Monostable AIM: To understand the operation of a monostable and be able to choose component values to give a required time period PRIOR KNOWLEDGE: Resistor and Capacitor values, period and frequency www.pfnicholls.com

Introduction A monostable produces a pulse for a given time period (T) when it is triggered A monostable is an example of synchronous logic and is described using timing diagrams Monostable circuits are usually triggered by a falling edge (when the trigger input goes from high to low) The trigger input has a falling edge from HIGH to LOW The output comes on for set Period of Time (T)

Ideal Monostable The ideal monostable should: Trigger on a falling edge Have a time period that depends on R & C only Not “restart” if the monostable is triggered again before the end of the time period. Once the monostable is triggered, nothing should affect the period of the output The second falling edge has no effect on the output The output comes ON when the trigger falls from HIGH to LOW The output goes OFF even if the trigger stays LOW

555 Monostable Circuit Good: The 555 based monostable is easy to build and provides enough current to drive LEDs, bulbs, small motors and buzzers etc Bad: The output stays ON if the trigger is held LOW (by holding the button down for instance) which is not what a monostable should do If Pin 4 is connected to 0V the output stays LOW The output can provide 100mA of current Pressing the button makes the Trigger go LOW Pin 5 is usually connected through a 10nF capacitor to 0V

555 Monostable Calculation The time period is given by: T = 1.1 x R x C T in seconds, R in Ohms, C in Farads Example: If R = 10kΩ and C = 470µF what is the time period? R = 10 x 103Ω (convert kΩ to Ω) C = 470 x 10-6F (convert µF to F) T = 1.1 x 10 x 103 x 470 x 10-6 = 5.2 seconds When the monostable is triggered, the output stays on for 5.2s LEARN this equation Make sure you can convert to standard form

555 Monostable Calculation Example calculation: A monostable is required to have a period of 10ms. What values of R and C should be used? Rearrange the equation for R: R = T / (1.1 x C) Guess a value for C: Use C = 1µF Convert to standard units: T = 10 x 10-3s, C = 1 x 10-6F Calculate the value of R: R = 10 x 10-3 / (1.1 x 1 x 10-6) R = 9100Ω Check that R is suitable: R > 1kΩ  Use R = 9.1kΩ and C = 1µF Educated guess Resistor values must be R > 1kΩ and R < 1MΩ

Pressing the button makes the Trigger go LOW NAND Gate Monostable Good: Output goes OFF again even if trigger is held low and so behaves as an ideal monostable Bad: More difficult to build and output can only provide a small current of ≈ 10mA which is enough to illuminate an LED Output Pressing the button makes the Trigger go LOW Timing components

Change short time periods to milliseconds NAND Gate Monostable The time period is given by: T = 0.7 x R x C T in seconds, R in Ohms, C in Farads Example: If R = 220kΩ and C = 33nF what is the time period? R = 220 x 103 Ω (convert kΩ to Ω) C = 33 x 10-9 F (convert nF to F) T = 0.7 x 220 x 103 x 33 x 10-9 = 0.0051 seconds When the monostable is triggered, the output stays on for 5.1ms Change short time periods to milliseconds

Summary A monostable is triggered on a falling edge The output stays HIGH for a Period (T) determined by the values of the timing components R and C The equation for a 555 timer is T = 1.1 x R x C A 555 based monostable is easy to build but does not behave correctly if the trigger is held low The equation for a NAND gate monostable is T = 0.7 x R x C A NAND gate monostable behaves correctly and the output stays on for the correct Period even if the trigger is held LOW Monostables are described by timing diagrams

Questions What does ‘Period’ mean? How is a monostable triggered? What is the disadvantage of a 555 monostable? For a 555 monostable, R = 47kΩ and C = 100µF. What is the time period? Using the values in question 4, what is the time period for a NAND gate monostable? Using C = 22µF, a 555 monostable needs to have a period of 400ms. What resistor value should be used?

Answers The period is the length of TIME that the output stays HIGH Monostables are triggered by a falling edge (HIGH to LOW) The disadvantage of a 555 monostable is that the output stays HIGH if the trigger is held LOW 5.2 seconds 3.3 seconds 16.5Ωk or 16k5Ω