The 555 Timer and Applications

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The 555 Timer and Applications

• Zvi Roth
• FAU

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Block diagram representation of the internal
circuit of the 555 integrated-circuit timer.
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555 Timer Features

• Single DC supply (that can run from 4.5V to 18V).
• Can provide relatively large currents to a load
  (hundreds of mA).
• Can reach frequencies of up to hundreds of KHz,
  or as low as fraction of 1Hz.

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555 Timer Pins Arrangement
8
6
3
2
4 Reset 5 Control
7
1
5
Two Basic Applications

• One-Shot The timer creates a single specific
  constant width pulse in response to a triggering
  pulse.
• Oscillator Timer generates a square-wave signal
  with a specific frequency and specific
  duty-cycle.

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Pins 4 and 5

• In the two basic applications pins 4 (Reset) and
  5 (Control) are unused.
• Unused Pin 4 must be connected to the Dc supply
  VCC.
• Unused Pin 5 must be connected to ground via a
  capacitor C10nF.
• Both pins play important roles in more advanced
  applications.

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555 Inner Structure - Comparators
Comparator 1 has (-) threshold of 2VCC
/3. Comparator 2 has () threshold of VCC/3,
unless modified via pin 5. Comparators outputs
is either HIGH (slightly below VCC) or LOW0.
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555 Inner Structure Flip-Flop
Flip-flop has two complementary outputs If Q is
HIGH then Qbar is LOW and vice versa. Two inputs
SET (activated if comp. 2 output is HIGH) and
RESET (activated if comp. 1 output is HIGH or if
pin 4 is LOW).
Output Q is tied to pin 3
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555 Inner Structure Transistor
Transistor Q1 is driven by the Qbar output of
flip-flop. If QbarHIGH then Q1 is ON and has a
low collector-emitter resistance. If QbarLOW
then Q1 is in cutoff.
Q1 collector is tied to pin 7 (Discharge)
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555 Operation - 1
Normally Trigger (pin 2) is held at a voltage
that is larger than VCC/3. If Trigger (pin 2)
becomes lower than VCC/3 then comparator 2 turns
HIGH and flip-flops output is SET to HIGH.
Q1 cuts off.
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555 Operation - 2
If Threshold (pin 6) becomes higher than 2VCC/3
then comparator 1 turns HIGH and flip-flops
output is RESET to LOW. Transistor Q1 turns ON.
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555 Timer configured as One Shot

• Pin 6 (Threshold) and Pin 7 (Discharge) need to
  be connected to each other.
• External R,C are used to create the one-shots
  pulse width. R connects to VCC and to C. C
  connects between R and ground.
• Pins 6,7 are connected to the node between R and
  C.
• Pin 2 receives an input from the triggering
  circuitry (to be explained soon).
• Pins 4,5 are unused (see earlier slide).

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555 One Shot Direct Triggering

• We may connect pin 2 directly to a VPULSE
  oscillator (which may be implemented by a 555
  oscillator circuit).
• The triggering square wave must satisfy three
  conditions HIGH must be above VCC/3, LOW must be
  below VCC/3 and LOW-time must be small enough (a
  narrow low pulse), narrower than the pulse that
  is generated by the one shot.

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555 One Shot CR Triggering

• The triggering square wave signal could have any
  levels and widths.
• The square wave goes into a CR network that does
  some sort of a signal differentiation.
• Negative spike is used to trigger pin 2 of the
  555. Its peak must be lower than VCC/3. Positive
  spike is unused.

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555 Timer One Shot Configuration and signals
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555 One Shot Explanation of Operation

• Pin 2 is triggered ? Comparator 2 sends SET to
  flip-flop ? Output pulse starts ? transistor cuts
  off ? Timing RC network starts to charge C from
  VCC via R.
• As C voltage reaches 2VCC/3 comparator 1 sends
  RESET to flip-flop and output pulse ends ?
  transistor turns ON and does a fast discharge of C

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Key Design Formula

• C starts at 0 and charges exponentially, with
  time constant RC, to a value of 2VCC/3.
• After pulse ends, need to allow for some small
  recovery time, allowing Cs voltage to go back
  to zero.

