Triplett MM350 tryout

I received the Triplett MM350 from DigiKey on Wednesday:

It's a little smaller than the Metex ME-11 - I was expecting 3 x AA cells but they are AAA cells.  The ones supplied are carbon zinc so I'll have to keep an eye on them - don't want a leak.  The battery door is a little easier to open than the Metex which has an awkward slide and captive machine screw.  I did try the temp sensor which works fine but it was hard to insert the banana plugs, a tight fit.

But the thing I really wanted to try was the frequency counter and duty cycle percentage as they might be used to measure Morse code speed and element weight.  So I hooked it up to a PK-4 keyer and nothing !  Then I realized that the counter needs an AC waveform not a positive pulse train.  So I connected a 100 uF cap in series with the output and voila, I've got frequency:

The red LED display is the Morse Speedometer from the project page:

http://wb9kzy.com/projects.htm 

The MM350 reads 13.71 Hz which is a period of 72.9 milliseconds (1 / 13.71 Hz = 0.0729 seconds).  72.9 ms is 32.9 WPM (2.4 / .0729 seconds = 32.9 WPM).  So with a conversion table the MM350 becomes a Morse speedometer.  Here is a quick spreadsheet of WPM versus frequency:

IOW multiply the frequency of the dits by 2.4 to get the code speed in WPM

 

In addition I tried the duty cycle % reading with dits:

The string of 33 wpm dits has a 50% duty cycle as expected.

and dahs:

The dah reads as a 25% duty cycle due to the inversion of the signal through the output transistor.  Also the red LEDs display the speed as 11 due to reading the dah mark (3x a dit mark).

So the Triplett MM350 works for measuring both Morse code speed and weight.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm