There are several limitations which a PIC (or any microprocessor) implementation will have relative to a 555 (unless there is specific one-shot hardware in
• With any processor emulation
of a 555, there will be a delay
between the trigger and the
output pulse. The delay will be a
multiple of the instruction
execution time, plus a fraction of
the instruction time due to the
phase difference between the
triggering edge and the CPU
• The available pulse widths will
always be a multiple of the period
of the CPU clock.
• The power supply voltage is
limited to a maximum +5V.
Depending on the PIC you are
using, the VDD may be as low as
• The output current is limited to
50 mA — sink or source — at least
in the PICs which I have been using.
If the first two of these limitations are not acceptable
for your application, then you will want to use the 555
itself. If the last two are an issue,
an external buffer transistor will
handle higher voltage as well as
Mims Circuit 1
Circuit 1 in the Mims book is
the Basic Monostable Circuit and
is redrawn in Schematic 1 for
reference. The formula for
determining the output pulse
width of a 555 is:
PW = 1.1 R1 C1
where PW is in seconds; R1 is in
ohms; and C1 is in farads. Or, to
make it more practical: PW in
milliseconds; R1 in Kohms; and
C1 in microfarads.
In this second of four articles, I will review the circuits in the 555
Timer IC Circuits by Forrest Mims which use the 555 specifically
as a simple one-shot (circuits 1, 3, 4, 5, 8, 9, 10, and 13), as well
as discuss how to use the PIC 555 replacement (December 2016
issue of Nuts & Volts) in the same applications. I have also
included three dedicated one-shot programs that can be modified
as required for your own projects. The third program is written
using interrupts so that you can embed it within your own
program. The third article will discuss most of the circuits which
use the 555 as an astable multivibrator in audio applications.
The fourth and final article will examine the remaining circuits.
Analog — Part 2
By Larry Cicchinelli
48 May 2017
To post comments on this article and find any associated files and/or
downloads, go to www.nutsvolts.com/magazine/issue/2017/05.
SCHEMATIC 1. Basic 555 monostable
When a PIC can Replace a 555