Finished kit

     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 block (blue).

Frequency Counter Assembly

Crushed capacitor    Although no 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.

Joy of Measurement

    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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