Schmitt trigger astable multivibrator.
This is some basic calculations for Schmitt trigger astable multivibrators.
In addition there is a list of threshold voltages for some CMOS gates.
Fig.1: Astable multivibrators with logic gates and waveforms.
The waveform shown above has exactly 50% duty-cycle ( it is a calculated example with ideal components ).
If you need 50% duty-cycle, you should use a divide-by-two circuit after the multivibrator.
For a CMOS gate where Voh≈VDD and Vol≈0:
There is no need to go to extremes and select a very high value for R in order to save power.
This circuit bias the gate's input stage in the linear region and supply current is much higher than many will expect from a CMOS gate.
The plot below shows the supply current vs. input voltage for one CD40106 gate at 9 V supply.
More modern and faster gates will draw much more current than this.
A 74LVC14 gate draws, as far as I remember, around 5 mA when it is used in this way.
Fig.2: CD40106 supply current vs. input voltage for one gate at 9 V supply voltage. Unused gates' inputs on GND.
Fig.3: Astable multivibrators with OP-AMP / comparator.
Fig.3a is an OP-AMP or comparator based multivibrator for use with dual supplies.
Fig.3b is an OP-AMP or comparator based multivibrator for use with single supplies.
Fig.3c is an open-collector comparator based multivibrator for use with single supplies.
If R34 << R and R31, calculation is like fig.3b. If not, calculations becomes somewhat complex as the time-constant differs in the 2 states.
Use a simulator.
Calculations and a list of Vl and Vh for different gates.
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