Stepped Tone Generator — took me a while to understand
what was happening.
As it turns out, that although the graph is correct, the
text is incorrect — the frequency rises and the step size
(DF) increases as R3 is reduced.
To duplicate Mr. Mims results, I used a 50K pot for R3
to get better resolution. I set R3 so that the pulse width on
pin 9 was about 200 µs and
adjusted R1 such that the input and
output frequencies were the same
at 2 kHz.
I then rotated R3 through each
of the step frequency changes and
measured its resistance.
The results are shown in Graph
2 which closely matches Mr. Mims
results. As you can see, the output
frequencies are sub multiples of the
2,000 Hz fundamental.
Section 1 of the 556 is wired as
an astable multivibrator with a
frequency range of about 282 Hz
to 12. 8 kHz. Section 2 is a
monostable triggered by Section 1,
with an output pulse width range of
about 16 µs to 5 ms.
The operation of Section 2 is
that of a frequency divider (see Part 2 of
this series for a detailed explanation).
For any single setting of R1 (period),
as R3 is changed, the output frequency
will be constant until a pulse width
threshold is reached (N*period), at which
time its output frequency will jump to its
Changing the period of Section 1 will
change the output frequencies of Section
2 at which it jumps. You should also note
that even though the monostable
frequency does not change until the threshold is reached,
the tone sound will change because the duty cycle of the
pulse is changing.
Using two PIC 555 replacement ICs will operate
exactly in the same way if you set one to mode 4 or 5
(astable) and the other to mode 0 (monostable). Also, the
program of Mims Circuit 21 (discussed later in this article)
will yield similar results if all you want
is an output that steps among several
Program NCO_Stepped Tone
Gen.asm in the downloads emulates
this circuit by implementing essentially
One uses the NCO of a 16F18313
to emulate a 555 astable mode; the
second uses a timer to enable
emulation of a 555 in its monostable
Schematic 9 shows the circuit
which duplicates the stepped tone
generator where R1 changes the
astable frequency and R3 changes the
GRAPH 2. 556 stepped
frequency vs. R3.
42 February 2018
SCHEMATIC 9. PIC stepped tone generator.