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How to Create an LED indicator light for uncoupler

JK Joe KOGR Forum Member

I want to add an LED indicator light to my fastrack uncoupler. This light will be "on" only when the uncoupler magnet is engergized.

 

I need help choosing the LED and resistor (and anything else I may need) for this circuit. I will run the uncoupler off of 18V AC track power, and maybe switching to 16V AC auxillary power from a Lionel 1033 in the future.

 

I would need to connect one side to the wire powering the magnet (which only has 18V when the pushbutton is pressed), and the other side to ground. The fastrack connections on the underside of the track are easy to access, and I have plenty of connectors.

 

Once I have a the parts, I will build a prototype, test, then take pictures to post on the forum. There have been many people having problems with uncouplers sticking in the "on" position, resulting in uncoupler failure. In the case of fastrack, the track melts and you have a $35 mistake. I have lost 4 uncouplers so far.

 

thanks for any help: Joe K

 

Any help, and a wiring diagram would be helpful.

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Boilermaker1Boilermaker1OGR Forum Member

Does the magnet get stuck on because the button sticks? If so, just toss the button and replace it with a momentary on SPST toggle switch, They're spring loaded back to Off so it'll only go if you hit the lever. 
The circuit would need a rectifier to convert AC to DC, and a resistor in series with an LED. For a 3V white LED, you'd need an 820 Ohm resistor. For colored ones, you'd need a bit more than that because the colored ones require less power. There are also a number of chips on the market that will output the constant 20mA needed to keep an LED lit, and then you can skip the step of figuring out the needed resistor.

Here's a decent single LED calculator

http://led.linear1.org/1led.wiz

 

gunrunnerjohngunrunnerjohnOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

Actually, since you're feeding it AC with a single diode, the minimum resistor values are halved.  You'll only get 1/2 the effective power with a single diode, so for 18 volts on the track, size the resistor like you had 9 volts available.

 

Truthfully, for stuff like this, I normally use a 1k resistor anyway, there's no reason to run the LED at it's limits for this kind of application.

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