DIY project: simple TTL metering

[Spyderman]

Hi,

I've been thinking about TTL metering for Zorki or FED for quite long. But just yesterday this idea suddenly came to my mind. I mean the circuit that I could use to make the TTL meter... And I'd like to hear opinions from those of you who understand electronics more than I do ;)

So this is the circuit:
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=43729&stc=1&d=1176360959

The circuit uses one CdS cell for metering and 2 LEDs for display. There is one variable resistor for calibration. An IC with 2 comparators (click) (http://en.wikipedia.org/wiki/Comparator) (LM393N (click) (http://www.ortodoxism.ro/datasheets/philips/LM393AFE.pdf)) is used to compare the voltage of CdS and the variable resistor. When the first is greater then a LED will light. Notice the 2 comparators' inputs are inverted (once the CdS is connected to the inverting input and once to the non-inverting input, thus when the resistance is not equal one LED will always be turned on)

The CdS cell will be mounted in the camera body facing the shutter curtain with painted white (or grey) spot and it will receive the light reflected off the curtain.

The basic idea is that both LEDs will turn off (or on) when the resistance of the CdS and the variable resistor are equal. The resistance of the CdS can be controlled by aperture.
And if we assume that the shutter speed is 1/(film speed) we've got a "sunny f16" meter which turns both LEDs off when the aperture is correctly set.
(otherwise we would need to add one variable resistor that would change with shutter speed selected)


From purely electrical engineer's point of view - do you think it will work?

[drewbarb]

I'm no electrician, so I'm afraid I can't help you much with the wiring chart, but you should check out Huw Finney's site. Those of you who frequented Pnet's Leica Forum a few years ago may remember Huw's "Mad Projects", one of which was to put a TTL meter into an old Leica IIIb. There's a link on his site here: http://huws.org.uk/

BTW- I'm sure I'm not alone in wondering of the last "Mad Project"- to convert an old M2 to digital- will ever see the light of day. Here's hoping. Anyway, good luck with your project, and keep us updated!

[Spyderman]

I've seen his site and also his projects. But he uses some digital programmable chip and it's all way too complicated. I was thinking about something a bit simplier ...

BTW: here is a Zorki TTL (click) (http://zenit.istra.ru/qa/tip-zorki-4-ttl.html) which uses comparator circuit made of discrete parts, but the basic idea is similar: 2 LEDs, comparator... but it also has a set of resistors to simulate shutter speed.

[pvdhaar]

Caveat..

The comparator inputs are not going to carry a lot of current, and are their conductance is very temperature dependent. You'll have a hard time calibrating the circuit.

You'll need to connect both the CDS and the variable resistor between power and ground to get a proper (though small) current, and therefrom predictable voltage levels.

Something like this..

[Spyderman]

Thanks Peter, I think I know what you mean...

EDIT: is this (the following picture) what you mean ???

http://www.rangefinderforum.com/forums/attachment.php?attachmentid=43734&stc=1&d=1176363906

[pvdhaar]

Definately better..

[thafred]

Very cool project Ondrej!! don´t have the skills to comment on your sheme but wanted to give a big THUMBS UP!
we need more DIY projects!! :cool:

[Spyderman]

Thanks ;)

BTW: I'd like to mount the IC and the switch instead of the self timer of Zorki-4 or FED-3 (or even FED-5). That way the button that releases the self-timer would turn on the meter and it would be right under your finger when you need it. Only problem would be with FED-2s and Zorki-3 and 3C which don't have self-timer...
I also thought that the batteries (probably 2 SR-44) could be placed on the "floor" behind the lens mount (between shutter and lens mount)...
The CdS cell will be up on the left (when looking at the camera from the front) in a similar place as in new Bessas (R2, R2/3a, ...) and would be weighted towards lower left part of the frame - less influenced by sky in both horizontal and vertical orientation pictures...

Do you know what paint would be suitable to paint the white spot on the shutter curtain ?

[Spyderman]

bump ;)

Anyone else has something to say ? I'd really like to hear your ideas, comments... whatever comes to your mind...

[laptoprob]

If this works I could build it into my M2. Have to open it anyway...
I can ask a pal of mine, hang on.

[JoeV]

I'm just thinking out loud here...

So, since the CDS cell's response to light is only modified by the aperture, this will work in a Zorki or other rangefinder because the iris always changes "real-time" (unlike an SLR, which needs a depth of field preview button to stop down the lens prior to tripping the shutter).

