front panel LED help sought

tony duell ard at
Wed Jan 14 15:42:14 CST 2015

> Virtually all of the LED's on the front panel board had so much mouse pee
> that the leads on the LEDS actually corroded through, so I need to replace
> them all.
> Without schematics, can anyone suggest how I might go about determining a
> suitable replacement LED from an electrical perspective?

Long answer : 

Trace out the circuit for one LED and its driver.

Most of the time such LEDs have a series resistor. The +ve side of the combination goes
to a supply line, the -ve side to the collector of a driver transistor, the emitter of which is
grounded (this may be an open-collector gate like a 7406 or 7407). From that you can 
estimate the voltage across the LED+resistor (= supply voltage - saturation voltage
of the driver transistor (say 0.5V)). Then the current through the thing is 
(that voltage - forward voltage of LED)/series resistor. Substitute the forward voltage of
some possible replacement LEDs and see if the current is sensible (compare to the 
maximum current on that LED's data sheet).

Short answer : 

Go and get some standard (not high brightness, high efficiency, or anything fancy
like that) red 0.2" (5mm, whatever) LEDs. Solder them in. I will bet it will work fine.


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