I drew this up from an old issue of The Mix years back... I presume it'll be reproducable, it's not that unique a design
Costs something like a fiver to build and all the parts should be readily available. Germanium diodes are better, but they never arrived from place I ordered from on eBay, and they've gone quiet now.
You can add them to anything really. if there's microphone jack then wire that to the carrier, and if not (or also) build an oscillator to put in there as well. I like to make them patchable as well - if there's a ring modulator in there, you might as well make it openly useable.
Text from the magazine:
PARTS LIST
4 X 1n4148 diodes
2 x miniature audio transformer LT44 (Maplin code HX82D)
3 x 2.2 uF non-polarised capacitor
3 x mono jack socket without normalisation (has two tags instead of four)
Small amount of wire
A metal box
Superglue
BUILDING STEPS
The circuit diagram shows the construction in a semi-schematic and physical form. There is no need for the use of vero board.
1. Superglue the two transformers about 6cm apart inside the metal box with the three terminal sides facing each other.
2. Solder the diodes as shown in the diagram, taking care to get the polarity correct.
3. Solder the earth connections together and connect the earth to the metal box.
4. Make a connection between the earth wire and the screen connections of the three jack sockets.
5. Solder a capacitor onto each of the remaining tags on the jack sockets and connect the other terminal of the capacitor to an appropriate input/output wire on the transformers.