Mini-MOS Keyer

Printable Images of each Schematic Page
• Page 1 - Keyer Logic
• Page 2 - Clk, Dot/Dash Timing, Tone Gen.
• Page 3 - Power/Ground

Below is the schematic for the Mini-MOS Keyer that was published in the August 1976 issue of 73 Magazine written by Erich A. Pfeiffer, WA6EGY.

In that original article, there are two small errors in the output circuit for the Sidetone Oscillator output. I looked for an update in the 73 Magazine. I didn't find one, but that doesn't mean there wasn't one. I just failed to locate it.

Anyway, the Emitter of Q2 and the Positive (+) side of the 1.5 Volt Battery (B2) are shown going to −VDD. But there is no −VDD. Those two connections should really be listed as −VSS. I corrected this in the drawings below.

I am not going to say much in the line of circuit description. The original article does a pretty good job of that.

Mini-Mos Schematic, Page 1

Mini-Mos Schematic, Page 2

Mini-Mos Schematic, Page 3

Mini-Mos Timing, Page 1

Printable Image
• Timing Diagram

This is a replica of the Timing Diagram, Fig. 5 from the original article. I replicated it because it's a little clearer and easier to read and the "T" time marking make it easier to see how one signal relates to another.

For the sending of the letter "T" (T1) and "E" (T7), the Dash and Dot paddles are closed (tapped) for a short amount of time. However, the sending of the letter "K" uses the Iambic properties of the keyer. The Dash paddle is closed first (T11) and then the Dot paddle is just tapped (T11-T12). A Dash is queued up first. Then a Dot is inserted when it is complete. Because the Dash paddle is held closed through the generation of the Dot, a final Dash is added. Completing the letter "K".

Mini-MOS Updates - Mod. 1 - Chassis Ground for Paddle Common and Mod. 2 - Automatic Character Spacing

These are the modification from the March 1978 issue of 73 Magazine written by S. M. Allen, K4JEM. Rather than multiple small drawings I combined all of the mods into a single drawing.

Printable Image
• Mod 1, 2, & 3

Modification 1 deals with the common lead for your paddles. As originally designed, power is obtained from a 9 Volt Battery. The Positive side of the battery (+VDD) is connected to the VDD Input pin (14) on all IC's and Chassis Ground. The Negative side of the battery (-VSS) is connected to the VSS Input pin (7) on all IC's. This then requires the Dual Paddle Common to be at -VSS. (See the original drawing for page 1.) Should the Paddle Common, which is usually the paddle base, come in contact with your transmitters ground, the battery will be shorted.

This modification replaces the CD4011 Quad 2-Input NAND gate package, used in the Dual Paddle input circuit, with a CD4013 Dual D-Type Flip-Flop package. This allows the Dual Paddle input circuitry to be changed so that the Dual Paddle Common is at +VDD or Chassis Ground.

Referring to the schematic, the "Q" output, at U1A-1, goes to the input of U2A‑6 and U2B-8. But this is only true if Modification 2 is not installed. If Modification 2 is installed, U1A‑1 instead goes to U2A‑1 and U8A‑2. U2B is being reused in Modification 2.

Modification 2 adds Automatic Character Spacing to the list of features. And, the modifications is presented in two possible ways. In the first method, Mod. 2A shown below, an extras IC is added, U8 a CD4011 Quad NAND gate package. And, the Sidetone is eliminated and it's gates (U7C, U7C and U6D) are put to work as part of the Automatic Character Spacing modification. This modification also means the addition of U8 which is a CD4011 Quad NAND Gate package. All four gates in the package are used. The original article contains a reasonable good explanation of how it works.

Modification 2 can also be implemented without elimiating the Sidetone Generator, but would require adding two new ICs. U8 which is a CD4011 Quad NAND gate package and U9 which is a CD4001 Quad NOR gate package. The circuit may look a bit different, but it is functionally equivalent to Mod. 2A

Mini-MOS Update, S. M. Allen, K4JEM

Printable Images of each Schematic Page
• Page 1 - Keyer Logic
• Page 2 - Clk, Dot/Dash Timing, Tone Gen.

What follows is another schematic of the Mini-MOS keyer but with the modifications indicated. Mod. 1 is shown in it's entirety. But it only shows the wires that need to be removed and the external connections for Mod. 2A and Mod. 2B. I didn't make a new schematic with Mod. 2A and Mod. 2B simply due to space issues. Plus, it would only create confusion with multiple copies of the schematic. Two different copies shouldn't be a problem. I did not bother a new Power/Ground schematic. There is really nothing different there except for the addition of Power/Ground for U8 and U9 and a change to U1.

The drawing shows Modification 1 enclosed in a dashed box. For Modification 2, the wires with "X" on them are removed and re-routed to the circled signal names.

Mini-Mos Update Schematic, Page 1

For Modification 2, the wires with "X" on them are removed and re-routed to the circled signal names in the Mod 2A or Mod 2B schematic. If Mod 2A is used, which reuses the gates from the Sidetone Generator, remove the wires and components in the enclosed box.

Mini-Mos Update Schematic, Page 2