we do not want to operate the comparators at +Vcc/3 or +2Vcc/3. An external voltage applied to this pin (or by connecting a pot between this pin & gnd) changes threshold as well as trigger voltage
When not is use, this pin should be bypassed to gnd
with a 0.01F capacitor to prevent any noise problem
EE 503, SP
Pin 6: Threshold:
When pin 6 voltage > 2Vcc/3, the o/p
of the upper comparator high, S=1, Q=1,
discharge transistor On, O/P of the timer low
Pin 7: Discharge: This pin is internally connected to the collector of the discharge transistor.
Pin 8: +Vcc: The supply voltage of +5V to +18V
is applied to this pin w.r.t
ground (pin 1)
EE 503, SP
Operation as Astable
Multivibrator
EE 503, SP
Operation as Astable
Multivibrator
R1
R2
EE 503, SP
Operation as Astable
Multivibrator
Initially C is fully discharged which forces the O/P to high state.
Pin 2 < Vcc/3, R=1, Q=0, discharge transistor Off, the capacitor starts charging from +Vcc
through R1
& R2
.
When Vc
> 2Vcc/3, Q=1, discharge transistor on, C is discharged through this transistor & R2
When Vc
< Vcc/3, Q=0, discharge transistor off, C will again starts charging to 2Vcc/3
EE 503, SP
High & Low state time periods are generated by the charge (+Vcc/3 to +2Vcc/3) & discharge (+2Vcc/3 to +Vcc/3)
High state time period, Thigh
or Ton
=0.69(R1
+R2
)CLow state time period, Tlow
or Toff
=0.69R2
C
Time period, T = Thigh
+ Tlow
= 0.69(R1
+ 2R2
)Cf= 1 / 0.69(R1
+ 2R2
)CDisadvantage:1. Thigh is always > Tlow2. Thigh
& Tlow
both dependent on R2
EE 503, SP
Operation as Monostable
Multivibrator (Monoshot)
=0.01F
EE 503, SP
Operation as Monostable
Multivibrator (Monoshot)
Pin 2 (trigger) is initially kept at +Vcc. So O/P low. Now pin 2 is lowered
Pin 2< vcc/3 , Q=1, C starts charging through R.
When Vc
> 2Vcc/3, Q=1, C discharges through discharge transistor.
EE 503, SP
Operation as Monostable
Multivibrator (Monoshot)
EE 503, SP
Operation as Monostable
Multivibrator (Monoshot)
Everytime
the O/P is appropriately triggered, the O/P goes to high state & stays there for a time period taken by the capacitor to charge from 0 to +2Vcc/3.
The time period or the monoshot
pulse width, T,T =1.1 RA
CThe pulse width of the trigger I/P should be less
than the high time of the monoshot
O/P
555 Timer ICSlide Number 2Pin ConfigurationPin ConfigurationFunctional Block DiagramSlide Number 6Slide Number 7Main FeaturesDescriptions of the PinsSlide Number 10Slide Number 11Slide Number 12Slide Number 13Operation as Astable MultivibratorOperation as Astable MultivibratorOperation as Astable MultivibratorSlide Number 17Operation as Monostable Multivibrator (Monoshot)Operation as Monostable Multivibrator (Monoshot)Operation as Monostable Multivibrator (Monoshot)Operation as Monostable Multivibrator (Monoshot)