555 Timer Schematic Diagram

It consists of 23 transistors, 2 diodes, and 16

resistors.
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 Applications of 555 Timers
• Monostable &Astable
Multivibrators
• Waveform generators
• Burglar Alarms
• Measurement ,Process &
Control Circuits
• Missing pulse detectors
• Traffic light control
• Automatic Battery chargers
• Logic probes
• DC to DC Converters etc.

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 Simplified Block Diagram
representation of 555 Timer
• Consists of two
comparators
• Consists of an R-S
flip flop
• Consists of Q1
that is operated
as a switch

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 R-S flip Flop Review
R S Q Action
0 0 Last Value No change
0 1 1 Set
1 0 0 Reset
1 1 ? Indeterminate(forbidden)

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 8 7
6 5

1 2
3 4

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•Pin 1 (Ground):- All voltages are
measured w.r.t. this terminal. This is the
most negative supply potential of the
device

•Pin 2 (Trigger Terminal )

This pin is an inverting input to a lower
comparator. This is used to set the flip
flop which causes the output to go high

•Pin 3 (Output Terminal)

There are 2 ways to connect load to the
output terminal.
Pin 3 & Vcc :- Normally On load
Pin 3 & Ground:- Normally off load

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•Pin 4 (Reset):- To disable or reset •Pin 6 (Threshold):- This is an
the timer a negative pulse is input to the upper comparator.
applied to this pin. When this pin Used to reset the flip-flop which
isn’t used, it’s connected to Vcc. drives the output low.
Why ?? •Pin 7 (Discharge)
•Pin 5 (Control Voltage) -When the npn transistor
The function of terminal is to connected to it is turned “on,”
control the threshold and trigger the pin is shorted to ground
levels. The external voltage or a – The timing capacitor is usually
pot connected to this pin between pin 7 and ground and is
determines the pulse width of the discharged when the transistor
output waveform. When not in turns “on”
use, it should be connected to •Pin 8 (Supply Voltage):- A
ground through a 0.01uF capacitor positive supply voltage is applied
to avoid any noise problem. to this terminal
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 BASIC TIMING CONCEPTS
• A resistive voltage divider
consisting of 3 equal resistors
R1 is employed 2Vcc/3
• VTH= 2Vcc/3 for comparator 1.
• Flip Flop is reset whenever
threshold goes higher than Vcc/3
2Vcc/3.
• VTL=Vcc/3
• Flip Flop is set whenever the
trigger goes below Vcc/3.
• In set state output Q is high
(approx. equal to Vcc) and in
reset the output is low

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555 timer as a Monostable Multivibrator

Figure The 555 timer connected to implement a monostable multivibrator. (b) Waveforms of the circuit in (a).

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 Description of the 555 Timer as a Monostable
Multivibrator
• Consists of an external
resistor R & capacitor C
• In the stable the flip flop
will be in reset state, thus
Q will be high and Q low.
• Transistor Q1 is driven into LOW
saturation
high
• VCE≈ 0 so the capacitor is
shorted to ground. i.e. VC≈
0 and output of
comparator 1 is low

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 Description of the 555 Timer as a Monostable
Multivibrator (contd…)
• Vtrigger is kept high (higher
than VTL i.e. Vcc/3)
• Output of comparator 2
will also be low
• Flip flop is in reset state so
LOW
Q will also be low i.e. V0≈ 0
• To trigger the monostable
LOW
multivibrator , a negative
pulse is applied to the
trigger input terminal

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 Description of the 555 Timer as a Monostable
Multivibrator (contd…)
• As Vtrigger goes below VTL i.e. Vcc/3,
the output of comparator 2 goes
high thus setting the flip flop. i.e.
Q=1 Q =0, so the transistor is
cutoff
• Vtrigger is given for a short time so
output of compartor 2 goes low
again.
• Still the ouput is high because R=0 LOW HIGH
and S=0 result in Q to be in the
previous state.
LOW
• Capacitor C now begins to charge HIGH
LOW
through resistor R and Vc rises
exponentially towards Vcc.
• The high voltage at the ouput is
retained as long as Vc<VTH

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 Description of the 555 Timer as a Monostable
Multivibrator (contd…)
• Once Vc exceeds VTH , the
output of comparator 1
goes high. Now R=1 and
S=0 so Q=0
• The monostable
HIGH
LOW LOW
HIGH
multivibrator is now back
in its stable state and is
HIGH
ready to receive a new LOW

triggering pulse

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Derivation for the width of the pulse in
monostable multivibrator
• The width of the pulse , T is the
time interval that the monostable
multivibrator spends in quasi
stable state.
• Denoting the time instant at
which the trigger pulse is applied
at t=0 , the voltage across
capacitor Vc can be expressed as ,

• Substituting Vc=VTH=2/3Vcc
at t=T gives,
T=RC ln 3 =1.1 RC

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Figure 13.28 (a) The 555 timer connected to implement a monostable multivibrator. (b) Waveforms of the circuit in (a).

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 555 Timer as an Astable Multivibrator

The 555 timer connected to implement an astable multivibrator. (b) Waveforms of the circuit in (a).

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