Video by Andy Masters - The 6th Form College Farnborough
This circuit has two unstable states.
Its frequency is approximately: F = 1 / ( R C )
The circuit is simple needing only three components.
R must be big enough to ensure that the output current is limited to a safe level. 1K is about the minimum.
When the output is high, C charges through R until the input voltage rises to 55% or 60% of the power supply voltage.
At this time, the output goes low.
Now C discharges through R until the input voltage is 43% to 45% of the power supply voltage.
At this time the output goes high again and we are back at step one.
These percentages are based on the HEF40106B inverting Schmitt trigger data sheet.
The simulation uses threshold voltages of 33.3% and 66.7% or one third and two thirds.
The three oscilloscope views show the square wave output, the capacitor charging and discharging and and the X/Y plot showing the hysteresis.
Clicking Reset is interesting. It shows the circuit behaviour on switch-on. To get the X/Y plot, I used Output->Voltage/Scope Probes.
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