Hi kdawg,
What problems have you been having? I found it best to experiment with different types of LEDs. There is a massive range of LEDs, and the important things to look out for when doing meter illumination are:
1. Brightness- measured in mcd, (millicandela, sounds like a good porn star name to me :wink: ) With this, it's a matter of experimenting- brighter is not always better, and experimenting with 3 or 4 different types can be useful. In general I've found that getting LEDs of 1300mcd or so are good "all rounders" for illumination- you can adapt their brightness by adjusting the series resistor.
2. Viewing Angle- if you light an LED and place it flat on a piece of paper you will see a cone of light emanating from the top of the LED. The viewing angle is the angle (in degrees) of this cone. What it means is that if it's placed in a panel, the narrow viewing-angle types will have a sharp pin-point of light which quickly decreases as you move your eye away from face-on. Some are as narrow as 13 degrees. A wider viewing angle, such as 40 degrees, has a much wider "hotspot" of light production. I find that for VUs the wider viewing-angle types are much better, because the dispersal of light is much more useful. Usually you're mounting the LED very close to the meter face, so a narrow viewing-angle LED leaves a visible band of light across the face.
Note too, that the viewing angle and brightness are linked. Manufacturers measure light output from head-on to the LED. So, a 15 degree viewing angle LED will have a higher mcd value than a 35 or 45 degree device. They may actually emit the same amount of light in real terms, but their dispersal is very different. So going for the brightest LEDs doesn't always give the best results. Check the datasheets for the viewing angles before ordering.
3. Type of capsule- LEDs come in two types of capsule- "Clear" and "Diffused". Clear packages are either "water clear"- or coloured to match the colour of the LED (one thing to watch out for- when ordering a load of LEDs which are all "water clear" packages, make sure that the company you order from packages the LEDs in separate marked bags...there is no way of visually telling one LED from another w.r.t colour, viewing angle, brightness etc...you just end up with a bag of identical-looking LEDs... :? ...yep, been there! :roll: )
A clear-package is usually the best bet for illumination- the diffused types are for direct panel mounting as indicators. But sometimes a meter scale is better illuminated by a diffused type- it gives a better spread of light without any "light marks"- i.e. when you see lines and bands of light rather than a diffuse spread.
Other interesting LED info:
Blue isn't just blue, and green isn't just green- there are many colours inbetween the regular colours- if you need a match check the wavelength (nm nanometres)
You don't always need a series resistor for each LED- it's much less wasteful of the supply voltage to place a few LEDs in series, add their forward voltages, work out the current you want them to run on, subtract the combined forward-voltage from Vsupply, and work out the resistor value from that.
Be sure to calculate the power rating of the resistor correctly- P=I^2*R, and make sure you mount the resistor somewhere where its heat dissipation won't affect other parts of the equipment.
You can mount LEDs on a thin piece of stripboard/Veroboard, and it makes a neat "lightstrip" with the resistors mounted on too.
I love LED illumination- no more bulb replacement!
(have you seen those all-solid state domestic bulbs you can get now- a multiple array of white LEDs...amazing...at last, we're living in The Future! :wink: )
Mark
