The NOT gate monostable circuit project uses IC 7404, which has six inbuilt NOT gates. Initially the trigger input T is high, so the output from the first NOT gate U1 is Low. The resistor R and the capacitor C are connected together in parallel and also to the input of the second NOT gate U2. As the input to U2 is Low at logic "0" its output at is high.
When a logic level "0" pulse is applied to the trigger input T of the first NOT gate it changes and produces a logic level "1" output. The diode D1 passes this logic "1" voltage to the RC network and the voltage across the capacitor C, increases rapidly to this new voltage level. This in turn outputs logic "0" at U2 and the circuit stays in this Meta-stable state as long as the trigger input T applied to the circuit remains Low.
When the trigger signal goes high, the output from the first NOT gate goes low and the Capacitor, C starts to discharge itself through the Resistor R connected across it. When the voltage across the capacitor drops below the lower threshold value of the input to the second NOT gate, its output switches back again producing a logic level "1" at the output of U2. The diode D1 prevents the capacitor from discharging itself back through the first NOT gates output.
The Time Constant for a NOT gate Monostable Multivibrator is given as T = 0.8RC + Trigger in seconds.