[Elecraft] [MM] The W1 LED Experiment

[Dave Van Wallaghen]

I apologize for taking so long to conduct the LED experiment and get 
back with everyone interested. I had some lead times on parts I ordered 
not to mention getting some time to run everything through. I must 
mention first that I had to do numerous swaps of different LEDs and 
resistor packs and this simply would not have been possible to do in a 
timely manner without using my new Hakko 808 desoldering tool. I'm sure 
I would have damaged the W1 PCB trying to do it manually over and over 
again. That thing was worth every penny I paid for it.

In the end, as others have pointed out here, even though I used LEDs 
with similar specs as used in the existing W1 design, there are vast 
differences in brightness when placed side by side with each other. I 
was looking for the right combination of LEDs and resistor packs to 
obtain a good level of illumination as well as balanced brightness among 
all of the LEDs and keep the current consumption as low as possible. All 
of the LED part numbers and current measurements will be listed at the 
end of this message.

I did order the Avago LED from Digi-Key specified by Elecraft just to 
make sure the it was in fact the same and it was indeed the same LED 
used in the current W1. Despite the spec sheet, it is much brighter than 
any of the other LEDs I tested. Anyway, I went down the same road that 
Doug Joyce did and ordered a green, yellow and red set of similar LEDs 
from Lumex. I then ordered a number of different resistor packs to 
replace RP1 & RP1 with values of 2.0k, 1.0k and 470. These LEDs were 
much too dim with the existing 2.0k resistor packs installed.

I did manage to come up with the combination Lumes LEDs and 1.0k 
resistor packs that satisfied my requirements. They are still a bit 
dimmer than the existing red LEDs but when placed side by side, I could 
not really see that much of a difference with my eyes. I did drop the 
resistor packs down to 470 ohm and they were certainly bright enough, 
but the current consumption increased by more than double of the 
designed consumption.

Still somewhat confused, I did manage to find similarly spec'd Avago 
LEDs from Mouser that had part numbers similar to the red ones used in 
the W1. So I ordered those and installed them (along with the original 
2.0k resistor packs) and much to my dismay, the green and yellow LEDs 
were much dimmer than the red.

My test setup consisted of using my K2 into a coiled wire antenna to get 
a > 5:1 SWR when putting 1 watt into it through the W1. This got 18 of 
the 20 LEDs lit (trying for all 20 often switched the W1 up to the next 
power scale). I used a 13.5v bench supply and put a DMM in line to 
measure the current consumption during each iteration.

The slight differences in forward voltages of the various LEDs played no 
appreciable role in current consumption. So I'll just list the 3 
resistor pack values I used. I did look up the LM78L05 regulator spec 
sheet and found out it will produce up to 100ma output current. The 
current W1 design operates at far less than that. The LEDs are probably 
pulse width modulated (or simply polled) by the PIC also so the current 
draw per LED seems to be about half of what you would calculate with a 
voltage of around 3v and the resistor pack value.

Here is some of the data:

W1 idle current: 13.8 ma

Current draw regardless of LED type per resistor pack value (18 out of 
20 LEDs lit):

2.0k - 27.5ma
1.0k - 39.3ma
470  - 63.0ma

I wanted to try an 810 or 680 ohm resistor pack, but could not get them 
in time from Digi-Key. As I personally wanted to stay within the 50ma 
range, the 1.0k value works for me. Like I said, to my eyes in a well 
lit environment, this value seemed to work fine with the Lumex LEDs 
although the Avago red LEDs were a hair brighter.

Part numbers:
My configuration:

All Digi-Key part #'s

Lumex Red 2x5mm LED    - 67-1047-ND
Lumex Yellow 2x5mm LED - 67-1050-ND
Lumex Green 2x5mm LED  - 67-1046-ND

1k resistor pak 10 pin - 770-103-R1KP-ND

Other values tried:
1.5k resistor pak 10 pin     - 770-103-R1.5KP-ND
1.2k resistor pak 10 pin     - 770-103-R1.2KP-ND
470 ohm resistor pack 10 pin - 770-103-R470P-ND

Avago red LED used in W1 - 516-1281-ND

Avago green and yellow LED's - Mouser Part #'s:
Avago Green 2.5mm LED  - 630-HLMP-S501
Avago Yellow 2.5mm LED - 630-HLMP-S301

I hope this helps anyone looking to put a colored SWR scale on their W1. 
This is certainly not a sanctioned change by Elecraft, in fact, I don't 
know if anyone has even talked to them about it. So all of the usual 
disclaimers about destroying your equipment apply ;-)

73,
Dave W8FGU