ENTC%203320

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ENTC 3320

• 555 TIMER

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555 Timer IC Timer

• The 555 timer has essentially two modes of
  operation
• Astable (free running) multivibrator and
• Monostable (one shot) multivibrator

555 Timer
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Pin Functions

• PIN 1 Ground--usually connected to ground. The
  voltage should be the most negative of any
  voltage appearing at the other pins.
• PIN 2 Trigger--level-sensitive point to 1/3 VCC.
  When the voltage at this pin is
  brought below 1/3 VCC the flip-flop is set
  causing pin 3 to produce a high state.
  Allowable applied voltage is between VCC
  (pin 8) and ground (pin 1).

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• Pin 3 Output--level here is normally low and
  goes high during the timing interval. Since
  the output stage is active in both
  directions, it can source or sink up 200 mA.
• Pin 4 Reset--when voltage at this pin is less
  than 0.4 V, the timing cycle is interrupted
  returning the timer to its nontriggered state.
  This is an overriding function so that the timer
  can not be triggered unless reset is released
  (pin 4 gt 1.0 V). When not used, connect to
  VCC.

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• Pin 5 Control voltage--internally derived 2/3
  VCC point. A resistor-to-ground or an
  external voltage may be connected to pin 5 to
  change the comparator reference points. When
  not used for this purpose, a capacitor-to-ground
  greater than or equal to 0.01 mF is recommended
  for all applications.
• Pin 6 Threshold--level sensitive point to 2/3
  VCC. When the voltage at this pin is brought
  greater than 2/3 VCC., the flip-flop is
  reset causing pin 3 to produce a low state.

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• Pin 7 Discharge--collector of a transistor
  switch to ground (pin 1). It is normally used
  to discharge the timing capacitor.
• Pin 8 VCC--the power-supply voltage connected
  here can range from 4.5 to 16 V with respect
  to ground (pin 1).

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Astable Multivibrator
Reset
Discharge
Threshold
Trigger
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The charge/discharge equation for a RC circuit is
The charging circuit is through RA, RB, and C.
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The discharging circuit is through RB and C.
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The duty cycle will always be greater than 50.
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The charge equation for a RC one shot is
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