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Exe 2
By varying the value of either R or C the 555 astable multivibrator circuit can be made to oscillate at any desired output frequency. But what is the maximum frequency of oscillations we can produce from a single 555 timer chip.
To get the 555 to operate at its highest frequency in this 555 circuits part 1 tutorial, it is necessary to continuously retrigger it the instant the output changes state, from high to low, or low to high. The fastest switching speed can be obtained by removing both the R and C timing components and feeding the output signal directly back the trigger inputs.
By connecting the output, pin 3 to both the trigger input, pin 2 and the threshold input, pin 6, every time the output changes state it re-triggers the 555 to change state again. However, the output waveform will not be symmetrical or a square wave but a series of negative pulses.
The highest oscillation frequency obtained using this arrangement will depend on the supply voltage, the type of 555 chip used, TTL or CMOS and the manufacturer as the internal circuitry differs from manufacturer to manufacturer. But it is possible to produce an output frequency as high as 350kHz at 5 volts.
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