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Astable Pulse Generator<^< Introduction: 555 Timer | Course Index | Monostable Pulse Generator >^> 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, TR1, is in the non-conducting state. The capacitor, C1, will begin to charge with current supplied by means of series resistors, R1 and R2. 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 The capacitor, C1, will now discharge, with current flowing through R2 into the collector of TR1. 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 The output waveform produced by the circuit has the following properties: Time for which output is high: ton = 0.693 (R1 + R2) C1
Time for which output is low: t1 = 0.693 R2 C1 Period of output waveform: t = ton + toff = 0.693 (R1 + 2 R2) C1 Pulse repetition frequency: p.r.f. = Mark to space ratio: Duty cycle: Where t is in seconds, C1 is in Farads, and R1 and R2 are in ohms. Note that, because the high time (ton) is always greater than the low time (toff), 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 ton = toff. However, by making R2 very much larger than R1, the timer can be made to produce a reasonably symmetrical square wave output. <^< Introduction: 555 Timer | Course index | Monostable Pulse Generator >^> |