Pixie III

Up for a challenge? How about building a real minimalist station from scratch and then seeing how many stations you can talk to? I'm going to describe one of the simplest transceivers ever built. Despite having only 29 parts, including the headphones and key, it is capable of making contacts from Canada to Mexico using just a 9 volt transistor radio battery for power. It can easily be built for under $5 if you have any kind of a junk box. $10 if you don't. Takes a couple of hours to build, provides years of fun. There are no dials and knobs on this set. No lights or digital displays. Just pure radio. This is going to let you demonstrate your real operating skills.

Basically the rig consists of a 2 transistor transmitter and an audio amplifier. By using the output amplifier transistor as a mixer we have a direct conversion receiver. GM3OXX came up with this concept back in 1983 and used it successfully with his FOXX transceiver. RV3GM simplified the circuit even more in 1992 in his Micro-80 design. In 1995 WA6BOY trimmed it even further in his Pixie and Pixie II designs. Each design has been simpler and improved.

We begin with Q1 , a straightforward Colpitts crystal oscillator. It runs continuously whenever the rig is powered. In receive mode it provides the local oscillator drive. The key is a sub-miniature microswitch salvaged from an old computer mouse. Since the key has a double-throw configuration it can be used to shift the oscillator frequency for the proper transmit/receive offset. In the key “up” position C2 is shorted out of the circuit and the crystal runs at its natural frequency. The oscillator drives the PA transistor but, without a hard ground on the emitter, this drive just tickles the signal coming in on the collector enough to provide a frequency-mixed sum and difference signal to the emitter. The RF signal is shunted to ground through C6 and the AF is then fed to the audio amplifier through blocking capacitor C8.

When we press the key the short is removed from C2 and the oscillator shifts frequency up about 700 Hz. The emitter of the output transistor is pulled to ground, shorting out the audio and allowing Q2 to amplify the transmit signal. C7 keeps DC from the antenna system and passes the RF to the output filter, a Cauer type low-pass. C9, L3, and C11 comprise a standard pi filter and the added capacitor, C10, works with L3 to provide a series trap at the second harmonic frequency. I needed to add C10 to comply with the stricter FCC requirements that we have now.

The audio amplifier uses just one section of a two-section operational amplifier. Other designs used an LM386 audio amplifier which is noisier, lower gain, and costs more. This LM1458 has ten times the gain, draws less current, and withstands three times the input voltage. Additionally, we have an un-used section available for other uses. Sidetone generator? Audio filter? The value of R6 sets the gain. Too high (like 4.7 Megohm) tends to make it unstable and you might have it howling.

Diode D1 can be most any kind of power diode. The only purpose that it serves is to prevent an accidentally reversed battery from destroying your work. All of the resistors can be 1⁄4 watt or less, very non-critical. Likewise the resistance values can be changed without much effect. If you have it, try it. Capacitor values are not critical, either. C2 will shift the offset frequency differently and the values of the RF output filter should be close and they should be of ceramic or mica construction. C12 would work better if it were larger (like 100 to 500 uF) if a physically small one were available. C12 needs to have a high enough working voltage rating to accommodate your proposed power source voltage. C15 is just to keep DC from the headphones while allowing audio to pass, so it is not critical.

The transistors that I chose to use were the cheap plastic version of the ubiquitous 2N2222. You can often find them for a couple cents each. They are good for better than a half watt output in this rig but will give a solid 100mw performance with a 9 volt supply. You can use 2N4401s or 2N3904s if you have them but a 2N2222 is hard to beat.

The headphones that I use are the Dollar Store variety; the ones with foam pads and a headband. They work better than ear buds because you can adjust the volume by sliding them off your ears. Can't do that with ear buds. There is no AGC on this rig so protect your ears. I cut the plug off (for use elsewhere) and directly solder the leads to the board. Wire the headphones in series.

If you don't have a crystal for the frequency that you want you can get them from the following sources rather cheaply:

Expanded Spectrum Systems (Although they have HC49 packages available for cheap, check out their little Cylindrical style. Same price and fits anywhere!

QRPme sells crystals cheap. Check to see if they have the frequency that you want.

I would suggest 7.03 MHz for 40 meters. It is the new “watering hole” for hunting QRP stations. The old 7.04 MHz is still used but suffers from digital QRM and the Russian maritime beacons. Not to mention the DX SSB signals, etc.

The value of RF choke L1 is not critical. Since little current passes through it most any molded or surface mount device will work. Wind your own with an FT37-43 toroid with 15 turns of wire.

L2 can be from 15 to 25 microhenries but needs to be able to handle a quarter amp of current. You canwind one on an FT37-43 core with 7 turns of #24 AWG wire.

L3 can be a molded choke but will have less loss if you wind it with 15 turns of # 26 AWG or smaller wire on a T30-6 powdered iron toroid. That would be 17 turns of that wire on a T37-6 core or 14 turns of #26 AWG or smaller wire on either a T30-2 or T37-2 toroid.

If you find yourself building many RF circuits like this you will find it more convenient to get yourself a LC bridge to measure things exactly. You can often find them at swap meets or you can break down and buy a new one.

Build it however you please. I've constructed most of mine on a piece of bare printed circuit board with small pieces of board glued on as islands. This provides a good ground plane. The unit in the photo is built on a 1” X 1 1⁄2 “ piece. Place the “key” on an outer edge so that you can tap the button with a finger tip. That white thing on the right side is the key. It's a good idea to place the crystal out where you can access the solder connections in order to ease replacement for frequency change. The IC can be lain upon its back “dead bug” style with its legs up as soldering posts. Pins 3 and 4 are bent back and soldered to the ground plane thus providing ground connection and holding it to the board.Performance? Let's face it, Elecraft it ain't! I can hear anything better than a 1,000 uV and with a 9 volt battery it puts out 200 milliwatts while pulling 90 milliamps. On receive it only draws 6 milliamps.

Although the transmitted signal is clean and no chirp, the sidetone (if you would call it that) is somewhat reminiscent of a squalling baby. On the other hand, it always transmits and receives right there on frequency. If someone wants to call you then they know where to look. No worry about bumping the dial because there isn't one. Not to worry about the deaf receive if the other party is running power. Kind of evens things out so that if he can hear you then you can hear him. I have worked nearly 100 stations with it, all up and down the Western U.S.. It is just about as minimalist as you can get. Now THERE is a challenge.

de ND6T

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