Mc14433 Schematics

Joseph A, Gtockner WA6AXE/3 303I'D Brandt Court Fort Meade MD 20755

Morse Converter for DMMs

— super gadget for blind hams

A digital converter which will take the binary coded decimal (BCD) output of a digital multimeter (DMM) and convert it to Morse code would surely be an asset to any blind or partially bhnd person's workshop.

After graduating from a CREI home study course in electronics engineering technology (minicomputers and microprocessors as the major elective), 1 had a strong desire to make some practical use of the knowledge.

Shortly after finishing the home study course, I was talking with my dear friend of nine years, W6LZV\ He was mention ing that a Morse clock could be helpful to him in his daily on-the-air operations. After thinking about a Morse clock versus another type of device which could be converted to a Morse code output, I asked Jean if he could use a digital multimeter with Morse code output. Though Jean is completely blind, he is very active in the building and repairing of electronic equipment. Therefore, it was no surprise to me when he said that the DMM-to-Morse code converter would be more functional.

This article is for the person who wants the challenge of using an instrument which was originally made for the sighted person, This article can open doors that have inhibited the handicapped person in delving into a world of technical discovery — a world which can be just as satisfying for him as for the sighted person.

While researching what chips might be commercially available to handle the BCD-to-Morse code conversion in a very simplified manner, ! ran across the July, 1977, issue of 73 Magazine, which has an article entitled "The Morse Clock." After reading and rereading the article, trying to fully understand the logic, I decided that I would use the article as a starting point for my design of the DMM-to-Morse code converter.

The digital multimeter used for this prototype project was the Sabtronics 2000 DMM. At first, 1 had only a schematic of the DMM to work fromr but it was apparent that converting the output of the MC14433 AID converter chip into Morse code would not be too difficult. After building the DMM and seeing how the MC14433 chip worked, I decided that since the position of the decimal point depends only upon the function button and range button selected and upon whether the X10 range switch is used( I would disregard the decimal point. Only the minus sign and the 3Vi digits were dealt with.

Features of the DMM-to-Morse Code Converter

The converter was designed to provide the following:

1. Morse code output of the 3VS digits.

2. Morse code character "m"P which signifies that a minus sign is present.

3. An audible over-range indicator,

4. A method by which the user can perform "continuity tests."

5. Three selectable code speeds,

6. A Morse code zero (0) for a "blanked" first digit.

The converter at work. The DMM indicates —108.5 Ohms. (The minus sign is peculiar to the Ohms reading on this DMM.) When the INI button is pushedr the converter will respond with--fm/nusj


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