Just as a quick review, the first article in this series described a general-purpose 555 replacement using a PIC microprocessor: the 12F1572. The second article
(May 2017) reviewed the circuits and their PIC
equivalents, which used the 555 basically as a monostable multivibrator. Part 3 discussed the less complex
audio oscillator circuits.
References to the PIC replacement refer to version
1.4 of the program. This version makes three
enhancements to the original: all astable modes (modes
4-6) use pin 4 as a gate input; mode 5 replaces the off-time function with period; and mode 6 is a metronome.
Many of the circuits in this article refer to a piezo
element or buzzer. The units required for these circuits
are usually of the internally driven type which are
polarity sensitive and are labeled with a + sign for the
positive terminal. Where I have modified component
values in any circuits, I show the original values in
parentheses. Also, those components marked with an
asterisk (*) are additional components not in the
A note about the resistor values for selecting the
Range and/or Mode for the PIC 555 replacement: The
ratio of the resistors is what is important as long as the
parallel combination is less than 10K. For instance, the
schematics show 2.5K and 5.5K when selecting Range
and/or Mode 5 (or Mode 2). You can use any resistor
combination that has a ratio within 10% of 0.45 ( 2.5/5.5).
Values of 2.7K and 5.6K, ratio = . 48, will work just fine.
Mims Circuit 14
One of the applications of Circuit 14 (Schematic 1)
shows the 555 being used as a metronome which uses
This installment will examine the more complex circuits from the “555
Timer IC Circuits” by Forrest Mims which are variations on audio
oscillators. Some will use the PIC replacement from Part 1 (December
2016), while others will develop specific programs using a PIC to
emulate a particular implementation of a 555.
Analog — Part 4
By Larry Cicchinelli
36 February 2018
To post comments on this article and find any associated files and/or
downloads, go to www.nutsvolts.com/magazine/issue/2018/02.
When a PIC can Replace a 555
Engineers Mini-Notebook 555 Circuits by Forrest M.
Mimms, III ©1984. Also see www.forrestmims.org.
SCHEMATIC 1. 555 metronome or tone oscillator.