Thursday 30 July 2015

CW decoder – The electronics

I found a circuit online that refers to a QST article in 1999, that shows a CW zero beat decoder. It uses a LM567 that converts the audio signal and switches on a LED to show the operator has achieved a zero beat. The project has a variable resistor that can be used to tune or “swing” the centre frequency – This was a good start for me, and I created the following schematic for the build and set to work constructing it on the breadboard.

As you can see its very simple indeed. I did differ from the published design and found that I could use a single .22uF capacitor across the output pins. So here is a breakdown of the build :

C1       0.1µF
C2       0.1µF
C3       1µF
C4       0.22µF
C5       0.47µF
VR1     10kΩ
R2       470Ω
IC1      LM567
Arduino Uno – Rev 3 (or any suitable Arduino copy etc)
20x4 LCD display ( Im using a I2C display. This takes up less wiring to achieve the same result)

Once I had built the electronics, and before I tested it on the Arduino it was time to feed the circuit with some audio. Luckily I have a audio output from the FT-2000 which had enough volume to trigger the IC.

After some adjustments on the potentiometer the LED was flashing in time with the tones being received on the 20m band at 750Hz audio frequency, This was great, with no mistakes and on the 1st attempt the circuit burst into life. here is the breadboard layout of the 1st attempt. It is laid out subtly different - for ease of understanding only. (also be aware I'm using an I2C display - and in this version I was using a 2 line display)

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