CTT Reed Switch Circuit?

[BigJim]

I have some questions about the reed switch circuit used for activating crossing lights that appeared in the latest CTT magazine. The whole thing seems a bit flawed to me. Maybe some of the electro-wizards here can comment.

1) Here is what seems to me to be the main flaw in the circuit as it is drawn. Please correct me if I am wrong.
If a magnet is placed on the lead loco to activate the gates, won’t it then turn the gates off too soon?

2) Is the circuit as drawn Bi-directional? In other words, will it work if a train approaches in either direction?

My thoughts, again, tell me if I am wrong.
Use four reed switches, in two parallel wired circuits, forming an X shape (top left and bottom right are used to activate the circuit, bottom left and top right are used to turn circuit off) as follows;
East bound turn on reed is placed before crossing between middle rail and outside rail on engineers side (south side), East bound turn off reed is placed after crossing between middle rail and outside rail on fireman’s side (north rail).
West bound turn on reed is placed before crossing between middle rail and outside rail on engineers side (north side), West bound turn off reed is placed after crossing between middle outside rail on fireman’s side (south side).
Place magnet on lead truck of loco offset to the right side to activate gates. Place magnet on rear truck of caboose offset to the left side to turn gates off.

Reeds could be placed in a way that the activation switch is placed further away from the crossing than the turn off switch, giving gates time to come down before train enters the crossing and going up just after the caboose clears the crossing.

Another question arises here, will the magnet be too strong as to try and close the reed switch on the opposite of where the magnet is placed?

[3railguy]

The reed switch setup has some advantage in that it can sense trains in one direction by offseting the magnets. But, your cars will only be unidirectional. It seems like a lot of work and expense to avoid control rails. Especially an AC crossing gate where a control rail and a length of wire is all that's required or a DC Xing gate where one relay is required verses two that the article shows. One relay shuts the other off or something like that. The author states the reed switches have an advantage in they allow you to use DC relays. You can use 12 volt DC relays on AC control rails by rectifying the AC at the coil with a bridge rectifier (less than $1.00) You can also buy an AC 12 volt power supply for a bank of relays for $8.00 (phased into track power supply). One poster posted concerns about magentraction tripping the reed switches in another thread. What if you have 100 cars and 20 engines to attach magnets to? G scale uses this setup but G scale is two rail and doesn't have the luxury of a third control rail.
John Long

[spankybird]

Jim,

Did you see this thread on the same subject [?]

http://www.trains.com/community/forum/topic.asp?TOPIC_ID=22355

[Big_Boy_4005]

QUOTE: Originally posted by spankybird

Jim,

Did you see this thread on the same subject [?]

http://www.trains.com/community/forum/topic.asp?TOPIC_ID=22355

Thanks Tom. Actually that topic didn't start out that way. Tim liked the article, but I found fault with it, just like Jim here. I don't think Tim expected to have his topic commaneered like that, so the title doesn't reflect the content.[:0][:p][;)]

[lionelsoni]

You can control DC accessories and relays, using a DC supply, directly from an isolated running rail, even though the trains may run on AC.

[FJ and G]

Bob,

Can you do this in command control mode? Also, wouldn't the DC current turn on the horn/whistle or bell?

[lionelsoni]

When the train is not on the control rail, the only connection to the train's supply is a common return or ground. So no current can flow between the DC circuit and the AC circuit powering the train. When the train is on the control rail, the DC accessory current just flows from one running rail, through the axles, to the other. Both rails are then at ground potential, so there is still effectively only one connection. Thus the train sees the same voltage that it normally would between the running rails and the center rail and is not affected.

The only way I can see that you could get into trouble is if you were using the same control rail for two purposes, each of which had a separate supply. For example, using the anti-derailing rail of an AC-powered switch also to control a DC-powered crossing gate, or vice-versa. Even in that case, only the accessories would misbehave and the train's power would not be involved.

[BigJim]

"Did you see this thread on the same subject "

No Spanky, I hadn't seen that post, for the simple fact that the title is "THANKS CTT", but , thanks for the link.

I will be the first to admit that I am "electrically challenged", that's why I brought the question up. After thinking about my original post, I don't think it would work right bi-directionally either.

[ChiefEagles]

I agree with 3railguy. Seems a lot of trouble when there is a simple way to make accessories activate as a train crosses and insulated track section. I just laughed and went on to another article.