The under wagon uncoupler now works. Essentially a piece of "reversing" memory wire pulls a cam which in turn deflects the Alex Jackson wire downwards.
The memory wire comes from MERG, and is different to the more common one-way wire available from other UK sources. "Reversing" wire does not require a big spring to return it to normal length, it returns to length when the heat & current is removed. Thus, only the lightest of spring action is needed to keep the wire reasonably straight. In my case that spring is the Alex Jackson wire.
(click picture for enlargement)
A = cam which pivots in bracket glued to wagon floor.
B = actuating arm of cam, fitted with insulating sleeve (shrink wrap). This presses down on the Alex Jackson coupling, and opens the coupling on the right of wagon.
C = Crimp tag around memory wire. Memory wire is just visible leaving crimp heading towards (D)
D = bolt and washer into soldered nut. Memory wire is looped around this and pulled almost taught before bolt is pinched tight.
E = Phos-bronze wiper pickup onto wheel behind brake gear, all four wheels have similar pickups.
F = bus point for "red" side of wagon pickups. Matching bus on opposite side for "black".
The above photo was taken after blowing up the first DCC chip due to a short circuit, and thus has more insulation than the version in the video below. I also plan to change the wiring of the function output; I will move the output (purple) wire to the series resistor (56 ohm, 0.5W, behind bottom right wheel. 0.5W is under-spec, the circuit dumps 2W through it and it gets HOT, but for intermittant use it should be OK). I will try using a half-wave supply from one of the track buses rather than the blue full-wave supply, this might mean changing the series resistor value.
The video below shows the test wagon working on the bench. The chip is still remote from the wagon (hence no track and some crocodial clips showing). Also, sorry for the slightly shakey camera, my tripod was put away when decorating and I haven't dug it out again!
BUT, there is always a bug somewhere. And this case its TCS decoders. I have a FL2, bought because of the "momentary action" described in their instruction manuals (set a CV for momentary action, and another for the duration of up to 4 seconds). That CV does not work !! Further, digging deep through the manuals on the TCS website, I eventually found http://www.tcsdcc.com/pdf/CVguide.pdf which says that CV62 is no longer supported ! So, a feature described in their main decoder manuals isn't there any more. Email exchange with TCS confirmed that the function has been withdrawn on decoders and they haven't updated the online manuals. GRRRRR!!! I needed the time limit to prevent the current limiting resistor from burning out (its running way over current specification, but a larger one has a space problem in fitting below the wagon). Work around for me is probably to map the output onto Function Button 2, which is non-latching on my Digitrax throttles.
I will inspect Zimo and CT decoders to see whether I can get one at a not too high price which has a function output with a time control (both do have such features, but a £30 loco decoder seems a bit excessive for a 4-wheeled wagon with only one uncoupler !!). The alternative is DIY decoders, either the MERG version, or those developed by Paul Harman.