lecture 4: Timing circuits Creating Signals & Controls

Lecture outline Astable Schmitt Multivibrator 555 Timer Monostable 555 Astable 555 Reading: 1)Hambley Ch ) Lab 3 handout 555 Timer

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

inverting schmitt with RC chargeup This is an astable multivibrator Negative feedback with RC causes switching f=1/(2RC ln(3)) Hambley p

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 Comparators give signal to flipflop Flipflop R high, Q low=reset Flipflop S high, Q high=set

555 monostable On-time to be announced by . Hambley p

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 Prelab exercise 10kHz and 75% duty cycle Hambley p Why are trigger and threshold tied together?

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.