Sunday, July 23, 2017

Mini-Project: Discreet Transistor CW Keyer; Part 1

Summertime in SE Michigan is painfully short, so I try to make the most of it while it lasts, reserving my hibernation-in-the-shack time for the seemingly endless winter months.  There are days, though, when the humidity and, here in swamp country, the mosquitoes make outdoor activities somewhat unpleasant.  So, I like to keep a few short-term projects in the queue for those occasions where I need to spend some time in the air conditioning.

My favorite mode is CW, and even though I'm a proud member of the SKCC (#3173), I don't mind admitting that I prefer using an electronic keyer.  And, while I love my old AEA "Morse Machine", there's something inappropriate about using it with my homebrew rigs.  I mean, if I can design and build my own SSB/CW transceivers, I damned well ought to be able to build a decent keyer, right?

Actually, I've done it before, but that one uses a microcontroller.  It works fine, but I want to do something "Old School", using no ICs.

When I first started "noodling", my thought was to, basically, build a solid-state version of the old Hallicrafters HA-1 "TO Keyer", so I immersed myself in the manual until I understood what made it tick.

The concept is incredibly simple: The DITs are formed by an astable multivibrator, and the DAH's formed by "ORing" the output of the DIT circuit with that of a bistable multivibrator.  Cool.  That gives me a starting point, now to make it happen in silicon.

Actually, it's been done - Hallicrafters had their HA-4 and Heath had their HD-10, but I want to come up with my own circuit rather than copy someone else's.  I also want to run this thing from a 9V battery, and both of the aforementioned designs require "split" positive and negative supplies.  So, while there is going to be some similarity to these earlier designs -  there's only so many ways to make a transistor multivibrator - this project won't be a clone.

I'm going to do something a bit different than I have in the past in that I'll post to this blog as I go, rather than waiting until the project is complete to do a "wrap-up" series.  Because of this, there are bound to be some mistakes along the way, but that's part of the fun, right?

Anyway, might as well start with the easy part: the DIT circuit:

Transistors Q1 and Q2 form the astable multivibrator, which is essentially a free running oscillator that produces square waves by feeding the output of each transistor to the input of the other.  The frequency and duty cycle are determined by C1, C2 and R1, R2 and R3.  By using variable resistors for R2 and R3, we're able to vary the speed and weight (element to space ratio) from the front panel.

When the DIT key is open, transistor Q3 is turned-on through R8, which forces Q2 off by forcing it's base low.  When the key is closed, Q3 is turned off, allowing Q2 to be turned on by Q1 and allowing the multivibrator to multivibrate.

R7's purpose in life is to create the "self-completing" dits by creating something of a "Wire OR" with the key input: If either the key OR the collector of Q2 are LOW, Q3 is turned on.  So, if the key is opened halfway through a dit, Q2 will keep Q3 on until the dit is complete.  Cool stuff, no?

This is what's running on the solderless breadboard pictured earlier - I'll be tweaking the timing resistor values as I move along, but this is a start...


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