Wiring diagram for single track warning lights.

[daan]

Hi all,
As promised a wiring map for a single track double direction warning light system.

I used 2 switches to switch the power on and off, the trick is that it doesn't matter if switch 1 is activated first, or switch 2. The switch which starts the warning lights automatically changes the function of the second switch, which then can be used to reset the warninglights. I know it can be made easier with some electronics, but the use of single coil relais makes the wiring understandable and easy to build. The switches will be normally open single pole switches, like reed contacts or pressure sensors.

The way it works is explained in the text field below the drawing..
(I noticed the drawing is a bit small when clicked on. If you're interested, please give an email, I'll sent back the original diagram)

[webenda]

Thanks Daan. Clever circuit. The text is hard to read, but it is not required--the schematic is self explanatory.

[daan]

Sorry for the small drawing.. Shutterfly only hosts jpeg files, so the bitmap in jpeg is a bit small..

[lionelsoni]

I told Daan that I thought I saw a problem in his circuit. Suppose switch a closes. That operates relay B, which latches, starting the crossing signal. Later, switch b closes. That operates the "reset all" relay, through the relay-B contacts, which releases relay B. Once relay B starts to release, it disconnects the "reset all" coil. However, the "reset all" relay will probably stay operated long enough that relay B will unlatch, as desired. But once it has released, it will connect switch b to operate relay A! The only thing that will prevent relay from operating is that switch b close just long enough to get relay B unlatched but no longer. So the train must be moving at the right speed.

Daan tells me that his scheme works for "any normal speeds", which I can believe. He mentioned that he tried adding about 1000 microfarads across the reset relay's coil, to increase the reset time by about 1 second.

As you may imagine, I have a counter-proposal, that uses four switches, one relay, and a resistor. Daan suggested that I post this. He prefers to keep the number of switches small, since he uses pressure switches and doesn't want to have to adjust any more of them, but thought it might interest someone else. Here is the relay circuit:

supplyvoltage---resistor---WT1---relaycoil---WT2---NOcontact---ground

There are two switches on each side of the crossing. Each of the two outer switches connects wiring tiepoint WT1 to ground, unlatching the relay if it is latched. Each of the inner switches connects wiring tiepoint WT2 to ground, latching the relay if it is not latched.

A second normally-open contact pair on the relay operates the signal in any convenient way.

When a train approaches the crossing, it first closes an outer switch, which grounds the resistor momentarily but otherwise does nothing. When it closes the inner switch, however, it operates the relay, which latches through its normally-open contact. The relay stays latched until the train reaches the other pair of contacts. When the train closes that inner switch, nothing happens, because the relay is already latched. When it closes the outer switch, however, it removes the supply voltage from the relay coil by
grounding the resistor, releasing the relay.

The resistor, relay coil, and supply voltage must be coordinated so that the relay has sufficient voltage to operate with the resistor in series. The resistor power rating should tolerate continuous grounding, in case the train stops right an an outer switch.

If the supply voltage is DC and suitable for the crossing signal, one relay contact can be eliminated:

+voltage---resistor---WT1---relaycoil---diode>---WT2---NOcontact---ground
+voltage---signal---WT2

The diode in series with the relay coil prevents current from the signal from flowing backward through the coil when an outer switch is trying to release the relay.

It is also possible to eliminate the resistor by using a second relay, operated by the outer switches. The relays are K1 and K2:

supply voltage---K1coil---WT3---K2NCcontact---WT2---K1NOcontact---ground
supply voltage---signal---WT3
supply voltage---K2coil---WT1

In this case it becomes possible simply to connect the signal in parallel with the original relay's coil, without any diode and whether using AC or DC.

Although four switches are needed, this scheme has the advantage that they can be located so that the point where the signal starts and stops can be customized for each direction. The switches can be the sort that Daan uses, or magnetic reed switches, or, my favorite, isolated control rails, although I think I would just isolate a chunk of rail extending some distance to either side of the crossing and operate the signals directly from that, without any relays or electronics.