pulse width is set to 980 µs so that it can be
calibrated to read full scale with a 1 kHz input
signal. Graph 1 shows the frequency response at 1
kHz full scale. The circuit is quite accurate even
though it’s quite simple.
If the input frequency is higher than about 1.02
kHz, the meter reading becomes erratic until the
frequency is high enough for the circuit to act as a
2:1 frequency divider, at which time the display will
stabilize at half scale.
Graph 2 is the same circuit but with the pulse
width set to about 81 µs so it can have a full scale
reading of 10 kHz. One of the reasons why these
two pulse widths are not closer to the ideal (1 ms
and 100 µs) is because of the trigger delay time of
the 555 replacement.
Using a 555 IC would allow you to set the
pulse widths closer to those ideal values.
My goal with these articles is to show how a
processor emulating a 555 can be used in quite a
few different circuits. The Forrest Mims’ book
mentioned is an excellent resource for anyone
wanting to learn the how’s and why’s of the 555.
Using the book as a guide for this article definitely
proved to be a very interesting and rewarding task. I
hope you have found these circuits as helpful and
educational as I have.
The source files for the three sample programs
as well as the files for all the schematics are
available at the article link. I use Dip Trace for my
schematics and printed circuit boards. NV
GRAPH 1. A 1 kHz full scale frequency response.
GRAPH 2. A 10 kHz full scale frequency response.
May 2017 55
Introducing the ME Labs D-Stick!
PICBASIC PRO is a trademark of Microchip Technology Inc. in the USA and other countries.
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The ME Labs Standard D-Stick provides all the functionality of Microchip’s 40-pin PIC16F1937 in a hardware module
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alternative to connecting a serial port, programmer, power supply, etc. to a solderless breadboard for project
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