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"block A" : digital clock registers with display
 
 

logic probe
NE555 timer
counters + decoders + displays
zero-crossing detector

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

digital logic probe
 

use this probe to determine logic levels at ttl circuit nodes
know how to limit the current at the led
assume the voltage across the forward-biased led to be 1.2 volts
current is maximum when the probe tip is connected to either of the two power supply buses

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

NE555 timer

       LM555 (timer) + resistors + capacitors + LED

o a general purpose circuit to provide square wave of adjustable frequency
o it can also be used to provide one positive-going edge or one negative-going edge at a time
o capacitor charging and discharging times provide the integration functions that control the frequency of the oscillator

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

counters + decoders + displays

        74ls90 - 0 to 9 counter
        74ls92 - 0 to 5 counter
        74ls47 - seven-segment decoder

o cascading counters with their respective display circuits
o note the operation of pins 2&3 and pins 6&7 in a 7490, and pins 2&3 in a 7492
o also note the operation of the 7447

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

time base : zero-crossing detector
 

o catching the instant of a "zero-crossing" to provide the time-base for the digital clock
o note the resistor values and the diode configurations
o the npn transistor is used as a switch to provide the timing edges