Basic accessory controling

[Anonymous]

Hello. I am interested in any system that controls your accessories ONLY. I already have a good transformer and Id rather have another source powering the accessories. Now explain to me what this CMRI thing is, and a few other things. I just want to know what all can these digital systems do.

Ryan

[rrinker]

The C/MRI is a complete system to interface your computer to the railroad. It was developed my Bruce Chubb and originally presented in 16 or so parts in Model Railroader in '85 and '86. Since then there have been many improvements, and a new 4 part series ran in Model Railroader this past Spring. It is TOTALLY isolated from the control system be it DC or DCC - even the detectors are electrically isolated, at least the DCC ones are. The beauty of it all is that is it VERY well documented so you can do with it what you will. Bruce Chubb sells two HUGE books with tons of information. One is the User's Manual which describes the components, how they work, how to build them, etc. It also covers most types of interfacing to signals and so forth. The other book has even MORE information on how to hook up various elements.
You can find out more on the Yahoo CMRI Group, and at the JLC Enterprises web site, http://www.jlcenterprises.net.

--Randy

[cwclark]

the CMRI is new and there is a chatroom at yahoo where the folks can explain it better...the one's i've dealt with are "train detection circuits"...by the way a train detection circuit is going to need more than one power source to operate it properly...there are basically four types of train detection...one was a mechanical system that used magnets glued to the engine to connect the circuit through reed switches placed under the track...they didn't work too well and it's old technology....then came linn wescott's "twin T"..which basically used a relay, set of diodes, transistors, and a resistance across the rails when the train entered the block to trip the signal relay...it's old but very reliable...then there's the bridge rectifier / optisolator NAND gate type train detection circuit..it works by an equal voltage in an unoccupied block which 'Drops out" when a train enters the block to trigger the accessories...it's also a reliable train detection circuit, and finally there are the photocell and infra red sensors which either work from the presence or absence of light or a beam is broken by a passing train in the infrared type....they all work well ...the CMRI is new and what i've heard is an extremely expensive train detection device....there is a step by step instruction on how to build a train detection circuit at www.gatewaynmra.org go to the project library and look for the train detection circuit...there are three articles that show you how to build one and what you will need to build one...chuck

[rrinker]

The CMRI isn't new, it goes back 20 years now (and Bruce was using it on his OWN layout before writing up the series). I took al ook at the Gateway page, the actual Chubb detectors are LESS expensive than those. And there are alternatives. ROb paisley has a nice detector that works with DCC and can cost as little as $4.50 per detected block, WITH all the parts, that's not just a blank circuit board. Also Rich Weyand at Trac-Tronics has some nice CMRI compatible stuff.
The only detectors that need all sorts of power supplies are the ones using 393 comparators for DC use. And old Twin-T style detectors, which need a bias voltage. Bruce's DC detectors need a +12 and -12v supply because of the comparator chip. After the experience with my senior lab project in college so many years ago, I have a 'thing' against 393 comparator chips. I really hate the little buggers [:D]. Rob's circuit uses a 556 dual timer, which only needs a SINGLE power supply and had the added benefit of a far higher output drive capacity than the 393. Circuits that detect through diodes or some other source are needed for DC, for DCC they are a waste. Both Rob's DCC detector and Bruce's use a current transformer, completely isolating the detector fromt he track power. No electrical connection, AND no voltage drop as through a diode detector.
Another source for detectors and some signal operating hardware is *** Bronson at RR-CirKits. ***'s detectors also work well with CMRI, his signal control circuits are not technically CMRI compatible. But it's an alternative.
Considering what you get, I don't think the CMRI is very expensice at all. It seems like it because the SMINI card is relatively expensive (but still under $100), but you are getting 24 input pins and 48 output pins with that. You can buy dedicated signal control cards from others for $10-$15 or so, but add up all those $10-$15 cards - if you are controlling 10 blocks you're over the SMINI cost! ANd the system STILL won't be as flexible, because the CMRI is software driven, you can change the signalling parameters to fit whatever prototype you want, or fit your own operating requirements.
Finally, for powering all this stuff, CMRI or otherwise, cheap and almost free power supplies are available - grab a cast-off old computer and use that. 5V and 12V at large currents AND regulated. Sure beats the cost of a 20 amp 18v transformer and a pile of 3-5A voltage regulators and the required heatsinks and mounting hardware.

--Randy