Elenco 130-in-1 Electronics Playground - Manual (Page 103)

-103-
You know that digital circuits produce low or high (L
or H) outputs (0 or 1). Now you’re going to create a
logic tester that shows 1 for high level (H) and 0 for
low level (L) on the LED display.
Slide the switch to position B and construct the
circuit. When you finish the wiring, slide the switch to
position A to turn on the power. The number 0 is on
the display because the test terminal (terminal 13) is
at low level when no input is exerted. Attach the test
terminal-to-terminal 122 to apply +4.5V. The display
alters to 1.
View the schematic. The operational amplifier works
as a comparator. The 22k
Ω and 10kΩ resistors
produce a reference voltage of 3V at its negative (-)
input terminal. When the voltage at its positive (+)
terminal exceeds this reference voltage, the
comparator’s output level goes high, turning off
transistor Q1. Now segments A, D, E, and F on the
display turn off, leaving a 1 on the display.
Notes:
EXPERIMENT #84: LOGIC TESTING CIRCUIT
Wiring Sequence:
17-18-19-20-44
86-79-63-21-23-45-132
43-80-82
67-81
68-83-85
119-124
122-131
69-89-13-CHECK POINT
121-25-70-90-84-14-CHECK POINT
Schematic