Cottage Tip: Building a small exposure meter-Part I

It has been said that necessity is mother of all invention, but I disagree. Other »mothers of invention«, in my view, are also vanity, frustration and other »virtues«, but they are often confused with the first mother, necessity. Anyway, there too many things to list humans invented just for the sake of their own satisfaction, not really necessity, and some of them are even nice gadgets.

I always »needed«-actually just wanted to have-a small, pocket-size exposure meter, but never wanted to spend a small fortune for one. I have my trusty, almost 20-year-old Minolta Auto Meter III which never let me down. But it is a bulky meter. I just wanted to have a small meter when I get out with a 35 mm camera, with no bags or whatever. Thus, the meter should fit in a small pocket. You can even get an old used one for about 20€, but they tend to be unreliable and in many cases, not working. You can get the small and sweet Sekonic Twinmate, but I just never wanted to spend some 100€ or so for one. Nowadays even less so, since for the same money you can buy a film SLR in very good condition....

So, the plan is to build one simple but precise (enough) incident light meter for about 20€ or so in materials and components. The exp.meter must be:

-small

-lightweight

-simple (both in operation and circuitry)

-precise within ±0.3 EV

-usable at least from 15 EV (the »sunny 16« conditions) down to about 4 EV (exposure value, ISO 100)-this is also the range of many commercial meters

I already checked the availability/pricing of components, but they still need to be delivered...in the mean time, I'll lay down the theoretical aspects of this building issue.

The sensor

The easiest approach (and maybe even the most effective) is to use a CdS (cadmium sulfide) photoresistor (light dependent resistor –LDR). It has  the nice property of a logarithmic response (of its resistance) to lighting conditions, but inversely (the resistance decreases with increasing illumination).  The EVs we deal with, are also a logarithmic arrangement, since by every EV step the lighting conditions change by a factor of 2 (1 stop). We can thus get (with a bit of gimmicks) the electrical output (current or voltage) directly related to the EV value. But we also need to make first an appropriate light diffusor (and attenuator) for the sensor (since they are quite sensitive).

The display

The nicest way would be to use an analog milliammeter or millivoltmeter, but unfortunately such small meters are very hard to find, plus they are substantially more expensive than their digital conterparts-and then you can be quickly out of budget just for the meter! Digital millivoltmeters (we need them in the range of ±200 mV) are quite expensive (10-15€), more than half of the budget, but they include all the necessary circuitry for a precise metering. They usually run on 9 volts. You just provide a battery source and the measuring leads for your signal, and of course, some room in the housing of your meterJ The use of a 7-segment or LCD display (while cheaper) would dictate the use of the necessary A/D converter and drivers chips, which would substantially complicate the circuitry and eventually, you could spend (more or less) the same amount of money.

The battery

An ordinary 9 volt battery is just fine, but not of my taste-too heavy and bulky. Instead, you can get a 9V battery of the 23A-type(used in remote controls), but very rare. Or you can use its 12v counterpart (also used in remotes), since its easier to get and cheaper. Smaller than a AAA battery, and you can even adapt a AAA battery holder for it.

The housing

Any plastic case of suitable dimensions can do the job-you can even recycle some old stuff. Or you buy one for a few € or $. Just be sure it can fit the voltmeter, the battery and the circuit. Along with the housing, we also need a switch to turn on the circuit for metering.

The circuit

I really like simple things, so I did not include any integrated circuits, only discrete components, and the least amount of active (semiconductor) components as well. In the hand-drawn circuit draft shown below there are no actual values shown, since I don't know yet how the LDR will behave. Only after that I can calculate the actual values to fit my needs. But I do know that I want the output to be 10 mV/ EV (for increased accuracy); at 15 EV (»sunny 16«) I want to get as close as possible to 150 mV, while at the low end I want about 40 mV for 4 EV.

 

The resistors Rz1 and Rz2 are just regulating the currents for the voltmeter and the sensor circuit, respectively. The first Zener diode ZD1 serves to supply the 9V needed for the voltmeter, while the ZD2 will be set later, when the LDR behavior will be known, as also the values of other resistors R1, R2 and R3. But we do know that the output will be according to this formula:

U_OUT= (U_Z2 * R1)/(Rx+R1)  * R3/(R2+R3)

So we have 4 parameters to play with (U_Z2 and R1-3) to adjust them to our needs. Maybe we can later add a couple of diodes to adjust the input voltage level, but we'll see. Anyway, I like this simplicity and I just can't wait to do the actual test, next time.