Multivibrator

Multivibrator An electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. Note: Astable Not stable in either state, switch from one state to other state continuously Monostable Which one of state s is stable , but other state is unstable Bistable Stable in either state, triggered by two external trigger

IC 555 - + Control Voltage (5) Threshold Voltage (6) Trigger Voltage (2) Ground (1) Vcc (8) Discharge (7) Reset (4) Output (3) - + RESET SET Q COMP1 COMP2 Flip-Flop T1

Schematic of 555 RA RB C 5 Volts 3.333 V N/C Discharge 1.666 V Output Threshold / Trigger Ground Output 1.666 V 3.333 V RA RB C

555 Timer Design Equations Digital Electronics TM 1.2 Introduction to Analog 555 Timer Design Equations tHIGH : Calculations for the Oscillator’s HIGH Time The Output Is HIGH While The Capacitor Is Charging Through RA + RB. 5v 3.333 v Vc 1.666 v 0 v  tHIGH  Output HIGH LOW Derivation of the equation for t-HIGH. Project Lead The Way, Inc. Copyright 2009

555 Timer Design Equations Digital Electronics TM 1.2 Introduction to Analog 555 Timer Design Equations tLOW : Calculations for the Oscillator’s LOW Time The Output Is LOW While The Capacitor Is Discharging Through RB. 5v 3.333 v Vc 1.666 v 0 v  tLOW  Output HIGH LOW Derivation of the equation for t-LOW. Project Lead The Way, Inc. Copyright 2009

555 Timer – Period / Frequency / DC Digital Electronics TM 1.2 Introduction to Analog 555 Timer – Period / Frequency / DC Period: Duty Cycle: Summary of a 555 time design equations. Frequency: Project Lead The Way, Inc. Copyright 2009

Detail Analysis of a 555 Oscillator 555 Timer Digital Electronics TM 1.2 Introduction to Analog Detail Analysis of a 555 Oscillator 5v 3.333 v Vc 1.666 v 0 v RESET HIGH LOW SET T1 ON OFF Q Analysis of a 555 Oscillator: Slide 1 of 4 Project Lead The Way, Inc. Copyright 2009

Detail Analysis of a 555 Oscillator 555 Timer Digital Electronics TM 1.2 Introduction to Analog Detail Analysis of a 555 Oscillator 5v 3.333 v Vc 1.666 v 0 v RESET HIGH LOW SET T1 ON OFF Q Analysis of a 555 Oscillator: Slide 2 of 4 Project Lead The Way, Inc. Copyright 2009

Detail Analysis of a 555 Oscillator 555 Timer Digital Electronics TM 1.2 Introduction to Analog Detail Analysis of a 555 Oscillator 5v 3.333 v Vc 1.666 v 0 v RESET HIGH LOW SET T1 ON OFF Q Analysis of a 555 Oscillator: Slide 3 of 4 Project Lead The Way, Inc. Copyright 2009

Detail Analysis of a 555 Oscillator 555 Timer Digital Electronics TM 1.2 Introduction to Analog Detail Analysis of a 555 Oscillator 5v 3.333 v Vc 1.666 v 0 v RESET HIGH LOW SET T1 ON OFF Q Output Is HIGH While The Capacitor Is Charging Through RA + RB. Output Is LOW While The Capacitor Is Discharging Through RB. Analysis of a 555 Oscillator: Slide 4 of 4 Project Lead The Way, Inc. Copyright 2009

Improved 555 Duty Cycle By connecting this diode, D1 between the trigger input and the discharge input, the timing capacitor will now charge up directly through resistor R1 only, as resistor R2 is effectively shorted out by the diode. The capacitor discharges as normal through resistor, R2. Now the previous charging time of t1 = 0.693(R1 + R2)C is modified to take account of this new charging circuit and is given as: 0.693(R1 x C). The duty cycle is therefore given as D = R1/(R1 + R2). Then to generate a duty cycle of less than 50%, resistor R1 needs to be less than resistor R2.

Multivibrator Circuit using 555

Circuit Diagram R = 10k C = 0.1 uF 74HC14 Hex-Inverter Schmitt-Trigger

Circuit Diagram C = 560 pF RA and RB  RA > RB RA = RB RA < RB With dioda 1N 4148 (improve duty cycle