lecture 4: Timing circuits Creating Signals & Controls Lecture outline

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lecture 4:
Timing circuits
Creating Signals & Controls
Lecture outline
Reading:
1)Hambley Ch 12.1-12.3
Astable Schmitt Multivibrator2) Lab 3 handout
555 Timer
Monostable 555
Astable 555
555 Timer
Schmitt trigger
Standard Schmitt
Inverting Schmitt
Switches between 2 output rails +/-M.
Switch when inputs equal each other
Use resistors to control this
Inverting and non-inverting Schmitt triggers
inverting schmitt with RC
chargeup
Hambley p.809-812
This is an astable multivibrator
Negative feedback with RC causes switching
f=1/(2RC ln(3))
Astable multivibrator
start, output high, Vc=0
RC chargup slowly
When input reaches Vcc/2,
schmitt trigger output low
RC discharges slowly
Cycle repeats forever i.e. astable output
Review-RC circuit
charge/discharge
Vc=A+Bexp(-t/RC)
Vc(t=0)=0; boundary condition will be given
Astable schmitt analysis
555 timer
http://en.wikipedia.org/wiki/Flip-flop_%28electronics%29#RS_.28Reset-Set.29_flip-flop
Comparators give signal to flipflop
Flipflop R high, Q low=reset
Flipflop S high, Q high=set
555 monostable
Hambley p.818-819
On-time to be announced by email.
555 monostable
Trigger is high, S is low, RS reset state, Q is low
BJT is on as Q- is high, threshold shunted to
ground
When trigger goes low <Vcc/3, RS set Q is high
Q- is low now, BJT off, C charges up thru RA
When threshold reaches 2Vcc/3, RS is reset by
top comparator
Everything returns to start state
555 Astable circuit
Hambley p.820-821
Why are trigger and threshold tied together?
Prelab exercise 10kHz and 75% duty cycle
555 astable operation
Start with vc=0, so trigger low, RS is set
BJT cutoff, so C chargeup through RA+RB
When threshold reaches 2Vcc/3, RS is set
through top comparator, RS is reset, BJT is on
C Discharges thru RB to ground thru BJT
When trigger goes low <Vcc/3, RS sets, cycle
repeats
555 astable analysis
more amplifiers
frequency response cont.

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