FIGURE 6. A TTL Schmitt inverter
sine-to-square converter.
FIGURE 7. TTL switch debouncer, with
logic 1 closed output.
FIGURE 8. TTL switch debouncer, with
logic 0 closed output.
control input, to minimize current drain.
Dealing now with
the individual inverter
ICs listed in Figure 2,
Figure 3 shows the functional diagram that is
common to the popular
7404, 74LS04, 74HC04,
74HCU04, and 4069UB
Hex inverter ICs. Of
these, the 7404 is an
ancient standard TTL IC;
the 74LS04 is a modern LS TTL type in
which each inverter has a fan-out of 10,
the 74HC04 is a fast CMOS type, and
the 74HCU04 and 4069UB are
unbuffered CMOS types that are suitable for use in linear applications.
Figure 4 shows the functional diagram that is common to the 74LS05 and
7406 Hex inverters with open collector
(OC) outputs. The 74LS05’s OC outputs
can handle maximum outputs of only
5. 5 volts, but those of the 7406 can handle up to a maximum of 30 volts.
Figure 5 shows the functional
diagram that is common to three of the
most useful of all Hex inverter ICs — the
74LS14 TTL and the 74HC14 and
40106B CMOS Schmitt types. In the
74LS14, the output of each Schmitt
inverter is in the logic 1 state until the
input rises to an upper threshold value of
1.6V, at which point the output switches
to logic 0 and locks there until the input
is reduced to a lower threshold value of
0.8V. At this point, the output switches
and locks into the logic 1 state again,
and so on. Thus, a 74LS14 Schmitt
inverter can be made to function as a
sine-to-square converter by connecting it
as shown in Figure 6, where RV1 is used
to set the circuit to its maximum sensitivity point, at which a quiescent voltage
of 1.2V is set on the inverter’s input.
Figures 7 through 9 show more sim-
FIGURE 9. TTL supply switch-on pulse generator.
FIGURE 10. CMOS Schmitt
inverter sine/square converter.
FIGURE 11. CMOS supply switch-on pulse generator.
ple applications of the 74LS14 IC. Figure
7 is a practical version of a mechanical-switch contact-bounce debouncer; it
can be activated by a push-button (S1)
or toggle (S2) switch, and has an output
that goes to logic 1 when the switch is
closed. Figure 8 is a modified
version of the above circuit,
with an added inverter stage,
and gives a logic 0 output
when S1 is closed. Figure 9 is
yet another variation of the
basic circuit, and generates a
brief logic 1 switch-on output
pulse when the circuit’s supply
is first connected.
Regarding the 74HC14
and 40106B CMOS Schmitt
inverters, these have typical
upper and lower threshold voltage values equal to 60% and
40% of the supply voltage,
respectively. A CMOS Schmitt
inverter can thus be made to
function as a sine-to-square
converter by connecting it as
shown in Figure 10, where RV1
is used to set the circuit to its
maximum sensitivity point.
Alternatively,
either CMOS Schmitt
inverter can be used
as a switch-on pulse
generator (which gen-
erates a brief logic 1 switch-on output
pulse when the circuit’s supply is first
connected) by wiring it as shown in
FIGURE 12. Functional diagram of the
4049UB (unbuffered) or 74HC4049
(buffered) Hex inverter IC.
FIGURE 13. Functional
diagram and truth
table of the 4502B Hex
three-state inverter
with INHIBIT control.
May 2007 69
