One for the electronics experts part II...

[mpl330]

Hi guys,

Seeing as the FMC project is now on hold awaiting the tactile switches I have started looking at replacing the cards in the OH. I have successfully traced all the 'switches' / inputs to their respective wires on the ribbon cables and now need to do this with the LED's/outputs. My plan was to simply stick the voltage down the wire and see which LED lit up but:

a) Do OC master cards always output 5v?
b) If they do I am going to need to know what voltage the LED's will take to work out what resistors I need (and also when I am testing them to see which is which) so is it possible to work out what voltage a LED will be as I do not know as they are already in place and I can only see the soldered legs so I do not even know what colours they are?

Foolishly I already played with sticking 5v down 1 wire  - it lit up! Then fizzed and went off in a puff of smoke so I do not want to repeat this and have to replace all the LED's!!!

It's a learning curve!
Cheers
Mike

[jackpilot]

I think that if you test with 2 AA batteries in serie (3V) for a few secs nothing bad should happen.
JP

[Speedbird17]

Hi Mike

I use one of these

http://hobbyistcentral.com/electronics/2009/05/kemo-led-tester-review/

It's easier than trying to figure out how to connect two AA in series, (kidding!)

Ralph

[jackpilot]

Good point:
For this side of the pond:   http://www.allelectronics.com/make-a-store/item/LT-100/LED-TESTER/-/1.html ($8)

[ian@737ng.co.uk]

what oh mikey........
most common LED's will be 2.2v forward voltage, so if you are using 5vdc to test, simply solder a 330ohm resistor in you positive lead.    then your LED's won't 'pop'.
and if you don't have any, give me mail and i'll send you some.
good luck.....   ian

[mpl330]

Thanks again chaps!
I obviously had a stupid moment and just stuck 5v down the wire - just as I did so I remembered about the resistors (they dont use them with the fsbus cards for some reason)...
I thought I read somewhere that different colours may use different voltages - is that likely to be the case - in the OH there are green, blue and amber...

I'll give it a try with a 330...

Thanks again
Mike

[ian@737ng.co.uk]

hi mike.......
depends on the exact LED.   u can get LED's 1.7v thru 2.7v, but me and Mr Ohm sort of worked out that 330ohm was a good safe value.    i actually use as low as 220ohm if the LED seems a bit dim.   i have been known to go down to a 180 (i'm a little devil aren't i).
good luck matey....
regards ... ian

[jackpilot]

  i have been known to go down to a 180  ... ian

[mpl330]

  always living life on the edge Mr S!
Will give it a try - few little presents have been arriving in the post:
new glue gun
some connectors for the ribbon cables
and an AFT OH from Jakub in Poland - quick turn around on that one and he was on the ball for checking I received it ok (typically the UK couriers tried to deliver and took it back to the depot but never left a card 

One of these days I will fly this thing!!!
Cheers
Mike