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Astable Pulse Generator

<^< Introduction: 555 Timer | Course Index | Monostable Pulse Generator >^>(:nl:)

'''Click here to download TINA circuit file'''

In order to understand how the astable pulse generator circuit operates, assume that the output (pin-3) is initially high and that the internal transistor, TR'_1_', is in the non-conducting state. The capacitor, C'_1_', will begin to charge with current supplied by means of series resistors, R'_1_' and R'_2_'. Note that the 3 internal resistors all have the same value.

When the voltage at the threshold input (pin-6) exceeds two thirds of the supply voltage the output of the upper comparator will change state and the bistable will become reset due to voltage transition that appears at the R input of the RS latch. This, in turn, will make the output go high, turning TR'_1_' on at the same time. Due to the inverting action of the buffer on the output of the final output (pin-3) will go low.

The capacitor, C'_1_', will now discharge, with current flowing through R'_2_' into the collector of TR'_1_'. At a certain point, the voltage appearing at the trigger input (pin-2) will have fallen back to one third of the supply voltage at which point the lower comparator will change state and the voltage transition at S will return the bistable to its original set condition. The output then goes low, TR'_1_' switches off (no longer conducting), and the output (pin-3) goes high. Thereafter, the entire charge/discharge cycle is repeated indefinitely.

The output waveform produced by the circuit has the following properties:

>>center<<Time for which output is high: t'_on_' = 0.693 (R'_1_' + R'_2_') C'_1_'

Time for which output is low: t'_1_' = 0.693 R'_2_' C'_1_'

Period of output waveform: t = t'_on_' + t'_off_' = 0.693 (R'_1_' + 2 R'_2_') C'_1_'

Pulse repetition frequency: p.r.f. =

Mark to space ratio:

Duty cycle:

Where t is in seconds, C'_1_' is in Farads, and R'_1_' and R'_2_' are in ohms. >><<

Note that, because the high time (t'_on_') is always greater than the low time (t'_off_'), the mark to space ratio produced by a 555 timer can never be made equal to (or less than) unity. This could be a problem if we need to produce a precise square wave in which t'_on_' = t'_off_'. However, by making R'_2_' very much larger than R'_1_', the timer can be made to produce a reasonably symmetrical square wave output. (:nl:)(:table style="clear:both":)