A capacitance meter.

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The block diagram shows the basic arrangement for the design of a capacitance meter.



                                                                             Range resistor

   Astable                 Monostable               Display                                                                                

  Multivibrator             Multivibrator                              




                                        Cx Unknown capacitance             



(1) An astable multivibrator is also known as a free-running multivibrator. It is called free running because it alternates between two different output voltage levels during the time it is on. The output remains at each voltage level for a definite period of time. If you looked at this output on an oscilloscope, you would see continuous square or rectangular waveforms. The astable multivibrator has two outputs, but NO inputs. The word “Astable” means not stable and so we know that an astable multivibrator is a multivibrator with no stable state.


The circuit diagram for the astable multivibrator using IC 555 is shown here. The astable multivibrator generates a square wave, the period of which is determined by the circuit external to IC 555. The astable multivibrator does not require any external trigger to change the state of the output, hence the name free running oscillator. The time during which the output is either high or low is determined by the two resistors and a capacitor, which are externally connected to the 555 timer.

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(3) If the frequency of the clock pulse generator is to be 100HZ and I needed to work out the time, I will calculate it by doing this:

T = 1/F    T = 1/100   T = 0.01secs

(4) If the mark to space ratio (or duty cycle) for the astable is to be 9:1 and I had to work out:

a) The time T1, when the output voltage is high, I will use the following method.



b) The time T2, when the output voltage is low, I will use the following method.


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