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The Pixie Hack Challenge

As suggested in my Making the Pixie QRP transceiver kit slightly less appalling video, minimalist rigs like the Pixie are best regarded as novelty projects. You can make contacts on them but it's hard work due to their lack of frequency agility, broad receivers and low power. Most Pixies end up rarely used on the shelf after the initial excitement has subsided.

However the Pixie kit isn't all bad. It's great training for soldering. It's given many their first experience at kit assembly. And the low price makes failure easier to bear.

Another less discussed benefit is how easy the Pixie is to adapt to form another project. Then you can have fun using it rather than enduring the tedium of unanswered CQ calls.

In this video I present the 'Pixie Hack Challenge'. You'll see ideas on different applications for the Pixie kit.

  The Pixie transceiver is basically an LM386 IC audio amplifier, a transistor crystal controlled oscillator and a transistor RF power amplifier. It has a few design subtleties. The most important is the use of the RF power amplifier transistor to form the diode mixer for the direct conversion receiver. While performance is compromised compared to a separate diode mixer, this arrangment avoids the need to switch the antenna (via the low pass filter) between transmit and receive. Secondly the required small frequency offset between transmit and receive is implemented by putting a voltage on the diode in the crystal circuit during receive. Thirdly a mute function on receive is provided by having the key short across the audio amplifier's audio input on transmit. More background here.


Project possibilities

These are the project possibilities mentioned in the video. I've given some hints as to how you may approach them but most have been untested by me.

1. Direct conversion receiver. Probably the simplest modification. Provide a few kilohertz of tuning range by wiring a variable capacitor and small inductor in series with the crystal to form a VXO. Two crystals of the same frequency in parallel with further increase tuning range. Or for even more coverage use a 3.5 or 7 MHz amateur band ceramic resonator. If you want optimised 80 metre results it could be worth doubling the low pass filter values to shut out reception of unwanted 7 MHz signals.

2. Low power AM transmitter. Here you could use the LM386 as a modulator with Pin 5 providing voltage to the power amplifier transistor's collector circuit. You should have a current limiting resistor otherwise the LM386 will overheat and be damaged. For ease of construction you will probably want to disable the receiver function. An electret microphone insert could be coupled to the LM386's input. Talking loudly should just produce sufficient modulation. Change the crystal to one in the phone part of the band. This is not a serious transmitter but potentially good for a novelty experiment, though I and others have had good contacts with a similar LM386-based modulation scheme for 500mW 160m AM transmitters.

  3. Low power FM transmitter. Similar to the AM transmitter idea, here you are providing frequency modulation by using the LM386 to impose a varying bias voltage on a diode used as a varactor. Not necessarily a purpose made varactor; people have used power diodes and even LEDs for this purpose. It may be difficult obtaining sufficient modulation. Results may be better with a ceramic resonator or wide range (eg two crystal) VXO.

4. On another band. Another fairly easy mod which I don't think I mentioned in the video is to put the Pixie on another band. 80 and 160 metres are probably the easiest. Basically you just change the crystal and the low pass output filter values. If the crystal oscillator doesn't oscillate you will need to change the capacitor values there until it does. This is most likely if converting to 160 metres - in this case you'd change the two 100pFs to 470pF. This video is an advance on the one above for the 40m AM transmitter conversion by describing an AM transmitter for 160 metres.

  5. Crystal tester. This arrangement would leave Q1 as a crystal oscillator but with the crystal connections brought out to a socket or clips to allow various crystals to be connected. You would take its output to a diode which would rectify the RF output, converting it to DC. This could switch on a DC switch and allow an LED to be lit if the crystal is oscillating. It may be possible to use either Q2 or the LM386 for this function.

6. High gain utility audio amplifier. The LM386 by itself often isn't enough for simple receivers, especially direct conversion types. It should be possible to not add the Q2 part of the transceiver circuit and instead use Q1 as an audio preamp, feeding the LM386 via the trimpot reconfigured as a volume control. This could be used as an audio signal tracer or audio stage in a simple direct conversion receiver. Add a diode balanced mixer, VFO and (preferably) front end RF filter and you'll be hearing HF signals from around the world.

7. Audio filter and amplifier. Similar to the above but you'd be adding a few more parts to the transistor stage to provide filtering. Either low pass filtering (3 kHz cut off for SSB, 1 kHz for CW) or band pass filtering (for CW) can enhance a simple direct conversion receiver (or even another Pixie!).

8. Crude optical communication. More to demonstrate the concept in a classroom than to be very practical (due to its low power) it may be possible to use the audio amplifier arrangement in (5) above to drive an LED. Another audio amplifier (maybe even another modified Pixie) and light sensor/receiver would be needed to receive the signal.

9. Audio tone generator. There's several ways to hack the Pixie to do this. Possibly the best is to use Q1 as an audio oscillator (look up 'Twin T oscillator' on the web) and feed its output into the LM386, possibly using the trimpot as a volume control. That should comfortably drive a speaker. Applications include some sort of alarm, code practice oscillator or audio signal injector for testing audio circuits. Another cruder approach is just to use the LM386 and experiment with resistors, capacitors (and even moistened fingers) between the input and output to provide feedback and thus enable oscillation.

10. Electronic music maker or light alarm. A variant of the above is to vary the resistance in the oscillator to change the pitch of the tone generated. This could allow multiple notes if it's connected to some sort of keyboard or stylus/resistor arrangements. Another possibility is to use a light dependent resistor to provide an alarm when a light is switched on.

The above are just a few ideas for projects based on a Pixie kit. With any luck you'll be able to come up with more. Please leave ideas you have in the comments after the above video. Even better make your own video of what you've done and link from your comment.

I suggest buying several Pixie kits and thinking about what you wish to build before starting soldering. It's wise to use a solderless breadboard or tack solder significant additions. Part of the challenge is being able to fit it all onto the Pixie's board. You may need to get creative. Eg consider soldering parts to the underside of the board. Or orient components differently and make additional wire jumper connections under the board (heatshrink tubing can help prevent shorting). For more adventurous modifications it may help to use a hobby knife to cut tracks in order to break no longer needed connections. The main thing is to spend time thinking and prototyping before building on the Pixie board to lessen desoldering and the risk of errors.


Pixie transceiver kits and related parts


Other components

To make the projects suggested you'll probably need a few extra parts not supplied in the Pixie kit.
A few bags of assorted resistors, capacitors and transistors, such as those shown below, should help.

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Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.


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