Boolean Algebra

 • Boolean algebra provides a mathematical framework for digital systems.
• A basic knowledge of Boolean algebra is indispensable to the study and analysis of logic
circuits.
• Remember that in the last lecture notes,  Boolean operations and expressions in terms of their relationship to NOT, AND, OR, NAND, and NOR gates were introduced.
• Variable, complement and literal are terms used in Boolean algebra.
• A variable is a symbol used to represent a logical quantity. Any single variable can have a 1
or a 0 value.
• The complement is the inverse of a variable and is indicated by a bar over the variable (overbar).
• A literal is a variable or the complement of a variable.

Monostable Multivibrator

           Multivibrators are special purpose digital circuit intended to perform specific circuit functions, specifically regarding pulse generation.

           They are further categorized into three: astable, monostable, and bistable.

       

MOSFET Monostable

Monostable Multivibrators have only one stable state (hence their name: "Mono"), and produce a single output pulse when it is triggered externally. Monostable Multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit.
Consider the MOSFET circuit on the left. The resistor R and capacitor C form an RC timing circuit. The N-channel enhancement mode MOSFET is switched "ON" due to the voltage
across the capacitor with the drain connected LED also "ON".

When the switch is closed the capacitor is short circuited and therefore discharges while at the same time the gate of the MOSFET is shorted to ground. The MOSFET and therefore the LED are both switched "OFF". While the switch is closed the circuit will always be "OFF" and in its "unstable state".

When the switch is opened, the fully discharged capacitor starts to charge up through the resistor,R at a rate determined by the RC time constant of the resistor-capacitor network. Once the capacitors charging voltage reaches the lower threshold voltage level of the MOSFETs gate, the MOSFET switches "ON" and illuminates the LED returning the circuit back to its stable state.
Then the application of the switch causes the circuit to enter its unstable state, while the time constant of the RC network returns it back to its stable state after a preset timing period thereby producing a very simple "one shot" or Monostable Multivibrator MOSFET circuit.