by Harry Lythall - SM0VPO

This is a circuit that I particularly like due to its simplicity and speed of construction. With this circuit, it is now possible to include a paddle keyer as an integral part of all, but the smallest, of QRP CW transmitters. It also satisfies my intense hate for CMOS!!


I have drawn the circuit direct from my lab-book, without the "modifications" I have recently been adding. The circuit, as shown, is based upon a 2.5 K ohm relay that was common in old military equipment but modern reed relays may be used, with the benefit of a reduction of the supply voltage.

The circuit relies on the fact that a relay will ENERGISE at a much higher DC voltage than that needed to HOLD it on. This results in quite a large amount of "hysteresis". A typical 12 volt relay will energise at about 9 volts, but once energised it will remain energised whilst the voltage is reduced to less than about 4 volts. The 18 volt DC shown was required for the relay I first used, back in 1968.

The two electrolytic capacitors determine the dit:dah ratio, but this may be adjusted with the 1K and 5K potentiometers. One word of warning; even modern electrolytic capacitors can have quite a wide tolerance, sometimes from -50% to +100%. A 100uf capacitor can therefore vary from 50 uf to 200 uf!! You may therefore have to "fiddle" with the capacitor values a little each time you build this circuit.

The only disadvantage I can find with this circuit is that pressing both keys simultaneously will give a constant series of DAHs instead of the "DITDAHDIT" you get from "top-of-the-range" keyers.


The paddle key can also be made up cheaply by using two of those small cheap ex-government keys that are almost too small to use. Bolt them together, base to base and mount vertically. You will then have one key to the left and one to the right. Fit two perspex paddles under the knobs. Works fine, especially if the base is quite heavy. A 1cm x 12cm x 7cm block of aluminium or brass is quite heavy enough. Four rubber feet complete the job.

Have fun, de HARRY, Lunda, Sweden.

Dont forget to visit my messageboard if you have any questions about this or any other project. I always look forward to receiving feedback, positive or negative. You do not have to register to post messages.

Very best regards from Harry Lythall
SM0VPO (QRA = JO89WO), Märsta, Sweden.
EA/SM0VPO (QRA = IM86BS), Nerja, Spain.

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