The "Talking" Pixie2 QRP AM Transmitter

The Pixie2 is a simple QRP CW transmitter that dozens of ham radio operators have successfully built. (QRP is ham jargon for low-power operations, and CW is the simplest method of sending Morse code merely by turning a carrier-wave on and off.) The Pixie2 is usually built for the 40 meter band but it will work on frequencies from 1000 kHz up to at least 15 MHz. It is said to output a couple hundred milliwatts of RF.

 

The circuit can be amplitude modulated quite easily. A small audio amplifier feeds audio current into the 8-ohm side of a transformer. The 1k ohm side of the transformer is inserted in the V+ supply going to the Pixie's output transistor.

 

This modified Pixie2 is called the Talking Pixie. It has 18 components (not counting circuit board, jacks, power supply and external audio amp). Building it on a prototyping board only takes a few minutes if all the parts are available.

 

The level of the audio fed to the transformer is adjusted until the best sound quality is achieved. The Talking Pixie will not sound as loud as commercial stations but the user must avoid the temptation to over-modulate; nobody will listen to an over-modulated signal.

parts list

C1: 100 pF
C2: 220 pF
C3: 82 pF
C4: .01
C5: .01
L1: 150 uH
L2: 22 uH
Q1: 2N2222 or 2N3904
Q2: 2N2222A (metal can type) or 2N3866
R1: 47K
R2: 1200
R3: 33K
R4: 10 or 15 ohms (experiment!)
T1: 1000 ohm to 8 ohm audio transformer

 

The frequency is crystal-controlled. A crystal for the frequency you're interested in will have to be ordered if you don't have one handy. The transformer must be rated to handle at least half a watt of audio; a very tiny transformer will not sound good and will have too much resistance on the 1K winding. L3, C6 and C7 form a low-pass filter to attenuate the harmonics generated by the circuit. Specific values for various frequencies can be found on the Medium Wave Alliance's filters page. L1 and L2 are factory-made axial molded chokes.