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lecture 4: Timing circuits

lecture 4: Timing circuits. Creating Signals & Controls. Lecture outline. Reading: 1)Hambley Ch 12.1-12.3 2) Lab 3 handout. Astable Schmitt Multivibrator 555 Timer Monostable 555 Astable 555. 555 Timer. Schmitt trigger. Switches between 2 output rails +/-M.

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lecture 4: Timing circuits

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  1. lecture 4:Timing circuits • Creating Signals & Controls

  2. Lecture outline Reading: 1)Hambley Ch 12.1-12.3 2) Lab 3 handout • Astable Schmitt Multivibrator • 555 Timer • Monostable 555 • Astable 555 555 Timer

  3. Schmitt trigger • 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 Standard Schmitt

  4. 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))

  5. 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

  6. Review-RC circuit charge/discharge • Vc=A+Bexp(-t/RC) • Vc(t=0)=0; boundary condition will be given

  7. Astable schmitt analysis

  8. 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

  9. 555 monostable Hambley p.818-819 • On-time to be announced by email.

  10. 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

  11. 555 Astable circuit Hambley p.820-821 • Prelab exercise 10kHz and 75% duty cycle Why are trigger and threshold tied together?

  12. 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

  13. 555 astable analysis

  14. more amplifiers • frequency response cont.

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