Graph 1.ods (available at the article link) also plots the
same information for a 556, which shows the same
characteristic as the 555 but with a slightly lower
inflection point of 1.2V.
As part of the investigation for this article, I found
that the resistors (labeled R) which form the internal
voltage divider shown in the 555 schematic can be
different among manufacturers. Out of nine
manufacturers I found, only four listed the resistor
values. Three of them were 5K and one was 100K. The
100K unit used FETs, while the others show bipolar
One way to determine the resistor values is to
measure the CtrlV and then place a 47K resistor from
the CtrlV to ground and measure the voltage again. If
the voltage doesn’t change much, then the resistors are
low value and probably 5K. All three resistors will be the
same whatever their value.
The reason for the discussion above is that the book
shows a 100K potentiometer on pin 5. If the voltage
divider uses 5K resistors, the 100K pot will have very little
effect on the voltage over approximately the center 80%
of its range.
I’ve included another spreadsheet (Voltage
Divider.ods) in the article downloads which shows this
characteristic in a table as well as graphically.
If you want to implement this circuit, I recommend
that a potentiometer of at most 5K be used; 1K would be
better or — better yet — drive it from an op-amp.
Also notice the presence of R5 which is not in the
book. It is inserted to ensure at least some resistance for
when the pot is at minimum resistance. Having the
internal discharge circuit connected directly to Vcc didn’t
seem like a good idea to me. I’ve done the same thing to
all of the 555 circuits in this series.
The equivalent circuit using the PIC 555 replacement
(PIC FM- 11) is
shown in Schematic
5. By tying pins 5
and 6 together, the
circuit will output a
50% duty cycle. For
a wide range of
the best range
selection for the
circuit is Range 1
(R3 = 6.5K, R4 =
1.5K): 10 µs to 10
theoretically yield a
frequency range of
50 kHz to 50 Hz.
However, with software delays, the shortest period is
closer to 60 µs, or about 16 kHz.
The higher frequencies will change more rapidly with
voltage than the lower frequencies due to the 10 µs
resolution/step size. For example, a 50 kHz signal has a
period of 20 µs. The next longer period (lower frequency)
available with this program would be 40 µs (except for the
delays mentioned above), which is a frequency of 25 kHz.
The step size is 20 µs because both the on time and
off time are changing simultaneously by the same amount:
10 µs per step.
I used a different circuit to drive the speaker, which
should be safer for the PIC which has less current
available on its outputs than a 555. The same circuit can
be used for the 555 if desired.
One of the differences between the operation of the
555 and PIC circuits is the overall frequency range. The
555 is usually rated up to a maximum output frequency of
3 MHz. The PIC replacement — without any software
December 2017 43
SCHEMATIC 5. PIC
GRAPH 1. 555 VCO frequency
vs. control voltage.