Nuts and Volts - February 2018

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 original.

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.