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Astable Pulse Generator

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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, 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 output go high, turning TR₁ 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, 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 output then goes low, TR₁ 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:

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 R₂ very much larger than R₁, the timer can be made to produce a reasonably symmetrical square wave output.

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