For everyone, who is enthusiastic for old analog cameras with build in light meter, there can appear one problem. Since most of these cameras are build in the 70s/80s, the used foto sensors are really old and may stopped work in a proper way.
In this instructable i'll give you the opportunity to change the old electro mechanic display against a LED light meter.
The toughest task was to implement the electronics plus battery in the small space inside of the camera and still have all the LEDs directly beneath the indication window (see picture). Therefore i added this instructable to the small spaces contest. If you liked this, please give a vote =)
In my case the camera is a voigtländer vito clr.
Step 1: The Old Light Meter
The old one works as a simple voltage meter. Behind a transparent plate of the camera is a sensor. This sensor is a solar panel/foto diode system, which appears as a current source, if light passes the active plane.
This sensor is connected to a coil system, which moves a needle.
If there is enough light on the sensor, the current causes a magnetic field in the coil and the needle starts moving. This is equal to old VU meters, used in several applications. With this technique, the caused photocurrent and movement of the needle are some kind of proportional and therefore this movement indicates the amount of light.
A big negative point of some of those old sensor types is, that they age with time and the output current per lux (unit for light intensity) becomes less with each year. Therefore, at some point of aging process, the sensor element cannot source enough current anymore and the needle wont move.
One can think of changing the sensor element with a newer one, but my experience was, that the sensors used in the 70s are made of some kind of toxic metal and are prohibited now and the newer ones either dont fit in the cam or they dont sources enough current into the old coil/needle system.
This was the point, when i decided to change the whole lightmeter to a newer one!
Step 2: Designing the New One
Since the old VU meters with coil and needle are now changed to newer LED driven ones, i decided to do the same.
The idea is, to measure the signal, which comes from a photo sensor, amplify it to a proper range, and display it with a row of leds.
To achieve this, i used the LM3914 IC, which is a pretty great tool for driving LEDs and sensing voltages. This IC senses an input voltage (against a reference) and displays it with a single led out of a row of ten LEDs.
This made designing the rest of the circuit really easy!! The hardest part is to fit the values to your sensor element. You have to measure voltages and amplify them in a proper range for the IC. You have to experiment a little bit and therefore need a multimeter.
I used a photocell (from an old calculator) and placed it behind the transparent plastic of the camera. Then i measured the current with no and maximum light (a few mA). Since i needed a voltage but have a current source, i implemented a transimpedance amplifier, aka a current driven voltage source (see Wikipedia for further informations). The resistor R4 defines the amplification of the current to voltage. A load resistance will cause less current to flow, so you have to experiment with your type of sensor, resistors and the amplifier. Make sure you connect the cell in the right way, if you measure nothing at the output of the opamp, change the polarity. I used something in the kiloohm range and got a voltage level from 0V to 550mV. R1, R2 and R3 define the reference voltage level from the LM3914.
If we want to measure the IC against 5V, we have to change their values to that range. With R1 = 1k2 and R2 = 3k3 (R3 = not connected) and got a reference of 4.8 V (see datasheet for further informations). With this reference, i have to amplify the signal i already have - this is also necessary to buffer the impedances caused by the current driven voltage source and decouple the source from the sensor element = making sure, the current stays stable and independent of the load resistance.
The necessary amplification in my case is at least 4.8V / 550mV = 4.25 - I used R5 with 3k3 and R6 with 1k.
The whole circuit will be driven by battery (i used 2 coin cells with 3V each, and a regulator to get stable 5V from these 6V.
Remark for C5 and C7: The fotoelectric sensor measures light, as you now already know. When i build up the first test board, i recognized that only one LED was on, if i measure natural light - this is what should happen! But as soon as i measured the light from lightbulbs, at least 3 or 4 LED where on and this is not what the system was supposed to do (since the indication is not clear now).
Lightbulbs are driven with 50Hz/60Hz mains and therefore the light flickers in this speed - too fast for us to see but fast enough fot the sensor. This sinusoidal signal causes the 3 or 4 LEDs to be active. To get rid of this, filtering the signal is absolutely necessary and is done with C5 in series with the sensor and C7 as a lowpass filter in combination with the opamp.
Step 3: Perfboard Build
I built up the first test on a perfboard. It is important to do that, because the size of the resistors have to be chosen from the measures you can only do with a proper working test circuit.
As soon as i used proper sized resistors and implemented the filter capacitors, the circuit worked pretty well and i designed the PCB layout.
You can try it with my choice of resistors, but it may not work properly.
I dont think that you can use a perfboard for your finished system, since the space in the camera is way to small. Maybe it will work if you think about using a SMD perfboard.
Step 4: PCB Build
The PCB has to fit in the inside of the camera, therefore one has to use SMD components (except for the LM3914, because i already had it available). The shape of the PCB is designed exactly for the dimensions of the camera. The opamp is a standard opamp (lm358) with single supply and the regulator is a simple 5V constant voltage low dropout regulator (LT1761). The whole ciruit is implemented on two single PCBs.
The battery part and the electronic part. I implemented everything on the same PCB, because i only have to order 2 times the same PCB, which is cheaper than buying two different types. You can see the footprint of the battery holder overlaying the other circuit parts in the second image.
The assembled PCB in the images shows the two sides of the electronic-PCB and the battery part. Both are screwed together and became a two-storied system.
A on/off switch is necessary, because the system will sink current from the battery even if no light is measured. Because of that, this battery had to be changed very soon. With a switch, the system only measures, if its necessary.
Step 5: Results
The results are shown in the images and the video attached.
I used a real light meter which i lent from a friend for calculating the right aperture @ shutter speed (see the drawn table on the cam in picture 3) by using a light source. I hold the sensor in the direction of the light until a special LED level (like LED no. 3) is reached and then measured the appropriate shutter speed at aperture with the professional light meter.table
I think you can use other methods, like an android app light meter, as well.
I hope you liked my idea and this instructable!
Greetings from germany - Escobaem