The SEI LED replacement light source.
Yer bog standard SEI LED
vanity version with extra LED
I had a think......
One white led filtered to about 3000K and for the deluxe version an
extra (adjustable) led just to illuminate the photocell for the vain
a working meter, there is only one of these in the whole world and it
owned by Roger Hicks.
For the filtering Lee filters had the solution and gave me a
swatch of some 250 lighting filters, one turned out to be perfect.
The LED comes from Marl, a bit pricey at
2UKP each but guaranteed colour temp. and quite bright, so bright the
warns of eye damage!, distributed by Farnell.
The LED has a negative (i.e. dimmer when hotter) temperature
coefficient, so when it's sunny and hot you would overexpose (perfect!)
a bit, the silicon diode (circuit D2) also has a negative coefficient
and is put in series with current detecting resistor chain (circuit R1,
2 and P1) so the current rises with temperature. Tests using my
freezer and oven at about -10C, 20C
and +40C (that was quite hot to hold) revealed no measurable change at
Certainly within the useable accuracy of the SEI (about 1/4 of a
of error when setting the match) thats about 1/12th stop (6%).
my L-308 moved from f8.0 at 20C to f8.2 at -10C.
A MAX1674 converts the 1.5V cell to 4-5V for the LED, the current
through the LED controls the output and is temperature compensated by
the silicon diode.
The MAX1674 circuit takes 40mA, this will give 450 hours using a 'D'
cell. The fun bit is soldering the MAX1674, it's in a uMAX pack,
the legs are on a 20thou (mils for the Americans, 0.5mm for the
Europeans) pitch, thats a 10thou gap. I have three 1674s as samples
from Maxim, they have survived de-re-soldering twice so far. The
choke is a bit big for the current required but the 1674 has a 1A limit
internaly, I will use the 1675 with a 500mA limit and a smaller choke
for the next lot. Assuming anybody wants one (or more).
To buy one of these hand built pieces of engineering joy, fully
calibrated, mail me firstname.lastname@example.org
Circuit diagram for MKII