Luxmeter problem

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?

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

George H.

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.

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 .

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!!

The better photographic exposure meters often have some method for converting incident readings into illumination measurements. If you have a photographer friend, ask him.

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 ?

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 snipped-for-privacy@ripco.com