2.2v LED off 12v battery, what do I need?

[Teflon-Mike]

Just bought an iluminated switch. Nievely I presumed it had a 12v bulb in it, like they always used to but getting it home, discovered it has a 2.2v 0.025a LED in it instead.
Need this to light up when I turn the switch 'on'.
So need to take feed off the switch out-put to the LED and thence to earth.
However, resistor networks are not my forte.
I THINK, from =IR, that it must equate to a resistance of 88 Ohms.
Now, I guess I need a resistor in series with it to drop the voltage.
But getting stuck on the maths. How big a resistor do I need?
And do I put it BEFORE the LED or after, or doesn't it matter?
Or is thre a better way of doing it, with a bridge circuit or something?
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[stinkwheel]

Simple ohms law question. V=IR

Your LED draws 0.025A. Needs 2.2V and is supplied with 12V

12V-2.2V = 9.8V

V = IR
9.8 = 0.025R

R= 392Ω

So a 400R resistor should do the trick. On either leg will be fine but traditionally on the +ve leg.

EDIT: That's for a stable 12V circuit. On a bike it'll be closer to 14V most of the time so 11.8 = 0.025R
R=472Ω
So a 480R resistor.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Codemonkey]

Adding to the post above, it will need to be rated at 0.3W or above since P=(I*I)*R

[Fortuna]

[Teflon-Mike]

OK, to get this straight, becouse I dissapeared up my own back-side in a frustrated rage if trying to remember where R=sum of 1/R's and whether I needed roughly 5x the resistance of the LED or 1/5th!

Bike +ve Supply-----SWITCH---------RESISTOR-------Switch LED--------- Bike Ground

That's the circuit... or at least the bit we're concerned with.

Switch LED = 2.2v, 0.025a (I am correct that's an 88Ohm resistance then?)

And at a notional 12v-ish I need a 470 Ohm resistor infront of it, to drop the volts to what the LED will handle.

Maplins: 330Ω to 910Ω Metal Film 0.6W Resistors..(for 24p!).. I take it I'm looking in the right kind of place?

Given comments over fluctuation in supply voltage, Lets say between 10v and 15v....
470/88 = 5.34:1 so (am I right in the maths here?) for 10-15v supply, 1.8 to 2.8v accross my switch LED?

And at nominal line volts of between 11.5 & 13.5, 2.15 to 2.5v? Does this sound about right?

I Put a single 1.2v Rechargeable AA battery accross the LED and it didn't light, two, it did. So, switch LED should 'work' still on lower end battery volts, and not 'blow' on higher end?

OK. ANOTHER bludy trip over to Coventry for a chuffing penny part should have this 'sorted'! Its for the indicators on Snowie's Pup BTW.

It now has LED's all round, and an electronic flasher to govern the flash-rate. And a fuse box with protection JUST for the Indicator flasher in it! And the LED Dash-tell-tale, curtecy of two 22p diodes, now works, and we dont have the wrong-side indicators working through it! But she decided she liked the idea of Hazard Warning Lamps..... so two more 94p diodes later, and we by-pass the indicator switch and get all four indicators blinking, and dont have the wrong-side indiators working through the hazard circuit.... but only works when the key is in the ignition.... two MORE 94p diodes, and the hazards are working off the parking light circuit, and the parking lights aren't flashing with the indicators! Just dont have the 'independent' tell-tale for the Hazards demanded by C&U, hence light up switch!... and now a resistor! And I'm fed up of hearing "It'll be a great bike, when its done" yeah... you come get solder balls burning through your jeans into your nuts, then!

thanks all.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[WD Forte]

As said, the running voltage if bikes/cars is nearer 14V rather than the nominal 12V

I usually use 1K resistors on std leds and under run them a bit
for longevity
A 1k will run it at 14 millamps which is usualy good enough
If you run them at or near their max stated current they may get hot and fail sooner
I doubt you need to have stuff lit up like a christmas tree especially
at night when it may be distracting or annoying.
____________________
bikers smell of wee

[Ichy]

Mike you lost me by the fourth sentence, does it work?

I guess the trouble with converting flashers into hazards is that the current direction is controlled by the indicator switch, sending the current either left or right.

Might have been easier to fit a switch that bypassed the handlebar switch flashing both sides at the same time.

Actually you could have used a cheap ebay alarm, I remember the last one i fitted had a silent alarm option, basically flashed all the indicators.

[Teflon-Mike]

[Codemonkey]

[Fortuna]

And ½ Watt

[stinkwheel]

I'll just add.

For simplicity and to avoid going totally mental. When fitting LEDs to your 12v bikes system. It is far easier to work out which resistor your particular LED choice requires and solder one to each LED. You can then consider each LED to be effectively "plug and play" into the bikes electrical system.

It's not elegant and could be considered wasteful of componants but given the absurd cheapness of resistors, it makes life a whole lot easier. You just spend a half-hour session soldering resistors onto your bag of LEDs and they're ready to rock.

I get my electronic componants from CPC. A pack of 100x 0.25W 470R carbon film resistors is 54p (so £0.0054 each).

The trouble is you need to spend £40 to get free postage. That said they stock a truly awesome range of stuff, power tools, electronics, computer componants and so-on so I usually tack any consumables onto an order for something else.

cpc.farnell.com
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Teflon-Mike]