As a quick review, the first article described a general-purpose 555 replacement by using the PIC12F1572. The second article (May 2017)
reviewed the circuits and their PIC equivalents, which
used the 555 basically as a mono-stable multivibrator.
References to the PIC replacement are 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.
Circuit 2 (Schematic 1) from Mims’ book shows the
555 connected as an astable multivibrator. It shows some
of the internal circuitry of the 555 which will be helpful in
discussing the circuits that follow. The operation of the
circuit can be described briefly as:
1. Initial condition: Power off.
2. Apply power.
A. Initial C1 voltage is 0V which is below the
trigger voltage (1/3 Vcc).
B. Flip-flop is set: Output high, discharge circuit off.
3. C1 charges through both R1 and R2.
4. C1 reaches the threshold voltage (2/3 Vcc).
A. Reset the flip-flop.
B. Output goes low.
C. Discharge circuit on.
5. C1 starts discharging through R2.
6. C1 voltage decreases to the trigger voltage (1/3 Vcc).
A. Set the flip-flop.
B. Output goes high.
C. Discharge circuit off.
7. Cycle starts over at step 3.
This third article in a recurring series continues to examine several of
the circuits from the popular 555 Timer IC Circuits by Forrest Mims,
which are variations on audio oscillators. Some will use the PIC
replacement covered in the first article (December 2016), while others
will develop specific programs using a PIC which will emulate a
particular implementation of a 555. The remaining oscillator circuits
will be discussed in a future installment of this series.
Analog — Part 3
By Larry Cicchinelli
40 December 2017
To post comments on this article and find any associated files and/or
downloads, go to www.nutsvolts.com/magazine/issue/2017/12.
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. Basic
555 astable circuit.