If you need an operation such as
this, you can use the PIC replacement
in mode 1 (delayed one-shot) or
mode 3 (retriggered and delayed one-shot; refer to Schematic 2).
I have also developed a program
( MSMVx2.asm) which simulates two
555s as shown in Schematic 3. Both
one-shots have been programmed to
trigger on a negative edge, but this is
easily altered by changing either or
both of two definitions. It’s also
possible to enable the one-shots to
trigger on both edges, but the
program would need to be modified
a little bit. There are definitions that
enable the one-shots to be
Both monostable multivibrators
(one-shots) use one of the timers in
the PIC to determine the pulse width.
One-shot 1 uses Timer 0 and one-shot 2 uses Timer 2.
Both one-shots have approximately a 1000:1
pulse width span due to the 10-bit A/D (
analog-to-digital) converter. There are definitions in the program
which enable you to change the resolution (also
changes the minimum and maximum widths) of the
Mims Circuit 7
Circuit 7 (which is shown in Schematic 4) is titled
as an intervalometer. It’s simply a 556 with section 1
wired as an astable and section 2 as a monostable
driving a relay. The period of section 1 determines
how often the relay is activated, while the pulse width
of section 2 determines the length of time the relay is
The timing diagram in the book has the
output of section 1 inverted. The output
should be shown as high for most of the time
with a short low pulse. Also, the 555 (in
monostable mode) is triggered on a negative-going edge which the diagram would show if
the upper trace was to be inverted.
The diodes are for protection against the
high voltage inductive kick which occurs
when the relay is turned off. D1 prevents it
from reaching the 555, while D2 should
prevent the voltage across the coil from
exceeding the reverse breakdown voltage of
the diode. I believe a better placement for D2
would be what’s shown by D3 in Schematic
4. This would clamp the high voltage to one
diode drop above the power supply.
July/August 2018 59
SCHEMATIC 2. PIC 555 replacement.
SCHEMATIC 4. 556 intervalometer.
SCHEMATIC 3. PIC dual