Transcript 3.555 Working CW

UNIT-IV
Application of Special I.C’s.
The 555 Timer, 555 as Monostable, Astable
Multivibrator and Applications, Phase Locked
Loops, Operating Principles.
Inside the 555 timer there are
 Over 20 transistors, 15 resistors,
2 diodes, depending of the manufacturer.
 Supply voltage between 4.5 and 18 volt, supply current
3 to 6 mA,
 Sinking or sourcing 200 mA of load current.
 Rise/Fall time of 100 nSec.
 The Threshold current determine the maximum
 value of Ra + Rb. For 15 volt operation the
 maximum total resistance for R, i.e. (Ra +Rb) is 20
Mega-ohm.
 The temperature variation is only 50ppm/°C (0.005%/°C).
Inside the 555
S
Q
R
 Note the voltage divider inside the 555 made up of 3 equal
5k resistors
Salient Functional Features of 555.
1. Trigger (Pin2) <1/3 Vcc, sets Vo (Pin3) to “1”. Once triggered
even pin2 is made 1 or 0 there is no change in the output.
2. Threshold (Pin6) > 2/3 Vcc sets Vo (Pin3) to “0”
3. Reset “0” (Pin4) sets Vo to “0” any time. Usually it is at “1”
4. Initially (Pin7) is set to ground as the power supply is on.
Monostable Operation or Timer.
Initial Condition:
R(0), S(1)
1. Threshold Comparator.
Ref: Inv(-) at 2/3 Vcc
2. Trigger Comparator.
Ref: non Inv(+) at 1/3
Vcc .
Q2 is a PNP transistor used to
reset and make a short the
NPN transistor when Pin.4
(Base) is grounded. Usually
Pin.4
is connected to +
of power
R-S Flip Flop
S
0
0
1
1
S
Q
R
Q
R
0
1
0
1
OUTPUT Q
No Change
0
1
Not allowed
REMARK
FF Resets
FF Sets
Supply so that transistor is open. Pin.5 is at 2/3 Vcc. The
comparator output is –ve, Q is 1 making short of pin.7 to ground.
Note that the trigger pulse must actually be of
shorter duration than the time interval
determined by the external R and C. When pin
2 is held low longer than that, the resultant
output will remain high until the trigger input is
driven high once again. In the mean while
even pin2 is made 1 or 0 there is no change in
the output.
Monostable Operation or Timer.
DERIVATION
 The time t1 taken by the circuit to charge from 0 to (2/3) Vcc is
 t1 = 1.098RC
 The time t2 taken by the circuit to charge from 0 to (1/3) Vcc is
 t2 = 0.405 RC
 The time to charge from (1/3) Vcc to (2/3) Vcc is
 tHigh = t2-t1 = 1.098RC - 0.405 RC = 0.693 RC
Connecting a Load.
Current Sinking and Sourcing.
Charging a Capacitor
10V
TCLOSE = 0
1
U1
R1
2
8V
V
V
1
V
1k
6V
U2
V1
TOPEN = 0
Voltage
C1
4V
2
10V
Capacitor
1uF
2V
0V
0
0s
1ms
V(U2:1)
V(R1:2)
2ms
3ms
4ms
5ms
6ms
7ms
8ms
9ms
V(V1:+)
Time
 Capacitor C1 is charged up by current flowing through
R1
V 1  VCAPACITOR 10  VCAPACITOR
I

R1
1k
 As the capacitor charges up, its voltage increases and the
current charging it decreases, resulting in the charging
rate shown
10ms
Charging a Capacitor
10mA
10V
8mA
8V
6mA
Capacitor
and
Resistor
6V
Current
Capacitor
4mA
4V
2mA
2V
0A
Voltage
0V
0s
1ms
I(R1)
2ms
3ms
4ms
5ms
6ms
7ms
8ms
9ms
10ms
I(C1)
0s
1ms
V(U2:1)
V(R1:2)
2ms
3ms
4ms
 Capacitor Current
 Capacitor Voltage
 Where the time constant
5ms
6ms
7ms
8ms
9ms
10ms
V(V1:+)
Time
Time
I  Ioe
 t
V  Vo 1  e
 t


