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Luxmeter problem

Discussion in 'Electronic Repair' started by N_Cook, Oct 2, 2013.

  1. N_Cook

    N_Cook Guest

    ESM1 from 1985, no info found out there. Sort of things carried around
    by environmental health people for non critical measurement, combined
    Lux and contact thermometer.
    States 0 to 3000 Lux on the back of the instrument (for inside rooms) ,
    uses ICL7106 and 4.5 digit LCD with last 0 permanently on. So can read
    up to 20,000 lux, but as only stated to 3000 , is it only linear in that
    range. Has 4 presets , presumably 2 for lux range.
    Tried it this morning about 8am when there was 10 minutes of sun,
    holding so sun axial to the PBW21 sensor, well over range.
    With a 4 stop plus 3 stop neutral density filter , ie /(16x8) ,
    7660 lux, with 4 stop only ie /16 measured 15380 and with 3 stop only
    over range again.
    So next to useless above 3000, or is it to be expected and could be used
    with preliminary cross-calibration source and 2/3/4 stop filters with a
    good stable bright light source?
     
  2. N_Cook

    N_Cook Guest

    the BPW21 datasheet shows log-log plot of short circuit current v lux
    over 6 decades as linear, so something about short circuit compared with
    actual use?
     
  3. Guest

    I assume it's a photodiode. Then short circiut current is very linear. You'd have to share the circuit. But if they just use a simple resistor across the diode to convert I to V, then it becomes non-linear as the diode becomes forward biased and some of the current goes back though the diode and not the resistor.

    George H.
     
  4. N_Cook

    N_Cook Guest

    Sounds plausible.
    No known schematic but I'll dig out the data for the main chip and work
    out what applies to it and lux measurement, which should show which pair
    of presets and which is zero and which is range.
     
  5. N_Cook

    N_Cook Guest

    The photodiode is in series with 18K and one end of those 2 is connected
    to an opamp input and the other end to another opamp input. So cannot be
    fully linear. I.m reasonably confident which preset relates to which
    function.
    Overal zero and overall scaling, then one for scaling the temperature
    function and one for the lux. The extra CMOS for OR driving the always
    on 0 digit and the switching function and maybe auto-off function. As it
    stands with zero lux it reads -80 lux so something obviously adrift.
    Adjusting light levels up from zero , then from -20 to probably +20 it
    auto "zeros" to 0. The temperature scale is easy to check calibration,
    which leaves the lux scale.
    Looks as though the nearest i will get to a reference light source is
    taking the average of a number of 60watt mains tungsten lamps, but then
    www sources say that is betwwen 700 lumens to 900 lumens, mostly about
    840 to 870, and monitor 1 metre away. Accept that a reading of 20,000 is
    nearer 100,000 lux and produce a corelation chart via some unknown but
    constant value of bright source and 1,2,3, and 4 stop (1/2 to 1/16)
    neutral density filters over the sensor .
     
  6. Shaun

    Shaun Guest

    "N_Cook" wrote in message
    ESM1 from 1985, no info found out there. Sort of things carried around
    by environmental health people for non critical measurement, combined
    Lux and contact thermometer.
    States 0 to 3000 Lux on the back of the instrument (for inside rooms) ,
    uses ICL7106 and 4.5 digit LCD with last 0 permanently on. So can read
    up to 20,000 lux, but as only stated to 3000 , is it only linear in that
    range. Has 4 presets , presumably 2 for lux range.
    Tried it this morning about 8am when there was 10 minutes of sun,
    holding so sun axial to the PBW21 sensor, well over range.
    With a 4 stop plus 3 stop neutral density filter , ie /(16x8) ,
    7660 lux, with 4 stop only ie /16 measured 15380 and with 3 stop only
    over range again.
    So next to useless above 3000, or is it to be expected and could be used
    with preliminary cross-calibration source and 2/3/4 stop filters with a
    good stable bright light source?

    Well I wish I could shed some light on the subject..... but nothing comes to
    mind, just this pun!!
     
  7. The better photographic exposure meters often have some method for converting
    incident readings into illumination measurements. If you have a photographer
    friend, ask him.
     
  8. N_Cook

    N_Cook Guest

    Changed the socketed LM358, changed the photodiode, perhaps something
    wrong with the 4066 for switching function feeds as all that is left
    active to change. The main chip was within 1 deg C reading , 20 to
    100deg C on the temp function, but lux a good 1/10 of where it should be.
    Ended up changing 3 presets completely from what they were to get the
    readings to agree with a 60W lamp and c850 lux, 1m from it, including
    the overal ranging preset which obviously messed up the temp function.
    The room temp reads 80 deg C with minimum of its obvious preset, oh well
    , it was the lux I was after.
    Could a 4066 go awry over 30 years, consistent on-resistance changing
    value on one switch and not another ?
     
  9. Amazing it works at all.

    I don't remember what project we were working on but it was something that
    used (if I remember correctly) 4 of them in the circuit and literally were
    ordering them by the truckload.

    Different sources, batches, date codes.

    I know they were a staple of CMOS projects but seemed to be one of the more
    unreliable devices for the era. Many of them seemed to work at first but
    then die after repeated power cycles. Similar to light bulbs, usually
    worked, power down, power up, no worky.

    -bruce
     
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