The F5K with some paddles is not 100% reliable in its CW characteristics.  It will intermittantly miss an element.  The problem seems to be 2 fold.  First there is very low current that passes through the paddle when a contact is closed on the paddle, and second the F5K has a pretty good frequency response on its keying input and tries to decode the paddle bounce.  Not all bounce turns into CW, but I do think the bounce gives the program ambiguous information and will occasionally drop or add an element.

I tried several approaches to solve this problem and this is my final solution.  I use a NE556 as a switch de-bouncer (a.k.a low pass filter), one half for the dit or dah element, and then use the output of the NE556 to drive the F5K Key input with something that looks closer to a digital signal than I could get from my paddle.

At the small current level passing through the paddle, oxidization on the contacts cause the paddle contacts to behave as a semiconductor instead of a conductor.  The circuit I designed runs about 14 ma through the paddle blasting through any oxidation. Thus far the circuit has proven to be 100% reliable in my testing.  All the parts can be purchased from Radio Shack.

Parts List

 NE556
14 pin Dip socket
2x 100K resistor
2x 1K resistor
2x 150 ohm resistor
2x 0.01 cap
2x 0.1 cap
2x 2N2222 NPN transistor
Phone jack to fit your paddle
Phone plug to fit F5K
Perf board

The parts are not super critical.  A 100 ohm is as good as 150.. They are mostly what I had in the junk box.  The 100K resistor/0.1 cap sets the de-bounce period if you increase the value of the resistor or the cap, the de-bounce is longer.  I would use a fairly high quality cap here for reliable timing.  The values given are good to the 60WPM limit of the built in keyer.  The 1K resistor sets the amount of current through the paddle lowering this value will draw more current.  For example if you were running the circuit on 5V you might want to cut that value in half to increase the current.  At 14V 1K will give you 14ma.  I housed mine in an old empty pill bottle, and put 2 jacks on the bottle cap so I can have 2 paddles plugged in at once.     The Schematic is here

73 W9OY