Introduction

Printable Images of each Schematic Page
• Page 1 - Morse Code Generator Schematic
• Page 2 - Power/Ground

This circuit was included in a article by D. McBright, in the February 1989 issue of Elektor Magazine. The original schematic used "IC" and "N" as Reference Designators. I changed this to using a common "U" designator bacause my drawing library only allow "U". I also adjusted the text accordingly. I shuffled the schematic a bit to fit better in this format and moved the Power/Ground arrangement to a separate page. The Power/Ground page also includes a drawing of the suggested layout, minus a few minor details.

Morse Code Generator, D. McBright

Ideal for both the morse trainee and the experienced operator,
this low-cost, versatile, generator with relay output provides
automatic timing, at a user-defined speed, of audible dots and dashes.

In this circuit, four 74HC series ICs are used to produce either a string of dots. or a string of dashes. The dash has three time the length of a dot. Spacing between characters is equal to one dot. Characters are selected by shorting either of two contacts of a paddle-type key to ground. The generator allows the string of "automatic" dashes to be replaced by dashes whose length and frequency are controlled by the operator, whilst the string of dots remains unchanged. The frequency of the dots ranges between about 130 and 910 per minute. A variable-level sidetone is available at a fixed frequency to enable the operator to hear the characters he is sending.

Output from the generator is a normally-open relay contact for connecting to the CW transmitter. A switch is included to disable the morse relay and so reduce the current consumption while practising. The morse generator is powered by a 9V battery or by a mains adapter with an output between 8 and 15 VDC. An internal voltage regulator supplies 5.6 V for the integrated circuits.

Circuit Diagram

The circuit diagram is given in Fig. 1. When counting, the first divider, U1, divides the clock signal supplied by U3A by 16. The oscillator operates at 128 times the dot frequency, and uses the hysteresis of a CMOS NAND gate, U3A, to give a charge-discharge cycle for R-C network (P2+R8)-C8. The second divider in the circuit, U2, is uses its first 2 stages to divide by 4; the third stage to produce dots or the first third of dashes; the fourth stage - in conjunction with the third - to produce dashes. Gates U3C and U3D are the respective inverters and U4C inhibits the final two-thirds of the dashes when dots are required. Between counts, divider U1 is reset to 15, and divider U2 is reset to 3 so that, when the appropriate contact is shorted, a character is started on the next positive edge from the oscillator. The maximum delay between a contact being actuated and the start of a character is approximately 3.5ms at the slowest dot speed.

When either of the 2 key inputs of 3-Input NAND gate U4A is connected to ground, the load inputs of the dividers are taken logic high, and counting is started. The third input of U4A is used in conjunction with feed-back diodes D1 and D2 to ensure that any character is completed if a key contact is broked early (this does not apply to dashes in the manual mode). However, a full-length dash will only be obtained if the contact is released after the first third of the character is completed, otherwise a dot of correct length is produced.

Two of the three inputs of NAND gate U4B mix the pulses from the dot/dash inverters, U3C and U3D, whilst the third is keyed only in the "manual dash" mode to make non-automatic dashes. Automatic or manual operation is selected by toggle switch S1. The output of U4B controls the sidetone oscillator set up around U3B, and the relay driver, Q1. Optimum sound output from the passive piezo-ceramic buzzer Type PB2720 from Toko is stated to be at a frequency between 3 and 3.5 KHz. Reasonable sound levels are, however, obtained at lower frequencies also. The sidetone oscillator frequency can be adjusted to individual taste by altering the value of R7 between 68KΩ (min) and 220KΩ (max).

A stabilized +5.6V is provided by a 5V/100ma regulator, U5, whose output voltage is raised by 0.6V with the aid of a conducting silicon diode, D3, connected between the common terminal and ground. As already noted, the circuit can be powered from a battery or a mains adaptor with DC output. In stand-by mode, the generator draws 6-7ma from a 9V supply; with the relay muted and characters selected, current consumption rises to about 10ma. Total current consumption with relay actuated depends mainly on the coil resistance.

Construction

The generator is conveniently built on a small piece of veroboard or other prototyping board. Construction and wiring are straightforward. All ICs are fitted in sockets, and solder terminals are provided for the wires to the external controls. Whatever type of relay is used, a diode to suppress back-EMF mush be provided as shown in the circuit diagram (this diode is integral to most, but not all types of DIL reed relay operating from 5V). The coil resistance of RL1 should not be lower than about 500Ω

The accompanying photograph shows the completed morse code generator connected to an all-mode SW transceiver Type FT-200 from Yaesu. The generator fitted in a small metal enclosure, with all controls mounted onto the front panel and connected to the board via wires and solder terminals. The piezo buzzer is glued in place behind the vent holes at the inside of the top lid of the enclosure. The paddle-type key is connected to the generator via a short length of stereo screened wire and a 3.5mm stereo headphone plug (fitted on to the rear panel) and mating socket. The screening of the wire is connected to the centre contact of the paddle.

Schematic, Page 1

Schematic, Page 2