Re: Wiring LEDs to a DPST Switch

Vollrath, Don <dvollrath@...>

My mistake Rich.
For the RED LED, the cathode of the diode connects to the LED and the anode connects to the resistor and switch terminal. It is backwards in my description. But I think you have the resistor on the wrong side of the switch. It must be in the path to restrict current through both LEDs. It sounds like you are also trying to get them powered from DCC track power... Right? If so, there should be yet another diode to protect the LEDs from reverse voltage. I've put a connection drawing (worth a thousand words) in the files section. Look for RRampmeter LEDs.pdf Aug 29. It shows 2 diodes in series with the Red LED, just in case you have a high V green and low V red LED.
The Green LED will be on when it is safe to remove the RRrampmeter. Red when you are using it.

-----Original Message-----
From: WiringForDCC@... [mailto:WiringForDCC@...] On Behalf Of richhotrain
Sent: Saturday, August 29, 2009 9:40 AM
To: WiringForDCC@...
Subject: [WiringForDCC] Re: Wiring LEDs to a DPST Switch

--- In WiringForDCC@..., "richhotrain" <redking56@...> wrote:

--- In WiringForDCC@..., "Vollrath, Don" <dvollrath@> wrote:

Yes. But, does the switch need to do anything else besides control the LEDs?
If no, one way is to use two resistor fed circuits. One turns the Green On via the switch, while the other pole shorts out the Red LED turning it Off. Wire one lead of both resistors to a + voltage supply source and the other ends to the 'wiper' of the separate Double Pole switch sections. Wire the cathodes of the LEDs (point of the arrow symbol) together and to the negative supply lead. The anode of the Green LED lead goes to the Normally Open side of one of the switch poles. [turning the Green LED On when the sw is turned On.] The anode of the Red LED goes to the feed, or resistor side, of the other switch pole. The N.O. side of that sw pole goes to the common cathodes and (-) supply.

Can also be done using only one pole of the switch. Connect the cathodes of the two LEDs together and to the (-) supply. Connect a resistor to the feed of a SPST switch. Connect the anode of the green LED to the N.O. switch pole. [when the sw is closed the Grn LED will be ON.] Connect a diode (maybe 2) in series with the Red LED (arrows pointing in the same direction). (1N4148 or any 1N4xxx) Connect the cathode of the diode (string?) to the resistor/sw terminal. With the sw Off, no power to the Grn LED, but the Red one will be lighted. With the sw On, the Grn LED will be lighted, but the voltage drop across it will be limited to less than it takes to overcome the voltage drops of the Red LED + diode(s)... So Red will be Off.

I tried the second method that you suggested, but I must be misunderstanding your instructions because the LEDs are not lighting.

I soldered the cathode legs of the red and green LEDs together and wired them to the negative side of a transformer. Then, I soldered the anode leg of the red LED to the anode side of a diode and the cathode side of the diode to a 470 ohm resistor. So far, so good?

At that point, does the anode leg of the green LED connect to one of the DPST switch leads on the ON side where the bus wire is connected to the rail, and does the resistored leg of the red LED connect on the OFF side of the DPST switch where the bus wire is connected from the command station?

That's how I connected it but no luck. Obviously, I am doing something wrong.


Over night, I had a chance to review my wiring and to review my prior message on this thread.

Here is the current status of my efforts. I can get the green LED to light when the switch is turned on, but I get no light from the red LED at any time.

I wired the circuit as follows: The cathode side of both LEDs are soldered together and wired to the (-) supply. The anode side of the green LED is wired to the NO switch pole with a resistor wired between the LED and the NO switch pole. The anode side of the red LED is wired to the NC switch pole. Between the Red LED and the resistor on the NC switch pole side, I wired in a 1N4xxx diode with the cathode side facing the resistor.

What am I doing wrong? Incidentally, I double checked the red LED and it is operative (not burned out).



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