Tutorial 11 - Astables
We need to consider some definitions:
The mark time [t(H)] is the time at which the output is a 1.
tH= 0.7(RA + RB)C
The space time [t(L)] is the time at which the output is a 0.
tL = 0.7 RBC
The mark to space ratio = mark time ÷ space time.
Ratio = tH ÷ tL
The astable period T is the time taken for one complete cycle, the mark and the space times added together.
T = mark + space = tL + tH
The frequency = 1 ÷ period.
The time tH will be longer than tL, unless R1 is very small compared to R2. If this is the case, then tH will be approximately equal to tL, but not quite equal. We can say to a first approximation that the mark to space ratio is 1. This will result in a square wave output.
The duty cycle = mark time ÷ period.
What is the frequency of the square wave output from the circuit?
Use the formula
From the diagram we can see that:
We need to substitute these values into the formula:
f = ____1.4_____ = __________________ 1.4____________
(R1 + 2R2)C [4700 W + (2 × 2200 W)] × 2.0 × 10-6 F
= ____ 1.4_______ = 77 Hz
9100 × 2.0 × 10-6
An astable 555 timer has the following external component values: R1 = 100 kilohms; R2 = 47 kilohms; C1 = 10 microfarads.
What is the mark and space?
An astable has a 2.2
value of 10 k, and R2
value 20 k.
A NAND gate astable has a capacitor of capacitance 20 mF with a resistor of resistance 150 kW. What is the period of the astable?