as shown by the red dashed lines —
we can generate a pseudo sawtooth.
A 555 timer is an astable oscillator
that exploits the charging and
discharging of a capacitor. It has its
trip points — the points where the cap
starts to charge and ends charging —
at 1/3 and 2/3 the working voltage,
creating the waveform shown in
Figure 1c. Not perfect, but good
enough for most electronics designs.
The waveform is then buffered (Figure
2) and conditioned. The Frequency
pot changes the frequency and the
Wave Form control adjusts the wave
to keep the top and bottom of the
waveform from being clipped.
A more linear sawtooth waveform
can be generated using a digital up
counter with weighted outputs. Look
at the sawtooth generator in Figure 3.
The 4516 binary counter has
BCD outputs (1, 2, 4, 8) that represent
values from 0 to 16 in increments of 1.
Let’s set the current of the 1 output
(Q0) to 40 µA by placing a 130K
resistor on pin Q0 to ground. This
means that every time Q0 goes high,
40 µA will flow through the resistor.
Now let’s set the current of output 2 (Q1) to 80 µA — twice as much
current as Q0. When Q0 is on and
Q1 is off, 40 µA will flow. When Q0 is
off and Q1 is on, 80 µA will flow. But
when both Q0 and Q1 are on, 120
µA will flow — three times the amount
of current through Q0 alone. Does
that sound like the number 3?
By setting Q2 and Q3 to equal
160 µA and 320 µA, respectively, we
can represent all the values from 0 to
16 using currents. These currents
are summed at the node of a
non-inverting op-amp and output as
a voltage. Notice that again the sawtooth waveform isn’t exactly a linear
ramp, but a staircase of small
individual voltage steps, as shown
in Figure 1d. You can increase the
resolution and remove the coarseness of the waveform by increasing
the number of steps from four bits to
five bits using a 4518 (Figure 1e).
The greater the number of binary
digits, the better the resolution of the
The input frequency (LMC662
oscillator) determines the frequency
■ FIGURE 2
+ LMC662 Sawtooth
■ FIGURE 3
f = 300Hz
+ LMC662 Sawtooth
of the sawtooth waveform using the
formula f = 1/2n where n is the
number of weighed outputs.
QIn the 1968 winter edition of
the Popular Electronics
Don Lancaster published a
simple signal generator project called
the AMLIGNER, that used nothing more
than a diode, a resistor, a coil, and two
capacitors (see Figure 4). The diode was
a Motorola M4L3054 four-layer device
that is no longer available. Is there any
substitute for the M4L3054? If not, what
is your recommendation?
— Dennis L. Farkas
AWhile there is a four-layer
diode equivalent of the
M4L3054, I doubt it will fit
comfortably into the original
design. Aptly named a diac, this family
of breakover diodes are made to
trigger triacs in phase-controller circuits.
Unfortunately, they have a characteristic breakover voltage range of 27 to
70 volts — far in excess of the 12 volt
■ FIGURE 4
February 2007 21