  RC  R1 C1  1ms
Charging a Capacitor
10V
8V
6V
Capacitor
Voltage
4V
2V
0V
0s
1ms
V(U2:1)
V(R1:2)
2ms
3ms
4ms
5ms
V(V1:+)
Time
6ms
7ms
8ms
9ms
10ms
Charging a Capacitor
The time that it takes for the capacitor to charge to 63.7% of the
applied voltage is known as the time constant (t). It takes
approximately 5 complete time constants for the capacitor to charge
to almost the applied voltage.
Astable Multivibrator
Initially when the output is high capacitor C starts
charging towards Vcc through RA and RB. However as
soon as the voltage across the capacitor equals 2/3 Vcc ,
comparator1 triggers the flip-flop and the output switches
to low state.
Now capacitor C discharges through RB and the
transistor Q1. When voltage across C equals 1/3 Vcc,
comparator 2’s output triggers the flip- flop and the
output goes high. Then the cycle repeats.
Working Principle
1. Threshold(6) and trigger comparator(2) inputs
are joined together.
2. A capacitor is connected to ground from 2&6.
3. If the power is on, Pin2 will be < 1/3 Vcc. Pin3
is “1”. Discharging transistor is off and the
capacitor starts charging. When it is > 2/3 Vcc,
the FF is set, Pin3 is “0” Now the transistor is
short and capacitor starts discharging through
RB. When it is < 1/3Vcc. Output is again “1”
and cycle repeats. Hence the charging and
discharging is between 2/3Vcc and 1/3Vcc .
Astable Multivibrator
Tc = 0.693(RA+RB)
Td + 0.693RBC
T + tc + td = 0.693(RA + 2RB)C
Therefore the frequency of oscillation
Thus the total time
The output frequency, f is independent of th
Minimum component Astable (50% Duty Cucle)
This is a cheap and cheerful astable using just one resistor and one capacitor
as the timing components: Minimum. However, if you build this circuit, it is
probable that the HIGH time will be longer than the LOW time. (This
happens because the maximum voltage reached by the output pulses is less
than the power supply voltage.) Things will get worse if the output current
increases. The charging and discharging path is through Pin-3 of 555.
BISTABLE CIRCUIT.
 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  = (R1+R2)C1
  = 0.69(R1+R2)C1
 The capacitor voltage cycles back and forth between
(2/3)Vcc and (1/3)Vcc at times, and
1 = 0.69(R1+R2)C1
2 = 0.69(R2)C1
 The frequency is then given by
1
144
.
f 

0.693( R1  2  R2)C1 ( R1  2  R2)C1
APPLICATIONS OF 555
1. Extended duty cycle astable
Duty Cycle = ton/(t1+t2).
2. Pulse width Modulation.
• One shot triggered by
trigger pulses at Pin-2.
• Modulating Sine wave is
given at Pin-5.
PWM output is at Pin-3. Used for DC motor speed control.
PWM Waveforms
Optical Transmitter Circuit
Astable is used to produce carrier pulses at a
frequency we cannot hear (well above 20kHz)
3. Pulse Position Modulation (PPM)
+ve Pulses
Suppressed
Differentiator
In pulse position modulation, the amplitude and width of the
pulses are kept constant, while the position of each pulse with
reference to the position of a reference pulse, is changed
according to the instantaneous sampled value of the
modulating signal.
Pulse position modulated waveform
A 555 IC timer can be used to build a Pulse position modulator.
This pulse position modulator (PPM) is different from pulse
width modulation (PWM) which keep constant frequency.The
PPM does not keep constant frequency.
4. FSK
Generator
Digital Modulation Only.
5. Linear Ramp Generator
When a capacitor is charged with a constant current
source then linear ramp is obtained. This concept is
used in linear ramp generator.
The circuit is used to obtain constant current Ic is a current
mirror circuit, using transistor Q and diode D. The current Ic,
charges capacitor C at a constant rate towards + Vco But
when voltage at pin 6 i.e. capacitor voltage Vc becomes
(2/3Vcc), the comparator makes internal transistor Qi ON
within no time. But while discharging when Vc becomes (1/3
Vcc)/ the second comparator makes Qi OFF and C starts its
charging again. As discharging time of capacitor C is very
small, the time period of ramp is assumed practically same as
that of charging time of capacitor.
Fan/Motor Speed Control
By adding a comparator to the ramp generator we can create a very
nice variable duty-cycle pulse generator, much like we did in the
previous section. We will use this for a speed controller for our
little DC brushless fan.
VARIABLE DUTY CYCLE
555 Timer as a Schmitt Trigger
When a Sine wave is applied
Tripping points are 1/3Vcc
&2/3Vcc.
R1=R2
The upper comparator will trip at 2/3 Vcc while
lower comparator at 1/3 Vcc.
The frequency of square wave remains
same as that of input. The Schmitt trigger
can operate with the input frequencies up
to 50 kHz.
555 Timer Applications
 555 timer is used to produce an oscillating signal whose voltage
output is increased by the transformer to a dangerous level,
producing sparks. DO NOT DO THIS WITHOUT
SUPERVISION