On 3/1/02 at 2:52 PM Dave wrote: [QL membranes]
>No, this has to be a broken one, because I need to destroy it to measure >it. The internal measurements are vital. Why? A contact resistance of anything up to 200-300 Ohm will still work OK (but if the keyboard is on long wires will be uneliable due to transmission line effects). The membrane is transparent and the connections are very easy to trace. In fact, I think you can deduce them from the 'KEYROW' function description in the original QL manual, it's basically an 8x8 matrix. The extra lines are for the shift, control and alt keys, they only have an extra diode to connect to the IPC lines. If you are looking for the spec on spacing and membrane bubble size, it's not too relevant. Normally, for membranes it's an interplay between spacing and the rubber 'springs' under the keys. In the case of the QL even huge spacings will work because the rubber is so hard that you have to really push on the key, so once it 'gives' and gets in contact with the membrane, the force is sufficient to make contact on any membrane I have ever seen (and even on some TACT switches!). The bubbles are about 13mm in diameter and the spacing is, IIRC 0.27mm... those neurons are about to be reused so don't hold me accountable if I'm wrong. Several of the bubbles in a single row are connected together by virtue of narrow cutouts on the spacer - 5 or 6, usually. This reduces pressure buildup in the bubble, when it's pressed into. Samsung membranes were separable, had no connections between the 'bubbles', were made of clear material, and had thick silver lines you could even solder to with some attention, and the contact points had small graphite circles printed on them. Very erly sinclair membranes were very similar to the Samsung ones, just not separable and without the graphite pads. The largest majority of Sinclar membranes were glued together (with a glue that chemically interacted with the signal lines...), were made of milky white material that resembles drafting transparency, had very thin silver-on-thallium plating that would easily corrode (soldering is completely out of the question here), and would eventually go brittle with heat and crack. Essentially the same as with the Spectrum. >From a technical standpoint, Spectrum+ membranes are the most interesting - these are two sandwiched membranes, the top one would simulate the required 'shift' key being pressed for the extra keys on the Spectrum+ keyboard :-) Nasta
