Gary Andrews M0CWY builds a Morse keyer from just a couple of relays and three capacitors.

Back in the 1980s when I worked for British Telecom International as a young technician, I was always amazed by what the older guys could achieve with just a couple of relays. The circuits that they designed bordered on genius. Recently, some 30 years later, I decided to design a relay circuit to perform a simple task − to generate the dits and dahs of Morse code.

The circuit I developed is simple. It uses only two relays as shown in Fig. 1. The relays are Panasonic TQ2-12V low-power high-speed telecoms relays and they operate and release in about two milliseconds. Note that they have an integral diode wired across the coil to clamp the back EMF (Electro-Motive-Force). You must connect the coil as shown in the circuit diagram or it will not work. The coil resistance is 1024Ω. If you use relays with a different coil resistance, you will have to alter the capacitor values. Larger and slower power relays will not work in this circuit.

Capacitor C1 sets the space between the dits and dahs and is the same value as capacitor C2, which sets the dit length. Capacitor C3 sets the dah length and is three times the value of C1 and C2. Diodes D1 and D2 ensure that only C2 or C3, not both, are charged when the paddle operates.

When the paddle is in the centre position, the circuit is idle, both relays are released and the relay contacts are in the position shown in the diagram.

When the paddle is moved to one side or the other, C2 or C3 is charged, relay B operates, contact B1 closes, relay A operates and C1 charges, contact A1 opens and C2 or C3 discharge through the coil of relay B. After a dit or dah delay, relay B releases, contact B1 opens and C1 discharges through the coil of relay A. After a space delay, relay A releases and contact A1 closes. If the paddle is still operated at this time, the whole process repeats and another dit or dah and a following space is generated. If the paddle has changed sides, a dit becomes a dah or a dah becomes a dit. If the paddle has been released, the circuit returns to the idle state.

Contact B2 is used to key the transmitter.

Construction

I built the entire unit onto a wooden base measuring 70 x 80 x 19mm. One of my favourite methods of construction for simple circuits, explained in my June article, is to produce a rough circuit board using a high-speed rotary tool fitted with a circular cutting wheel. I cut straight lines into the copper forming rectangular pads onto which I solder the component leads. If you do this, make sure you wear eye protection because a lot of dust is produced. The circuit board measures 70 by 40mm and the finished item is shown in Fig. 2.

Next, I assembled the circuit board. I mounted the relays upside down and fixed them into place using a drop of super glue. I wired the capacitors and diodes between the relay pins and the pads, or the ground plane, according to the circuit diagram.

With the circuit board complete, I screwed it to the wooden base and glued the connectors at the rear. I fixed rubber feet in the corners of the bottom to stop the unit from sliding around on the desk.

I completed the job by wiring the connectors to the circuit board. The finished unit is shown in Fig. 3.

If you wish, you could add an audio oscillator, making a nice self-contained Morse practice unit, and then it could also be used with simple transmitters and transceivers that do not have sidetone.

Operation

Operation is simple. Connect power, plug in a paddle and connect the unit to the transmitter. When the paddle is moved side to side, strings of dits or dahs separated by suitable spaces will be produced. If the relative lengths of dits, dahs and spaces are not in the correct ratio of 1:3 or, if you prefer, some other ratio, you can pad the capacitors with additional parallel capacitance to increase the length or add parallel resistance to shorten it. The values shown in Fig. 1 result in a speed of about 23WPM (words per minute). I prefer a more leisurely pace so I changed C1 and C2 to 47µF and added 47µF in parallel with C3 to give a speed of about 15WPM.

The best thing about this keyer is the clattering sound of the relays as you send your message. In fact, with the noise of the relays you don’t need sidetone! Anyway, I hope you are as impressed as I was all those years ago with what can be accomplished using just a couple of relays!

This article was featured in the September 2018 issue of Practical Wireless