Oscillator NOT Gate

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When Vout goes high, V1 also goes high because the voltage across C can not change suddenly.
If V1 is high, V2 must be low.
If V2 ls low, Vc will fall towards zero as the capacitor slowly discharges through R2.
When Vc drops below half the power supply,
- V1 is interpreted as low.
- V2 goes high.
- Vout goes low.
- V1 goes even lower because the voltage across C can not change suddenly.
- The diode D prevents the potential at V1 dropping significantly below zero.
Since V2 is now high, Vc will slowly increse as the capacitor charges slowly through R2.
When Vc rises above half the power supply,
- V1 is interpreted as high.
- V2 goes low
- Vout goes high and we are back at step 1

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