DCC Block Detection

Puckdropper wrote:


Bet you are looking at the wrong gap. Remember, what is being detected is the occurrence of an electrical load. Dummy locos on the head end don't trigger detection, which set of wheels is drawing power has a bearing [Particularly with steam, where you may have 8 - 12" of loco before the tender wheels start picking up from the other rail.]
You may be seeing the time delay associated with the circuitry responding.
Chuck D.

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Ideally, *all* rolling stock should present a 'load': dummy locos, freight cars, etc. should have metal wheelsets (with one wheel insulated). Solder either a small 1/8 Watt carbon film or 'chip' resistor (either can be had cheaply in quanty from www.mouser.com or www.digi-key.com) from wheel to wheel. The value (resistance) of the resistor depends on the sensitivety level of the detector. The value should be choosen to not present a significant drain on the track power (and the current should be low enough not to cook the resistor!), but be enough to trigger the detector. Each piece of rolling stock should have 2 wheelsets with a resistor, one at each extreme end of the rolling stock.

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When we get something more going with signals, we'll start putting resistors on wheelsets. Two per car makes a lot of sense, especially with how inexpensive they are.
We have a small advantage as most trains run either with a flashing FRED or caboose, so upgrading the cabooses first will give us 60% of the benefit for 5% of the work.
Puckdropper
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Puckdropper wrote:

If the FRED, or caboose markers, are 'track powered', that's all thats necessary for that car. [I.E., they are already done!!] Chuck D.
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It depends what you're doing. if you want block occupancy detection, you're right - if the rear of the train has no load you'll show as a vacant block as soon as the engine gets off.
If you're using block entry as a trigger for some event (an automated turnout throw, for example), you can get away without adding dummy loads. I've never had a lot of luck with resistance wheels - either the value is too high and they don't register, or they work fine, but interfere with other things, like automatic reversing sections. *
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Start by taking a 1K ohm resistor and running the leads down the track. See where the signal changes. My bets are that there is a gap at that point on one rail. That rail will be the one that the detector is attached to. The circuit is the modified Twin-T design using diodes instead of transistors and it detects current flow to the track on one lead. The power leads don't matter as to which is attached to the track, normally with these circuits, and the IC is a quad comparater chip that detects the voltage across the diodes and converts it into a signal when there is aomething drawing current on the track. The circuit design has been around with minor changes in resistor values since the early '70s. The Twin-T design has been around since the '50s. -- Bob May
rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net
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