front panel LED help sought

Brent Hilpert hilpert at
Wed Jan 14 23:02:01 CST 2015

On 2015-Jan-14, at 6:53 PM, Alexandre Souza wrote:
> On Thu, Jan 15, 2015 at 12:44 AM, drlegendre . <drlegendre at> wrote:
>> Howdy gents,
>> On Wed, Jan 14, 2015 at 3:28 PM, Alexandre Souza <
>> alexandre.tabajara at> wrote:
>>> Jay, leds hasn't changed much from years ago. Electronicaly, i believe
>> any
>>> led of same size/format/color will fit.
>> As one who has recently rebuilt an early 70s Altair 8800, I can inform you
>> that a great deal has changed, in the LED world.
>> The very inexpensive 'standard' red LEDs which I obtained to replace the
>> failed units in the Altair were magnitudes brighter than the original
>> parts, when fed from the same power sources. I ended up having to increase
>> the value of the series resistors by hundreds or even thousands of ohms,
>> just to maintain a similar level of brightness vs. the old LEDs.
>> I'd have to take a look inside, but I believe the values jumped from 300R
>> to several K in most cases. The new tech is so much more efficient than the
>> stuff kicked-out in the 70s!
> drlegendre, he will change ALL of the leds, by what he said. So he doesn't
> need to worry about how bright they will be. Also, there are hi-bright leds
> and lo-bright leds, I believe he will use the least expensive, less bright
> ones by default.

Nonetheless, even 'normal' modern LEDs can be much more efficient than LEDs of the early-mid-70s, and even if replacing all of them for uniformity they might look out of character or non-original if not adjusted for brightness.

The target current for an LED in the 70s would typically be 20mA as I recall, while 10mA or less will generally do today.

A few months ago when that television series about the PC-era compatibles-vs-IBM story was being mentioned on the list, I went off to watch the preview on youtube. There was a scene where the engineer was hacking over the IBM-PC to get ROM-dumps or some such and had a bunch of breadboards filled with blinken LEDs. Being in critic/pedant mode, as soon as I saw the lit LEDs I immediately thought "LEDs didn't look like that in 1983".

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