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I always loved the Pititico CW Transceiver designed by Miguel – PY2OHH and wanted to incorporate an audio amplifier in the circuit. This way I could use regular headphones. Here it is finally, working like a charm. You want to get on air for cheap ? I think you can build this for $10, or even less than that. It’s not perfect, but it’s a simple and easy to build CW transceiver. Initially I wanted to try the Curumim, also designed my Miguel. Since he was not so satisfied with it, I decided to use a few more parts and make a better Pititico.

THE SCHEMATIC

I found the original schematic designed by PY2OHH long time ago and I built a few versions in the past. Recently one of the YouTube subscribers ( Jesus ) asked me for help in a school project. He had to build a very simple CW transceiver for the 160m band. I thought it would be the perfect time to test this idea and see if it works. I was happy to realize that it works amazing, even on the 160m band with little changes.

There is nothing original in the circuit. All I did was to combine two circuits together ( Fig. 1 ). Miguel’s Pititico CW transceiver, with an LM386 audio amplifier. Of course I had to make some adjustments in order to get it to work properly, but it was worth it. It feels a lot like a Pixie transceiver, but with less parts needed to build it.

Fig 1 – Pititico 2 CW Transceiver Schematic

SCHEMATIC DESCRIPTION

On RX the circuit works as a direct conversion receiver, where the 2N2222 acts as oscillator. The 10K resistor limits the circuit current to 1.5mA. The trimmer capacitor C5 will set your offset frequency. The trimmer capacitor C4, will help you adjust the TX frequency exactly on 7.030MHz. Otherwise the transceiver would transmit around 7.029Mhz. You can also replace them with a fixed value capacitor instead if you want.

In my case for example, having C5 – 33pF and C4 – 10 pF offered me a TX frequency of 7.030MHz and an offset frequency of around 800Hz. I find it a lot easier to use two trimmer capacitors instead and adjust them. It’s not a perfect solution and you may have to go back and forward adjusting them until you get it right. But is a simple and easy solution. This is also the reason why I kept it on a fixed frequency and I am not using a VXO.

In TX the circuit works as a low power oscillator, reaching 1W and currents of 100mA. The transistor supports this current well in CW, but do not keep the CW key activated too long as the transistor will burn out. Another problem is operating without antenna or dummy load, the transistor burns out right away.

The capacitor C7 should be between 120 to 150pF. I changed the value to 330pF increasing the power of the transceiver to 700mW. I also placed a modified Pi Network filter on the output that that improves the harmonics suppression. In the past and in the videos I was using a modified Pi Network filter that acted like a bandpass filter. It was a nice solution, but on transmit a bandpass filter is not a great solution. I tried at that time to remove the AM broadcast band interferences and that was the best I could do back then with the knowledge I had at the time. Now I am a lot happier with the results of the updated version.

To remove most of the AM broadcast band interferences I added C11 together with L2. L2 is important to separate the audio output from the oscillator. With ought L2 the TX frequency and the offset frequency would not work properly. So far it seems that I have very little interferences and I am happy with the result. So far this part is identical to the Pititico CW transceiver. Because a lot of readers didn’t want to use an external amplifier, on Pititico 2 I incorporated one on the PCB board. The audio output is for low impedance headphones.

PITITICO 2 CW TRANSCEIVER PCB BOARD

Using the toner transfer method, you can make a PCB board ( Fig. 2 ) in less than 20 minutes. In the video you hear me talking about the issues with the low pass filter and the AM broadcast band interferences. For that reason I updated the schematic and the PCB board with an improved version. So just build it as in the schematic and it should work right away.

Or if you can order PCB boards from PCBWay. I wanted to make it smaller, but it was hard to do so and still keeping it easy to make using the toner transfer method. So the reason it’s a little bigger is because the traces are on one side only. It can be build a lot smaller if you want to.

Fig. 2 – Pititico 2 CW Transceiver PCB Board

BUILDING THE PITITICO 2 CW TRANSCEIVER

In the videos here in the article and in Fig. 3 I have the older schematic and PCB board design. In the new one I no longer use a bandpass filter so there is no need to follow any instructions on that. I was also talking about R2 in the older schematic. That would be R1 in the schematic from Fig. 1. I am now using 0.6W resistors for all, so there is no need to make any changes on this part either.

I used 2N3904 transistors with good results in the past, but remember the transistor was heating up quite a bit. I prefer the TO18-3 ( metal can ) 2N2222A transistor as I can use it with a small heatsink. For the audio output, regular low impedance headphones are the best. A small speaker may also work, but the audio signal will be weak. It was designed for headphones.

Fig.3 – Pititico 2 CW Transceiver ( Older Version )

RF FILTERS AND DUMMY LOADS

Here you have a video with a short introduction to RF filters and dummy loads. Is important to learn why they are so important when building and testing any transmitters or transceivers. I hope is easy to understand by beginners. I also made another video on simple low pass filters that may help you.

ADJUSTING THE BANDPASS FILTER

The inductor that I made for the bandpass filter was made on a T37-6 toroid. It has 16 turns of 0.35mm enameled copper wire. Calculated using toroids.info for the value of 1uH it should be 18 turns instead. For me 16 turns offered me the best power output and the filter performs well. To adjust the filter once you finished building the transceiver, simply connect the transceiver to a a power meter and spread or tighten the turns on the toroid for the highest output power. With a 2N2222 transistor with an hfe of about 240 the output power in my case is about 700mW. It depends on the transistor you are using and the amplification factor of the transistor.

So all I can say now, is to wish you good luck building it and have fun using it. Is really tiny and easy to build and adjust, even for beginners. You can also learn a lot thanks to this simple QRP transceiver. Before I end the article, I have to say thank you to Jesus for asking me for help with his project. If he didn’t, for sure it would take me a lot longer to do this circuit. Also thank you to his teacher, Jacinto ( YV5EWX ) for giving us the hard task of finding a 160m crystal ( ha ). That was no fun at all. Building and getting it to work was a lot of fun though. If you want to discover even more, the Pititico CW transceiver started from a simple CW transmitter, modifier later on.

73, YO6DXE