Figure 1 shows the power-up sequence of a
555 astable multivibrator. The output is high while
C1 is charging. The first pulse is longer because the
capacitor has to charge up to the threshold value
(2/3 Vcc) from ground, while for the remaining
pulses, C1 charges up from the trigger level voltage
The actual ratio of the first pulse width to the
remaining pulse widths is about 1.7:1. You might
initially think that the first pulse should be twice that
of the remaining pulses, but the voltage rise is
exponential — not linear.
All the documentation I have seen for 555
astable operation states that the charge time will
always exceed the discharge time since the
capacitor is being charged through R1 and R2, but
is being discharged only through R2. Therefore, the
duty cycle will always be greater than 50%. The higher the
R2/R1 ratio, the closer to 50% you get.
If R1 = R2, the duty cycle will be 66%. If R1 gets low
enough, the discharge transistor will no longer saturate,
and the discharge time will be longer than the calculated
value which can lead to duty cycles less than 50%.
Schematic 1 also shows two options for changing the
charge time dependency on the value of R2. A diode in
parallel with R2 will decrease the charge time once the
voltage across R2 reaches the diode threshold value. This
is about 0.6V for a typical silicon small signal diode, as
low as 0.3V for a Schottky diode, and about 0.2V for
With a Vcc of 5V, the difference between the
threshold voltage and the trigger
voltage is about 1.66V. The diode
will short R2 during a portion of the
If you want to make the on
time totally independent of the off
time, you can put an FET like the
2N7000 across R2 and drive the
gate from the 555 output. The FET
will short out R2 when the output is
high, causing C1 to charge only
through R1. Using an FET, you can
easily get duty cycles much less
than 50%. You will want to make
sure that R1 is not so small that it
could cause so much current
through the FET that it is destroyed.
From a totally practical
perspective, I would never have less
than 100 ohms for either R1 or R2.
If you are using a pot for either or
both, I strongly recommend a fixed
resistor of at least 100 ohms in
series with each pot.
The PIC replacement circuit is
shown in Schematic 2. R1 and R2 should be selected to
force the 555 replacement into Mode 4 — one of its
astable modes. R3 and R4 are used to select the range
(see the first article for details); R5 and R6 set the off time
and on time.
Keep in mind that for both the mode and range, it’s
the ratio of the two resistors that is the determining factor.
I’ve written four sample programs (available at the
article link) that demonstrate astable multivibrator
operation using the PIC12F1572. Three were derived from
the three samples discussed in the second article of this
Program 1 (ASMV-1) demonstrates the simplest and
fastest astable possible, with a 50% duty cycle using
December 2017 41
FIGURE 1. 555 astable power-up.
SCHEMATIC 2. PIC 555