Make Electronics

Chips, Ahoy! 195

Experiment 19: Learning Logic

FundAmentAls

Rules for connecting logic gates (continued)

Not permitted:

• No floating-input pins! On CMOS chips such as the HC
  family, you must always connect all input pins with a
  known voltage, even if they supply a gate on the chip
  that you’re not using. When you use a SPST switch to
  control an input, remember that in its “off” position, it
  leaves the input unconnected. Use a pull-up or pull-
  down resistor to prevent this situation. See Figure 4-77.


• Don’t use an unregulated power supply, or more than 5
  volts, to power 74HCxx or 74LSxx logic gates.


• Be careful when using the output from a logic gate to
  power even a low-current LED. Check how many mil-
  liamps are being drawn. Also be careful when “sharing”
  the output from a logic gate with the input of another
  gate, at the same time that it is driving an LED. The LED
  may pull down the output voltage, to a point where
  the other gate won’t recognize it. Always check cur-
  rents and voltages when modifying a circuit or design-
  ing a new one.


• Never apply a significant voltage or current to the
  output pin of a logic gate. In other words, don’t force
  an input into an output.


• Never link the outputs from two or more logic gates. If
  they must share a common output wire, use diodes to
  protect them from each other. See Figure 4-78.

Not

good

Good

1

1

Not

good

Good

1

1

Figure 4-77. Because a CMOS chip is so sensitive to input

fluctuations, a logical input should never be left “floating,” or

unattached to a defined voltage source. This means that any

single-throw switch or pushbutton should be used with a pull-

up or pull-down resistor, so that when the contacts are open,

the input is still defined.

Not

Good

Good Good

Figure 4-78. The output from one logic gate must not be allowed to feed back into the output from another logic gate. Diodes can
be used to isolate them, or they can be linked via another gate.