Cottage Tip: Building a small exposure meter-Part II

I got (almost) all I need to build the actual meter (save the voltmeter, it should take quite longer to come, not really sure why). I got the LDR sensor, so the first thing I did was to build some kind of enclosure and light diffusor around it. I just recycled a piece of black plastics for the base and the middle of the (Fuji) film canister cap for the diffusor. I drilled two holes in the base for the sensor leads and painted the inside of the cap with white opaque nail polish (taking care to make an even layer). When the nail polish dried, I glued the sensor on the base and then glued the white-painted cap onto. I know, it is not exactly a dome-shaped diffusor like in commercial meters, but probably (hopefully) will do the job more or less in the same way. It is more like a »hybrid incident light adapter« between the dome-shaped and the flat diffusor (the ones used to asses the contrast ratio). See photos below.

Starting materials for the sensor: light-dependent resistor (LDR), black plastics for the base, white nail polish and a Fuji film canister cap.

LDR glued to the base, and the cut mid-section of canister cap painted inside.

The sensor assembled on the breadbord (don't mind the resistors nearby, they remained from a previous project).

The so-prepared sensor was ready for testing! Unfortunately I came home quite late, so I catched the last sun rays. There wasn't a 15 EV intensity anymore, but only about 13.5 EV.  Then, I measured the response-resistance down to about 4 EV at different values.  I then plotted the dependence of LDR resistance against light intensity (EV). The outcome was quite a nice exponential curve (as it should be) with a very good correlationship.

The testing rig: multimeter measuring sensor's resistance and the Minolta exposure meter for getting the actual EV value.

I then used the obtained formula of the curve equation to calculated the predicted resistance at a given EV value (also for the points I did not measure). Then, the calculated resistance values served to calculate an appropriate series of voltage values to be obtained between Rx and R1 (voltage drop across R1; see the previous post). For that purpose I used the first part of the formula:

U_OUT1= (U_Z2 * R1)/(Rx+R1)  

Where U_Z2 is the voltage of the Zener diode (supplying the voltage to Rx and R1), Rx is the value of the LDR and R1 is the chosen resistor value.

Now, I must confess, I wasn't really picky about the Uz and R1 values, but I tried to match them to what I have at hand (and/or combining various values), but anyway, I wanted to get satisfactory results, at least. So for Uz I chose a Zener diode with voltage drop of 3 V and for R1 I chose the value of 3200 ohms (3k+2x100 ohm).

I got this, quite a linear curve:

The curve equation now tells me that if I want to get the output of about 10 mV/EV I first need to add (offset) 1290 mV to this (voltage) signal and then divide it by a factor of about 28.4. Very luckily to me,  1290 mV is quite exactly the voltage drop of 2 regular diodes connected in series(cca 1.3V)! This is not necessarily the case, but luckily for me, it was. Otherwise, I would need to use another Zener diode and a trimmer to adjust the offset voltage, in a slightly different circuit arrangement. Using a different LDR  and light diffuser would certainly yield different values and curves. For the voltage divider I didn't use exactly the factor of 28.4, since the calculation  gave too much shift from the theoretical values, especially at high EVs (where the meter is used mostly). Given my resistor choices, I opted to use 1267 ohms for R2 (1k+220+47 ohm) and 47 ohms for R3. This gives a ratio of 1:29.57. By applying this ratio and the voltage bias of 1300 mV (two diodes), it gave me the following (calculated) measurement error at different values:

At first, it doesn't look like nice. But, we seldom use a meter below 6 EV (very dim light), and the error of around 0.3 EV is totally acceptable in practice for cameras and meters alike. Only between 11 EV and 13 EV the error is quite large, but as long as we know how much the error is, we can always correct for it. But clearly, all this is still theory only. The practical measurements will tell how good or bad the meter is.

Anyway, at least I came up with the final version of the circuit, with resistor and diode values to test, and hopefully, solder into the circuit board. See below (this is only the signal part of the circuit). But let me stress once again: this circuit is (should be) suitable for MY very own case of sensor, not necessarily (or likely) yours!

The signal part of the circuit I came up with.

The diodes D1 and D2 are just ordinary small-signal diodes. The Zener diode, as said, has  a drop of 3 volts, while the resistor Rz has been set arbitrarily at 4.7k, just to get a current somewhat higher than 1 mA, (at high EV values current can approach about 1 mA in this configuration). Voltage drop across Rz is 7.7 V (12-3-1.3), divided by 4700 yields about 1.6 mA. The photo below shows a more general (and probably also more appropriate) case of a signal circuit-using another Zener diode and a trimmer potentiometer to adjust the bias voltage. The latter is probably the largest source of measurement error in such a meter:

A more generic signal circuit.

During the weekend I'll test the sensor and circuit on the protoboard (»breadboard«), and I am quite anxious to get the results, which I'll promptly report to you. If time will permit, I'll also get in the final construction till next time.

Silver regards

Mitja

CORRIGENDUM: While the circuit in the penultimate photo (without the trimmer pot.) is in principle OK, your restless editor forgot for a moment a basic aspect of Ohm's law, and a vital coefficient....Therefore, the correct values for R2 and R3 are 1.267 M ohm and 47 k ohm, respectively. I apologize for that.