switch circuit question

[flipnap]

hey guys, so i was looking to wire in an LED into my throttle hold so I could see at a glance if it was on or off.. i hooked up a voltmeter to the leads on the bottom of the switch and in off mode i got a reading, on hold (switch off i guess) i was getting no readings at all.. im kinda stumped and i guess this is really more of a basic electronics 101 question.. but shouldnt current always be flowing through those two wires, pre switch? you know what im getting at?.. in other words, shouldnt my voltmeter sort of be "short circuiting" the switch and acting like a closed switch itself and picking up a reading regardless of the switch state? kinda confused on this one.. thanks

[JEEPWORLD2002]

no the way a switch works = well say its on the + wire of a batt. when the switch is open the voltage wont flow and when its closed it will flow. so bassically the one side of the switch you may have 5v and the other terminal goes to the device, switch open no voltage?? switch closed the voltage flows to the device. so with the v meter one end on ground and look at terminal 1 supply. then close the switch and you should have same reading on both terminal. so to wire a led you would attach a the + end to terminal 2 (the one that get voltage when the switch is closed) amd - led lead will need a step down reistor and attach to ground

[N017RW]

You will not get a voltage reading across a closed switch since there is no difference of potential (voltage).

Adding LEDs to your radio is risky without the proper understanding of what you are doing. But if you must add the diode (and a current limiting resistor) do it on the "load" side of the switch. That is the side with no voltage relative to common ground when the switch is off/open. Other wise it will always be ON.

You'll need to know the supply voltage present and the forward current & voltage specs. for the diode to select the proper resistor value.

[flipnap]

guys, so if i connect the negative lead of my multimeter to a ground on the unit, isnt that in effect what the "other lead" of the switch is doing, acting as a ground and completing a circuit? i thought maybe the MM was measuring the current across the switch. guess i need to reschool myself on whats what..

You will not get a voltage reading across a closed switch since there is no difference of potential (voltage).so basically when i was getting a reading i was seeing a "difference" in potential and the switch was actually open? and when the switch completes the circuit theres no potential because the current is flowing so my meter will read zero?

[N017RW]

I'll start with your second/last question: Generally speaking, Yes, kind-a. You need a/some resistance to "drop" a voltage across a component when current is flowing. It is this drop you are reading as voltage. When the switch is off/open you have infinite resistance and zero resistance (ideally) when closed.

As far as the first statement/question: Your DMM doesn't complete a circuit when used in voltage measurement mode, it is actually in Parallel.
When in current measurement mode the meter is connected in Series with the circuit being measured and does complete and become part of the circuit.

[flipnap]

ahh gotcha.. thanks for getting back to me.. trying to do some mods to my dx6i and couldnt figure that one out.. im about to put an EL panel behind my readout.. that should be fun!! ha ha (but it is much needed)

[kazager]

I wonder if there is a lighted switch out there to just replace the original, so you wouldn't have to put any holes in your TX. Interesting idea you have.

[N017RW]

Hey Flipnap,

Look around here, others are BL-ing with LEDs.

Isn't there a possibility of EMI with EL panels?