Self Test on Tutorial 8


1.           A 555-timer is used as a monostable and this is the circuit.                                                        


             (a)       Put a ring around the RC network system.                                                                                        

             (b)       Which pins are used for the positive supply voltage and the negative (0 V)?                     

                           (i) Supply Voltage                                                                                                                                              

                          (ii) Negative                                                                                                                                                          

             (c)       The LED has a voltage of 2.2 V across it when it is fully lit, and takes a forward current of 10 mA.  Calculate the voltage across the 1 k resistor.  Hence work out the voltage at pin 3.                                                                                                                                                                                                                                                                                                                                               

             (d)       Draw the output of the monostable when switch S is pressed.                                                                                                                                                                                                                                                  

             (e)       State and explain what, if anything, would be the effect of pressing the switch S again, if the output is still high          

             (f)        If R = 2.2 k and C = 1.5 mF, what is the time period of the monostable pulse?                    


2.           A 555-timer is shown in astable mode.




             (a)       Identify a potential divider on the circuit by drawing a ring around it.  Show it clearly.


             (b)       If R1 = 10 k, R2 = 6.8 k, and C = 150 nF, calculate the RC time constant before it is connected to the 555-timer 


             (c)       The output of the circuit is said to have mark time and space time.  Explain what these two terms mean.        

                         (i)  Mark                                                                                                                                                                           

                        (ii)  Space                                                                                                                                                                          



             The circuit is constructed so that R1 = 10 k, R2 = 6.8 k, and C = 150 nF   


             (d)       Calculate the mark time.                                                                                                                        




             (e)       Calculate the space time.                                                                                                                      



             (f)       Calculate the mark to space ratio.                                                                                                      



             (g)        Draw the output you would see on a CRO screen