Recommended components :

Transistors and diodes. Pretty much any sort will do, I have listed the part numbers that I use in the prototype.

Connections :

Calc. end: ----+
               +-------+---+\/-\
  2.5mm phono  |       |   |    >
  plug         +-------+---+/\-/
           ----+   |     |    |
                   |     |    +--- Tx
                   |     |
                   |     +-------- Rx
                   |
                   +-------------- 0V

A calculator to calculator cable is just the Tx and Rx crossed over. You can buy these from Casio UK (0181 4509131) for a couple of quid. If you make one you might find that the barrel of the plug prevents it from fitting all the way in to the calculator so cut about 1 mm of the length of the barrel off and it will work fine but look a little odd. Same applies for making the calculator end of the interface. You may like to buy a Casio calculator to calculator cable and put a chassis socket on your interface instead. For a suitable socket try :

Name			Part number (page)	Price	Notes
2.5 mm stereo socket	FT94C (642)		42	6mm hole required

Computer end: [This works for my Acorn A3010 and RiscPC, I assume this will work with
a real PC too because Acorn kit has had 'standard' serial ports for quite some time
now. Don't blame me if you fry your serial port (or anything else!) ]

-------------
\ 1 2 3 4 5 /
 \ 6 7 8 9 /
  ---------

9 way D type socket, viewed from back of computer.

+--- 1            (Data carrier detect)
|    2 ---- Rx
|    3 ---- Tx
+--- 4            (Data Terminal ready)
     5 ---- 0V
     6            (Data set ready)
     7 --\        (Request To Send)
     8 --+        (Clear To Send)
     9 --/        (Ring Indicator)

Other notes:

Resistors, probably worth buying just 4 if you don't keep stocks in or do much electronics.
Don't forget to observe the polarity on the electrolytic capacitors on the inverter, esp. on the output line.
You might not consider the 8 pin DIL socket necessary if you have complete confidence in your soldering skill.
I guess any old scraps of veroboard will do or you could even make a PCB if you have (access to) the equipment. If I get deluged with orders then I might do a one off production run of PCBs.
The toggle switch, LED and LED clip are just for show really.
The cables might be a little loose in the grommet, use a bit of superglue.
The case could be cheaper if you don't mind having a battery clip coming out of the box as well, or you could put in a chassis mounted socket (such as JK10L, 2.5mm power chassis socket, 8mm dia. cutout) and (assuming your AC adapter has a 2.5mm power plug (most do)) then you can feed it 6 to 12V DC and the voltage regulator will take care of it.
