An Electronic Straight Morse Key
Alpar Cseley HA8KT describes an interesting and fun constructional project to replace a traditional Morse key.
Alpar Cseley HA8KT describes an interesting and fun constructional project to replace a traditional Morse key.
Samuel F B Morse on his way back from a European trip drummed slower and faster with his fingers on the railings on board the Sully steamer… He invented the electric telegraph, the mechanical Morse key and the Morse alphabet.
The purpose of my recent experimental work was to replace the old-fashioned mechanical straight Morse key used by enthusiastic hand-key fans with one that is easier to use, quieter, smaller – suitable for holiday trips, could be used in hotel rooms without disturbing others and calling undue attention to your activity. Besides, it serves as ’proof of concept’ for further touch keyer developments.
Principle of Operation
Working with the straight mechanical key needs the trained movement of the wrist and help from (in principle…) three fingers. In order to increase the code sending speed, first mechanical ‘bugs’, then sophisticated electronic code generators were developed. These elkeys are manipulated by so called actuators. Actuators of the popular semi- or fully automatic elkeys need either pushing arms sideways or touching vertical sensor surfaces, necessitating the movement of two fingers.
The fully electronic straight touch key (more correctly actuator) described here replaces the simple on-off mechanical Morse key and is operated with vertically moving one finger, tapping on a sensing surface: on the touchpad. Tapping with one finger is a simpler form of motion than using two fingers or flexing the wrist.
Elkeys (code generators) with touch-sensor actuators are published in the amateur literature. Those are, in reality, switching circuits and the actuators (albeit contactless) built for them are ‘tappers’. Those actuator circuits could be sorted into three groups:
a) Using one touchpad: amplifying the (almost everywhere present) 50 or 60Hz EMI signals used for switching the associated circuit, which then keys the transmitter.
b) Keyers with a touchpad having two electrodes: the change of resistance between the electrodes as the finger(s) touching them results in the switching and keying.
c) Keyers with capacitive touchpad(s): touching the electrode(s) changes the capacity between the electrode(s) and the earth results in the switching – keying the radio.
While the simplest of the above, the group a) keyers, work well in most of the environments (radio shacks), lack of EMI signals from the mains renders them inoperative. Being on the hilltop, or out on the sea for /MM with radios fed from batteries, keyer type b) or c) from above is what we need.
Deviating from the traditional mechanical, straight Morse keys, an experimental electronic actuator named
TAPKEY was built, which is suitable for straight keying of transmitters of different types.
A touch-sensitive (A-type) circuit described in [1] was used as the starting point for experiments and improvements. By addition of a touchpad with two electrodes it was converted to a type-B device.
Description of the Circuit
Experimenting with a couple of circuits revealed some shortcomings: delays in keying, insensitivity, pulse distortion in the keying circuit. After some time finally, a suitable circuit was developed as the basis of a reliable electronic, tapping, straight actuator: the TAPKEY.
The block diagram, Fig. 1, explains the functions of the electronics. The circuit is built around the TTL logic SN7400 IC, which includes four NAND-gates.
By placing a finger onto the touchpad, the skin resistance ‘shorts’ the electrodes of the pad. The output of the connected Darlington amplifier goes to logic LOW. It flips (through two serially connected NAND gates) the output of the logic block from HIGH to LOW: it is the KEY DOWN state. If a connected transmitter’s key line needs to be LOW for transmit (positive keying), it would sense the KEY DOWN and transmits.
A diode (D2) at the logic block output isolates the keyer circuits from the transmitter keying line voltage.
Further isolation could be achieved by using the built-in reed relay for keying a transmitter. A jumper should be placed to position A for direct connection to a transmitter keying line, or to B connection to the reed-contacts. Keying by the relay helps in valve transmitter keying.
Unlike the mechanical straight key with audible clicks giving feedback for the operator about his/her tapping, a silent working electronic one does not. Still, the radio operator needs indication about the TAPKEY’s working, the tapping. Therefore, an LED (D3) is permanently connected to the TAPKEY’s logic and lights up at KEY DOWN. For audible feedback and code practice, a sidetone generator driving a piezo sounder is provided. This sidetone however, could be switched-off (switch S3), while the LED could not.
A further switch (S1) conveniently shorts the touchpad (KEY DOWN, turnson the transmitter) for situations when a continuous signal is needed for some adjustments (e.g. antenna tuning).
The circuit diagram, Fig. 2, shows the details of the electronics. The circuit following the touchpad includes resistor R2 and capacitor C1 for filtering out stray voltages (hum). For satisfactory touch sensitivity the T1+T2 Darlington should have high gain. In the prototype TAPKEY
BC179 (h21e = 432) and BC108 (h21e = 579) provide ample gain. In the ’no-touch’ case R3 pulls the input of NAND1 to HIGH (the NAND gates need >2V for HIGH and <0.8V for LOW). Inverted by NAND1 and NAND2 the keying output (pin-6 of the 7400) stays HIGH.
