Pititico CW QRPP transceiver designed for the 40m band. One of the smallest QRP transceivers designed around one 2N3904 transistor by PY2OHH.
A small CW QRPP transceiver with a consumption in RX of 1.5mA and a power of about 500mW. With a few modifications and using other transistors, the power can get as high as 1W. I actually built an older version as described in the video, having the output power over 1W with a 2N2222 transistor. Unfortunately it was heating up too much so I ended up lowering the power down to 500mW.
1. THE SCHEMATIC
I found the original schematic designed by PY2OHH long time ago and I always wanted to build it. I built a few versions using various schematics found online. One of the versions I have in the video presented on the bottom of the article. Version one here in the article is closer to the original design, and version two with little improvements. I’m really happy about how both work. It’s a great little project that it’s worth trying.
PITITICO CW QRPP TRANSCEIVER SCHEMATIC
2. SCHEMATIC DESCRIPTION
On RX the circuit works as a direct conversion receiver, where the 2N3904 acts as oscillator. The 10K resistor ( R2 in schematic one and R4 in schematic two ) limits the circuit current to 1.5mA. The capacitor C4 should be between 68pF up to 82pF and this value will set your offset frequency on RX. The value of 82pF generates an offset close to 800Hz, the one of 68 pF has an offset of approximately 1kHz. If you have issues with the offset frequency and is not quite enough, Manuel recommended me to install a 100pF capacitor in parallel with the CW Key. This worked well for me in some of the versions I built where I had Issues with the frequency offset.
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 C5 should be anything between 120 up to 150pF. In the first versions I built I increased the value up to 470pF increasing the power of the transceiver up to 1W. Unfortunately the transistor was overheating, so I ended up lowering it to 330pF. I also placed a modified Pi Network filter on the output that acts like a bandpass filter. This helped me get rid of the AM broadcast band interferences. The value of C5 ( 330 pF ) is important not to be changed, as it will modify the properties of the filter.
3. PITITICO CW QRPP TRANSCEIVER PCB BOARD
As you will see in the photos down bellow, I decided to build my Pititico CW QRPP Transceiver in a small wooden box. But I also made PCB designs that you can use if you want, with the toner transfer method. Version one is closer to the original design. Version two with the buzzer has a few modifications. I wanted a power LED ( green ), a transmit LED ( red ) that will light up when the CW key is down. Of course the buzzer was installed as TX monitor.
4. BUILDING THE PITITICO CW TRANSCEIVER
Since the Pititico CW QRPP transceiver it’s so simple, feel free to experiment and see if you can get better results out of it. For those like me still learning about building homebrew transceivers, this is a great way to learn and experiment. To listen to the audio output, you should use either a phone speaker ( 300 Ohm ), or even better an external powered speaker. I’m using an LM386 amplifier set to the higher gain possible.
As you can tell, this is not exactly my prettiest built, but it works perfectly. I wanted to test it before designing the PCB, plus that I really loved the wooden enclosure I had and I wanted to keep it like this. In the video down below is an older version. It worked fine but I still had AM broadcast band interferences. On the new version I built, I also moved the crystal on the top panel so I can change the crystal for other ( 40m ) frequencies.
5. ADJUSTING THE BANDPASS FILTER
The inductor that I made for the bandpass filter was made on a T37-2 toroid. It has 15 turns of 0.4mm enameled copper wire. To adjust the filter once you finished building the transceiver, simply connect it to a power meter and spread or tighten the turns on the toroid for the highest power. If you don’t have a power meter, just wait for a strong signal and adjust the windings on the toroid until you get the loudest audio coming from that signal.
In my case the filter helped me get rid of most AM broadcast band interferences. I may still hear some now and then, but really low in the background and you can barely notice them. I actually modified the Pixie transceiver also using a similar filter with the same good results.
David ( DL1DN ) from QRP Lifestyle built a modified version for the 10m band, so feel free to do modifications for other bands as well. I hope you like this project as much as I do. Now all I have to do is to learn the Morse code and get on air using CW on this tiny QRPP transceiver that I absolutely love.
73 DE YO6DXE