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555 One Shot Simulation
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GreenV(trig), PurpleV(cap), BlueV(out),
RedV(trig_in)
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555 Timer Oscillator Configuration
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555 Oscillator Explanation of Operation

• Initially C is discharged and flip-flops output
  is QHIGH and transistor is OFF.
• C begins to charge towards VCC via resistors RA
  and RB.
• When C voltage reaches 2VCC/3 flip-flop resets,
  transistor turns ON. C begins to discharge
  towards 0 via RB and the transistors ON
  resistance.
• When C voltage reaches VCC/3 flip-flop sets
  again.
• C forever charges and discharges between the
  above threshold levels.

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Key Oscillator Design Formulas

• TH is the time that output is HIGH.
• TL is the time that output is LOW.
• T is the oscillation period.
• Duty-cycle is always larger than 50.

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555 Oscillator Simulation
Enforce IC0 for C1.
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Simulated V(cap) and V(out)
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Extra Credit Assignments

• Each Exploration Activity, if done right, is
  worth 0.25. There are 7 exploration activities.
  Partial credit will be awarded to imperfect
  explorations, depending on the quality of the
  work done.
• Each Advanced Activity is worth 1, again with
  possibility of partial credits. Do no more than 2
  advanced activities.

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One-Shot Exploration Activities - 1

• What is the one-shots recovery time? Zoom in on
  capacitors discharge back to zero.
• Increase designed pulse width such that it is
  larger than the period of the triggering signal
  What happens when a second triggering signal
  comes in while the pulse is not over yet? Is the
  second triggering ignored?

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One-Shot Exploration Activities - 2

• Explore the triggering CR circuit (by trying
  different incoming signals voltage levels and
  different RC values). When does the triggering
  work right and when does it fail? Keep in mind
  that very often the square wave that goes into
  the CR circuit comes from another 555 timer,
  configured as an oscillator

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Oscillator Exploration Activities - 1

• Let pin 4 (Reset) be connected to VPULSE that has
  V1VCC and V20. Select some value t1 for TD, and
  let PW be larger than the simulations final
  time. Watch how at tt1 the reset kicks in and
  stops the oscillation.
• Let pin 5 (Control) be connected to VSIN with
  some large amplitude and relatively small
  frequency. Watch how the frequency of the
  oscillator can be modulated. Can you create a
  siren wave?

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Oscillator Exploration Activities - 2

• What is the highest frequency that we can
  generate? Why is that so? Hint Can you measure
  the ON resistance of the transistor?
• Can you generate a square-wave signal with
  duty-cycle which is close to 50? Should you
  simply take RAltltRB?

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Advanced Activities - 1

• Car Burglar Alarm Circuit When car door opens a
  contact switch is activated causing the driver to
  be given with a 30 seconds delay before an alarm
  siren is sounded. When driver acts properly (say,
  keys in some code within this half a minute), the
  alarm circuit is reset. Complete the design.

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Advanced Activities - 2

• Analog Frequency Meter A 50 duty-cycle
  square-wave signal (VPULSE) has a frequency that
  needs to be measured. That is, circuits output
  is (at steady-state) a DC signal proportional to
  f. Explore the meters limitations.
• Hints 1) The time average of a one-shots output
  is proportional to f (Why?), 2) Averaging can be
  done with a low-pass filter. You may use LOPASS
  from the ABM library.

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Advanced Activities - 3

• Analog Capacitance Meter An unknown C needs to
  be measured. That is, circuits output is (at
  steady-state) a DC signal proportional to C.
  Explore the meters limitations.
• Hint Embed the unknown C in some 555 basic
  circuit that needs a capacitor. Is it better to
  use a one-shot or an oscillator? DC output should
  be linearly proportional to C.

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Advanced Activities - 4

• Missing Pulse Detector Create some specific
  periodic VPULSE signal. Create a single pulse
  (like the ones in the above sequence of pulses).
  You may subtract the two signals to create a
  sequence of pulses in which one pulse is missing.
  Can you design a 555 timer circuit that detects
  (i.e. creates some special pulse) a sequence in
  which one of the pulses is missing?