When touched, the changing resistance between the touchpad’s electrodes drives T2 transistor into saturation, then the NAND1 input goes to LOW. The logic state changes through NAND1 and NAND2, thus the output of NAND2 (pin-6) goes LOW: shorting the connected keying line of the transmitter to ground (KEY DOWN). The two serially connected NAND gates provide options for negative or positive keying. However, within this prototype TAPKEY the keying mode is wired (and tested) for straight (positive) keying of an FT-897 transceiver. Regarding the keying line of this radio, +5V (logic HIGH) is present on the socket when not keyed, and supplies 1mA at KEY DOWN. Therefore, a diode (D2) is inserted in the output (pin-6) of TAPKEY isolating the TAPKEY circuitry from the transceiver.
On the right side of the circuit diagram a symbolic jack plug is depicted, wired as required for keying the FT-897. Only the tip and the shaft of the 3.5mm plug are used (the transceiver must be configured for straight keying).
The reed relay (marked by a yellow dot in Fig. 6) works in paralell with the logic output and closes the input contact to ground for KEY DOWN. It is recommended that the reed relay is placed into an IC socket. It could simply be left out from the PCB when not needed – or inserted any time later without soldering and changing anything in the circuit. One note here: the given type of relay in the parts list has no protective diode across the relay, therefore a diode (D1) is paralelled with the solenoid on this PCB. Nevertheless, there are relays with built in diodes, albeit for more expense.
While NAND1 and 2 drive the keying, the other two NANDs switch on the LED and the sidetone while KEY DOWN. Pin 3 of the sidetone generator IC 555 drives a piezosounder (it does not have internal oscillator). The audio is not exactly a sinewave, but is acceptable to the ears.
The sidetone’s frequency with the components indicated is 998Hz (measured). The frequency could be changed by using other values of R4, R5 and C4. If the resistors are in kilohms and the capacitor in nF, the following formula gives the tone’s frequency in Hz: The minuscule difference between the measured 998 Hz and the calculated one with the given parts is 991Hz, caused by the tolerances in component values. Because of the significant gain by the front end Darlington, needed for reliable sensing of the tapping finger, for circuit stability and TTL levels, the supplied power was stabilised to 5V. The circuit could, though, be powered from a 9V battery, or a suitable plug-in power supply (with >5V out). The power consumption was found acceptably low:
• KEY-UP
9mA Not touching
• KEY DOWN
22mA Touching (w/o relay and sidetone) 26mA (w/o relay, w/sidetone)
28.5 mA(w/relay and sidetone)
The KEY DOWN currents depend on the resistance across the touchpad’s electrodes. KEY DOWN resistor R1 (shorting the touch pad electrodes) should be adjusted for preventing excess collector currents in T1 and T2. Its value depends on the transistors selected. For a longer time of inactivity the power could be disconnected by the S2 switch.
Building and Use
The intention is to build a TAPKEY as flat as possible for travelling and portable operation. A further intention is using only a single-sided PCB for easier reproduction. One-sided PCB requires the touchpad on the solder side like this prototype TAPKEY has, Fig. 3. Consequently, the components need to be on the underside of the reversed board, increasing the height of the finished TAPKEY. To be able to construct a flat TAPKEY, one possible solution is to use a separate touchpad attached (glued) to the component side of the PCB and have the component side up. This is the subject of further creative work. The component placement of my recent TAPKEY can be seen in Fig. 4.Touchpad (Fig. 5) cleanness is a basic requirement for perfect keying. Tin- or gold-plating of the electrodes are viable soutions albeit with very different price tags.
The finished, though still not boxed, TAPKEY is shown in Fig. 6. The size of the PCB is 60 x 150mm (approx. 2.4 x 5.9in).
The TAPKEY in Practice
The resistance of the operator’s skin on his/her tapping finger switches on the transistors and changes the states of the following logic gates. The lower the skin resistance, the more sensitive the circuit is for the tapping. Experience shows that a lighter touch works better with the middle fingers than with the index fingers. This is most probably due to the thicker skin on the more frequently used finger’s tip (there could be a difference between the right and left hand’s fingers,too). The middle finger may be as agile as the index one, worth a try.
Code sending with the TAPKEY needs some minimum practice: the tapping finger should not be left resting on the touchpad!
As originally intended, the circuit was tested with a Yaesu FT-897. The output of the TAPKEY was connected directly (with the jumper on the PCB in position A) to the transceiver KEY socket (observe wiring of the 3.5 mm plug). Keying was correct, fast, without delays. It is quiet, thus could be used during the quiet night hours (even in the bedroom, YL permitting…).
Reference
[1] Bassó Andor: Amatőr kapcsolások (Érintésre működő elkey vezérlő)
Fig. 2: Rádiótechnika, 1978/12, pp. 563-4