How to add in an LED or two?

[Lee Alkureishi]

Hi all,

I have a control box wired into my VFD, which provides a reverse-off-forward (SPDT) switch and a 10k potentiometer for speed control. There is also a SPST switch wired in series between the VFD and the SPDT switch, which gives emergency off control.

I'd like to add in an indicator LED to both the main control box and the emergency on-off switch box. Today, I picked up a couple of green LEDs (2.1V, 20mAmp) and a couple of resistors (0.5W, 470 ohm). My understanding is that the 470ohm resistor will allow me to run the 2.1V LED on the VFD's control circuit (which I think is 10V - at least, that's what's marked on one of the terminals that the SPDT switch is connected to).

My question is how best to wire in the LEDs to each of these parts of the circuit? I'm thinking it'll need to be in series in each case.

Right now, my circuit is:

VFD --> SPST --> SPDT --> VFD

Can I wire it all in series? Like this: VFD --> SPST --> 470ohm resistor for LED --> LED for SPST --> SPDT --> 470ohm resistor for LED --> LED for SPDT --> VFD

If this looks ok, which of the terminals on each switch would the LED be wired into?

I'd like the LEDs to indicate when each of the switches is in the "on" position. For the reverse-off-forward SPDT switch, I'd like the same LED to light up for both forward and reverse.

I'm a little concerned that the resistors in series will affect the functioning of the switches - but, I have very little electronics savvy, so I'm hoping to tap the collective on this one!

Thanks in advance,

Lee

[Joe Shinall]

Ummm....no clue.

[Steve Milito]

First you need to know the voltage; I'd measure it.
It been a very long time since I've done electronics, but an LED will have a 0.6V drop across it when forward biased, which is when it shines light. So you need to make a circuit diagram that shows the voltage source, the resister, and the LED and use it to calculate the V drops, current, and power to make sure everything is OK. You should not put the LED in series, but rather shunt it to ground.
Your 470 ohm resister will drive the LED at it's maximum current if the control voltage is 10v. Personally, I'd use a bigger resister. Your LED just won't be as bright.

[Steve Milito]

Correction: Your particular LED will have a 2.1 v drop across it when forward biased.

[David L Morse]

You should get double pole switches and keep the LED circuitry separate from the control stuff.

[Lee Alkureishi]

Thanks for all the advice - double pole switches it is :)