10.20.2013

which (switch)witch is which?

Didn't think I'd taken a photo of this - on film no less!

One quick scan and a digital cleanup later...

About 15 years ago, I had made up a demo control display for the Erin Mills Model Railroad Association using N scale track and turnouts.

The top "Inglenook" was controlled by a single capacitor-discharge circuit (which I thought came from a plan at Rob Paisley's excellent circuitry site, but didn't) that routed power via a diode matrix to a set of Atlas #200 "Snap" relays which, in turn, routed power to a set of Tortoise machines under the plywood. This was slow motion control of three turnouts with sinlge-button route selection. The snap relays had pretty low current draw and virtually no mechanical binding, so a single cap-D was OK. A bonus was the ability to manually slide any snap relay lever and control each turnout separately if required.

The bottom ladder used Peco PL-10 twin-coil solenoids supplied with current from one-per-turnout TTL input capacitor-discharge circuits made from plans at the TracTronics site (see SwitchWitch), also via a diode matrix. Instant and reliable snap-action regardless of how many turnouts move simultaneously, without routing heavy current through the tiny Radio Shack 0.5 Amp momentary N.O. pushbuttons (which would then arc internally and weld themselves 'closed'). Brilliant, low current control circuits for use with something like a Launchpad micro controller? Hint hint...

I added a momentary on-off-on toggle switch to the leftmost TTL input cap-D (on the lower ladder) to show individual (local) control of turnouts in this option as well. The SwitchWitch circuits can be built active-high or active-low to suit your driver output configuration. The toggle merely grounded either input.

The demo board can be seen here sitting on top of my 2.5'x5' N scale portable layout which was made for DCC and two-cab control clinic at the Erin Mills club. For some reason, I have long since disassembled both of these units and scattered the components to the four winds. Probably seemed like a good idea at the time.

A lot of modellers at that point (and probably still now) had trouble "getting" some of the electrical concepts I was talking & handwaving about. In the end, it was easier to build up some working demo units that could be mulled over in real time. Not sure how many people acted on any of the stuff I presented, though.

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