Make Electronics

Experiment 23: Nice Dice

216 Chapter 4

Now we come to the first new and difficult fact about the 74LSxx generation

of TTL chips that makes them less desirable, for our purposes, than the 74HCxx

generation of CMOS chips that I have recommended in previous projects. The

modern and civilized HC chips will source 4mA or sink 4mA at each logical

output, but the older LS generation is fussier. It will sink around 8mA into each

output pin from a positive source, but when its output is high, it hardly gives

you anything at all. This is a very basic principle:

• Outputs from TTL logic chips are designed to sink current.


• They are not designed to source significant current.
  In fact, the 74LS92 is rated to deliver less than half a milliamp. This is quite
  acceptable when you’re just connecting it with another logic chip, but if you
  want to drive an external device, it doesn’t provide much to work with.
  The proper solution is to say to the chip, “All right, we’ll do it your way,” and set
  things up with a positive source that flows through a load resistor to the LED
  that you want to use, and from there into the output from the chip. This is the
  “better” option shown in Figure 4-104.

Not

so good Better

1 14

74LSxx

series

logic

chip

1 14

74LSxx

series

logic

chip

Figure 4-104. Most TTL chips, including those in the LS generation, are unable to source

much current from their logical output pins (left) and should usually be wired to sink cur-

rent from a positive source (right).

The only problem is that now the LED lights up when the counter’s output is

low. But the counter is designed to display its output in high pulses. So your

LED is now off when it should be on, and on when it should be off.

You can fix this by passing the signal through an inverter, but already I’m get-

ting impatient with this inconvenience. My way around the problem, at least

for demo purposes, is to use the “Not so good” option in Figure 4-104 and

make it work by connecting a very-low-current LED with a large 4K7 load resis-

tor. This will enable us to “see” the output from the counter without asking it to

give more than its rated limit, and if you want to create a more visibly powerful