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time
 
 

1 logic probe
2 clock time base : zero-crossing detector
3 NE555 timer
4 counters + decoders + displays

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

clock time base
 
 

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

 
 
 
 
 
 
 
 

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