Elenco Digital / Analog Trainer Kit Version - Manual (Page 49)

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CIRCUIT DESCRIPTION - DIGITAL SECTION
THE DATA SWITCHES
There are eight data switches labeled SW1 through SW8. The circuit is very simple. To perform the desired
functions, there is a double throw double pole switch, wired as a single pole double throw. One end is connected
to the 5V, the other to ground and the center lug is connected to the output.
THE LOGIC SWITCHES
The logic switches are also DPDT switches wired as SPST switches. The logic switches perform the same
function as the data switches. That is, they produce high or low states. But there is one big difference. When
switching the data switches, many pulses may be produced due to bouncing of the contacts.
In the logic switches, only one pulse is produced at the IC output no
matter how many times the contacts bounce.
This is extremely
important if you are producing pulses for counting circuits. Figure R
shows the wiring of the logic switch. The two NAND gates are connected
so that when the X input is grounded, the X output goes high. Opening
and closing the ground at X will not change the output. Only when X is
grounded will the output change to low. Thus, only one output change is
produced with one movement of the X switch. There are two outputs
from each logic switch, X and X or Y and Y.
THE LOGIC INDICATORS
There are eight logic indicators. Figure S shows the circuit. It consists of
a 74HC04 IC. When the input is over 2.8V, the output of the IC will be low,
drawing current through the LED indicator. The 120
Ω
resistor limits the
current in the LED to less than 20mA. When there is no connection to the
input of the logic indicators, the two 100k
Ω
resistor bias the input to GND.
This insures that the LED will be off.
Figure R
X
IC7
X
X
X 6 5
4
1
2
3
IC7
100k
100k 120
74HC04
LED
5V
Figure S