Frequency Counter Kit:
Variants of this kit are available from different sources. The one
I purchased from Banggood was $11.78, including the plastic enclosure
and a USB power cable. Shipping was free. In the photo above the
completed kit is shown in crystal checking mode. Observe that the
displayed frequency count agrees with the number stamped on the crystal
(3.686 MHz). The green jumper at the lower right corner selects either
a crystal or a signal source that is plugged into the adjacent terminal
instructions are included with the kit, assembly is straightforward.
The PCB is well-annotated and a parts list identifies values for all
components. One tiny ceramic capacitor was crushed in the
tightly packed bag of parts. However, I had one of the same
value, so that caused no delay.
A 20 MHz crystal connects to the PIC
16F628A microcontroller’s OSC/CLK pins. Pressing the yellow button
(the only button) for a second or so produces a menu that, best I can
tell, allows adding or subtracting a constant from the displayed
frequency count, a sort of calibration feature. That was not necessary,
however, as the frequency count appeared to be in close agreement with
expected values for the various crystals tested. A 20 meter crystal
stamped 14.060 read as 14.058 MHz. A 10 MHz crystal read 9999 point
something—close enough. I mean, this is not a high precision frequency
meter. It is claimed to read up to 45 MHz. However, the highest
frequency crystal available for testing was 20 MHz.
This composite image above summarizes an
exercise in which the frequency counter displays the output of a 555
and test RC. The measured (or true) value is compared to the
frequency computed from marked R and C values. The concurrent
oscilloscope trace is in approximate agreement with the frequency
counter kit reading. This sort of exercise is helpful in conveying
basic timing principles. One natural follow-up would be to measure the
R and C values independently and re-compute frequency based on their
measured rather than marked values. Resistance can generally be
measured to within 1% using an ordinary volt-ohm meter. Similarly, such
a test rig could be used to estimate an unknown capacitance, given a
known resistance and measured frequency.
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