# Indicator light



## CHscenic (Jan 2, 2008)

I was wondering if any one had an idea how I might add a light to a draw bridge that will come on when the bridge is open and the track connection in broken, then goes out when the bridge is closed and track power has been restored? I suppose having a light that is on as long as the bridge is closed and goes out when open is another option but means the light is on all the time. The drawbridge is part of our club modular display and I have wired it so that when the bridge is open power is killed to the modules on both sides of the bridge. I would just like a light that is visible to confirm either when it is powered or the power is broken. Thanks DC


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## Dan Pierce (Jan 2, 2008)

Install a microswitch that when the bridge opens, the contact is made. This should be the C and NC contacts. When the bridge is closed, engaging the microswitch only the NO contact conects to the c contact. 

c= common to both no and nc 
NC is normally closed 
no is normally open 

This is the static state of an electrical switch. 

This can be done with a reed switch and magnet also. Just be sure the reed sensor is normally closed version and when the bridge is closed with the magnet near this reed, it is an open switch. When the bridge is raised, it goes to the normally closed position, thus being a switch in the on position.


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## SoCalStu (Dec 27, 2007)

DC, 
Another way is to break one wire and solder on two metal tabs at the bridge's pivot point. Bridge down: Tabs seperate, light out. Bridge up: Tabs slide together making the connection.


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## Dan Pierce (Jan 2, 2008)

The reed switch I mentioned above has no moving parts, and the reed can be on the fixed pier, magnet on the moving bridge. 

I use the reed switches encased in weather proof plastic and some are flat cases with 2 mounting holes.


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## stanman (Jan 4, 2008)

I use a magnetic proximity switch - the same kind used in security systems. They're widely available and not expensive; I think I got mine at Home Depot, but not sure.


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## CHscenic (Jan 2, 2008)

Thanks for the good ideas. I have been thinking a bit more... 
What I would like to do is to have the indicator light come on when power to the track is lost and go out when power to the track is restored. I think it could be done with a reed relay but the relay would trigger when there is power to the track and turn the light on. I would like it to do the opposite. Thanks DC


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## krs (Feb 29, 2008)

Are you running DC or DCC or possibly both at different times? 

You could always use a reed relay with an NC (normally closed) contact but I think it makes more sense to just use a transistor that's driven from the track via a diode bridge. 
When there is power on the track, the transistor turns on and it in turn turns off the LED. 
When there is no power to the track the LED is normally on. 
Or you could set it up to use two LED's, one red and one green, or a bipolar one - and switch between red and green depending if the track is powered or not - all sorts of options you can play with.


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## CHscenic (Jan 2, 2008)

We use DC to the track. 
The red and green light does sound like a great idea. Exactly what do I need and how do I put it together? Thanks
Also, do you think the dwarf double target lights on this web page might work. DC
http://www.sbsignal.com/Large_Scale...Scale_Easy


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## krs (Feb 29, 2008)

To meet your original request - LED on when bridge is open, ie track power off, I would use something like this:








With no track power, transistor Q1 is off and LED lights, with track power of more than about 2 1/2 volts, transistor Q1 turns on - that essentiall shorts out the LED which turns off.


To use two LEDs where one is one while the other one is off, you can wire that between the collector of the transistor and a power source via the appropriate resistor of course.


Resistor values depend on the DC power source that is available and the LEDs chosen.

With this arrangement you can't use the dwarf signal because of the common cathode arrangement of the signal, but with a bit mor circuitry you could get that to work as well.


You could for instance use a small relay with a forn C contact, use the circuit above up to Q1 to drive the relay and then control the two common cathode LEDs using that form C contact.


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## CHscenic (Jan 2, 2008)

KRS, that diagram looks pretty cool but guess what? I don't really have much knowledge of electronic circuitry. Could you tell me what each part is? Are they parts I could pick up at the local Radio Shack? Thanks DC


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## krs (Feb 29, 2008)

You need to decide on the "mechanical" part first. 
I always have the most difficulty with that myself, building models is not exactly my forté, electronics are straight forward. 

So - do you want a single light that turns on and off or a light that changes from red to green. 
With the red to green option, you have two more choices, either two LEDS, a red and a green one like the dwarf signal, or one LED that changes colour from red to green. 
It also depends if the indicator chosen has integrated current liniting resistors or not. 

So pick your preferred indicator first and we can then do the circuit to control it. In general, those parts should be available at Radio Shack - these are all run-of-the-mill components.


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## CHscenic (Jan 2, 2008)

I think just a single light that changes from red to green would be great.

If this helps, power to the track is supplied by a Crest Everest PS (15a 24v) coupled with an Aristocraft Train Engineer. DC

Posted By krs on 16 Jun 2010 12:08 PM 
You need to decide on the "mechanical" part first. 
I always have the most difficulty with that myself, building models is not exactly my forté, electronics are straight forward. 

So - do you want a single light that turns on and off or a light that changes from red to green. 
With the red to green option, you have two more choices, either two LEDS, a red and a green one like the dwarf signal, or one LED that changes colour from red to green. 
It also depends if the indicator chosen has integrated current liniting resistors or not. 

So pick your preferred indicator first and we can then do the circuit to control it. In general, those parts should be available at Radio Shack - these are all run-of-the-mill components.


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## krs (Feb 29, 2008)

Hi DC - 

If you need more than just some general suggestions, I would suggest we continue this off line. 

It's just too cumbersome to do this via the MLS forum. 
For one, I only check the forum occasionally, often not even once per day, and posting any sketches involves uploading them to some server and then linking back from the forum - in an email I can attach them directly. 

The forum is great for information that is of interest to many people or for a general request - we'r past that point here. 

- Knut


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## CHscenic (Jan 2, 2008)

Knut, I have sent you a private message here. I look forward to you ideas. DC


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