lecture 4: Timing circuits Creating Signals & Controls

Lecture outline Reading: 1)Hambley Ch 12.1-12.3

Astable Schmitt Multivibrator 2) 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

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

555 astable analysis

more amplifiers frequency response cont.