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Relay? help?

EvergreenCM

Jan 18, 2013
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Hello,

I need to build a flashing railroad crossing signal. I would like to use 12v trailer lights and have them flash back and forth - just like a real railroad crossing signal.

I would like to just be able to plug it into a standard 110v outlet. I have wiring experience, just never worked with relays etc. before.

Any ideas on how to convert etc.?

Thanks!
 

davenn

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Sep 5, 2009
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Hi there
welcome to Electronics Point forums :)

I am assuming ( because of the lack of info) that this is for a model railway ?
Trailer light globes would be pretty huge in physical size for such a job
LED's would be much better and are readily available at well less than 50c each
A simple oscillator circuit could drive the LED's to have them flashing alternately

a 110VAC to 12VDC plugpack to power the driver circuit and LED's

Dave
 

EvergreenCM

Jan 18, 2013
2
Joined
Jan 18, 2013
Messages
2
Hi there
welcome to Electronics Point forums :)

I am assuming ( because of the lack of info) that this is for a model railway ?
Trailer light globes would be pretty huge in physical size for such a job
LED's would be much better and are readily available at well less than 50c each
A simple oscillator circuit could drive the LED's to have them flashing alternately

a 110VAC to 12VDC plugpack to power the driver circuit and LED's

Dave

Hi Dave,

Thanks so for your reply! t's actually for theater use. It will be about 7 feet tall.
Any ideas for that? Thanks!
 

davenn

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Sep 5, 2009
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Hi Dave,

Thanks so for your reply! t's actually for theater use. It will be about 7 feet tall.
Any ideas for that? Thanks!

ahhh OK :)

stick with your trailer type globes

The power supply and oscillator can be the same
make sure the puer supply has sufficient current rating to power the globes you decide on
The oscillator can be a 555 timer that drives power transistors for the higher current requirements of the globes

do a google search on 555 timer as a monostable multivibrator

Dave
 

KrisBlueNZ

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Nov 28, 2011
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I've taken the liberty of designing a circuit for you!

I guess it must be a slow night ;-)

attachment.php


I've shown two versions. The first one uses high-current MOSFETs to switch the lamps, and the second one uses a grunty relay. The second one is simpler and probably more reliable, but it will make an audible clicking sound, will eventually wear out (after a few years, probably) and is slightly more expensive.

You'll need to decide on the light bulbs you want to use, and find out how much current they draw. Then rate the power supply for at least twice that much, because incandescent bulbs draw a lot more than their rated current for a short time as they warm up each time you turn them on.

Any other questions, just ask :)
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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The problem with flashing incandescent bulbs is that it subjects their filaments to a lot of thermal stress. This leads to them failing very quickly (in relative terms)

The cure is to always have some current flowing through them. Not enough to light them more than perhaps a dull red glow, but enough to keep them hot.

This has the other beneficial effect of reducing the spike of current as they are turned on each time.

The resistor will be different for each type of bulb, and the easy way of determining its value required a variable power supply.
 

KrisBlueNZ

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Interesting and helpful advice Steve.
 
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