555 Timer Monostable ©Paul Godin Updated February 2008

555 as a Monostable ◊The 555 timer can also be configured as a monostable. ◊The 555 Monostable has interesting characteristics that may be used in specific applications. 555mono.2

Monostable Multivibrator Operation of the 555 Timer 555mono.3

Monostable The Trigger is active low. Input must be below 1/3 Vcc. The RC controls a charge cycle only. The Trigger must be brought high to allow the latch to reset. Animation 555mono.4

Timing Diagram Tw The Trigger is active low. Input trigger Output pulse Output pulse created when input trigger voltage is less than 1/3 Vcc If the trigger is held low beyond calculated pulse width, the output pulse follows the input trigger 555mono.5

Monostable Filter Cap 0.01μF Calculated Values 555mono.6

Calculations: Monostable Notes: The value 1.1 is a k-factor associated with the 555 timer. The trigger is active-low (not edge-triggered), and must be brought high before the end of the pulse width. Tw 555mono.7

EWB and Multisim ◊The 555 timer as a monostable in EWB and Multisim is problematic. The simulation tends to run too slowly and the application may crash. ◊There seems to be no way around this problem. Recommend multiple saves with different file names so that you may revert if needed. 555mono.8

Sample Monostable Calculation ◊Design a Monostable that produces a 5S pulse. Use a 100F capacitor. 555mono.9

Design Exercises 555mono.10

Design Exercise 1.Using a 555 timer, design a monostable with an output pulse of 4 seconds. Use a 100μF capacitor. What happens if you leave the trigger in an output low state? 555mono.11

Animated Slide The following slides contain animations to demonstrate the operations of: 555 as a monostable 555mono.12

Monostable 0 + < Vc Wait-state. The transistor discharges the Capacitor 0 A low is applied to the trigger input. The comparator provides a logic high; the latch is set > - 1 Q’ goes low; Q high. The transistor is “off”. Cap charges. +> - Capacitor voltage > 2/3 Vcc. High comparator output high. 1 Latch reset. Q’ logic high. Capacitor discharges, waits. 1 0 Animated 555mono.13

©Paul R. Godin prgodin gmail.com 555mono.14 END