Make Electronics

Experiment 19: Learning Logic

196 Chapter 4

In the 74HCxx logic family, each input of a logic gate consumes just a micro-

amp, while the output can source 4 milliamps. This seems paradoxical: how

can the chip give out more than it takes in? The answer is that it also consumes

power from the power supply attached to pins 7 and 14. That’s where the ad-

ditional electricity comes from.

Because the logical output from a chip can be greater than the logical input,

we can put the chip in a state where it keeps itself “switched on” in a way which

is similar to the way the relay in the alarm project was wired to lock itself on.

The simplest way to do this in a logic chip is by feeding some of the output

back to one of the inputs.

Figure 4-79 shows an AND gate with one of its inputs wired to positive and its

other input held low by a pull-down resistor, with a pushbutton that can make

the input high. A signal diode connects the output of the chip back to the

pushbutton-controlled input. Remember that the diode has a mark on it indi-

cating the end which should be connected to the negative side of the power

supply, which in this case will be the end of the 10K resistor.

The schematic in Figure 4-79 shows how the circuit should look in breadboard

format. Figure 4-80 shows it in a simpler format.

From this point on, I won’t bother to show the power regulator and the capacitors

associated with it. Just remember to include them every time you see the power sup-

ply labeled as “5V DC Regulated.”

When you switch on the power, the LED is dark, as before. The AND gate needs

a positive voltage on both of its logical inputs, to create a positive output, but

it now has positive voltage only on one of its inputs, while the other input

is pulled down by the 10K resistor. Now touch the pushbutton, and the LED

comes on. Let go of the pushbutton, and the LED stays on, because the posi-

tive output from the AND gate circulates back through the diode and is high

enough to overcome the negative voltage coming through the pull-down

resistor.

The output from the AND gate is powering its own input, so the LED will stay

on until we disconnect it. This arrangement is a simple kind of “latch,” and can

be very useful when we want an output that continues after the user presses

and releases a button.

You can’t just connect the output from the gate to one of its inputs using an

ordinary piece of wire, because this would allow positive voltage from the tac-

tile switch to flow around and interfere with the output signal. Remember, you

must never apply voltage to the output pin of a logic gate. The diode prevents

this from happening.

If you’ve grasped the basics of logic gates, you’re ready now to continue to our

first real project, which will use all the information that I’ve set out so far.

74HC08

AND

gate

5VDC Regulated

10K

1K

Figure 4-79. Using a diode, the logical
output from a gate can be allowed to feed
back to one of its inputs, so that the gate
latches after receiving a brief logical input
pulse.

Figure 4-80. The breadboard-format sche-
matic in is simplified here to show more
clearly the way in which a gate can latch
itself after receiving an input pulse.