Of course, you'll have to place the CDS cell between the lens and shutter such that it doesn't intefere with the film's exposure. Perhaps just above the edge of the frame, as close to the curtain without interference? You need it as close to the center of the lens' coverage without getting in the way of the picture or the shutter.

How do you intend on calibrating the variable resistor dial; in EV values, based on a fixed EI (exposure index)? What if you change film speeds or shutter speeds; how does that effect the quench meter system? I suppose you could carry with you a conversion chart, for altering film and/or shutter speeds from the pre-calibrated EV's that the system was calibrated for.

Where in the camera body will you place the circuit board and batteries, and make the battery accessible for replacement?

[ray_g]

I seem to recall someone on PNet planning to do something similar to a barnack. You may want to do a search there. Sounds like an interesting project. Good luck!

[laptoprob]

Earlier in this very same thread: Huw's site:
http://huws.org.uk/
It's all about the sunny 16 rule.
I just might put something similar in my M2. My pal the electro wizard is confident.

[mad_boy]

It has been some time since I have been in electronics,
but from what I recall the opamp would be a better main device than the comparator.

on one input you would put the potentio meter to have a reference,
on the other imput the CDS in series to a fixed resistor.
Then you need to loop the output back to the negative input to reduce the ammplification to a factor 50 or so and then go through a resistor to your led.

There are IC's with 4 opamps in them. if you use one for each led with a slightly different reference setting you can also have both leds illuminate at correct exposure.

I so not have time just now to draw the circuit but if you are interested, I could have a go in a day or 10.

mad_boy

[Spyderman]

I had a different idea in mind...


It is supposed to work this way:

- the CdS should be FACING the shutter curtain and meter the light reflected off the curtain - similar as on Bessa cameras (explanation for JoeV)

- both LEDs should turn off just when a certain amount of light is hitting the shutter (this "reference" will be calibrated by the variable resistor)
-> by changing the lens aperture we are changing the resistance of the CdS cell, and when the resistance of CdS and the variable resistor are the same - the LEDs should turn off

- let's suppose the shutter speed is 1/ISO => the aperture metered will be independent of the film speed, because the shutter speed compensates for film speed (slower film - slower shutter speed)


and as I wrote before: Huw's project is nice, but it seems a bit too complicated for me, because he used a programmable IC with A/D converter and light-to-frequency converter... something way out of my league :D But this analogue circuit with comparator seems like a usable and DIY-able ;)

[bobomoon]

I met someone in Seoul, Korea last winter who had an m6 meter installed in his a super brassed black M3. His classmate from high school runs the local authorized Leica repair shop.

He told me his friend took about a year to do it in his spare time and afterwards told him: 'never again.'

[hobbim]

Multiple outputs (like -1, 0, +1, +2 ..) would be very nice indeed. However, it will easily be bulky with all the leds. Only a few leds are easy to fit, even with surface mount leds. I hate the over-under metering with any rangerinder, including the ones I own.

The proposed circuit should work. I'd choose a single op-amp circuit though. One op-amp as the comparator and one led connected from the output to gnd while the other connected to the vcc (with resistors of couse). (One led on when output high, the other on when comp. low). That would simplify the circuit a bit and allows a single tiny surface mound op-amp to be used.

Choosing very bright leds is essential for the battery life. Calibration can be done with a gray card agains another meter. The painted area with the sensor placement defines the metering patterns. Any flexible paint on the curtain should do, for example acrylics. The layer should be thin of cause.

[laptoprob]

Not only would multiple outputs be sensible, multiple parameter settings would make the meter more versatile. In other words: an ISO setting switch. And the LEDs: would a bi-colour green/red be useful? As in wrong vs. correct metered exposure? Or even triple: under, correct and over.
The metering system in the Bessas is nice, but impractical. Since under and over are both red, there is no way of knowing which it is while your eye is at the viewfinder.

[rpinchbeck]

Tell you guys what, if a consensus on a circuit can be agreed to, I am willing to do the CAD for Schematic and PCB. It's what I do for a living dontcha know...

If we want to take it to an actual PCB then we need form factor and a BOM (bill of materials) of readily available parts to design with. I may be able to suck up to one of my vendors for a huge favour and get some PCB's made. I for one wouldn't mind retrofitting the meter in a Kiev 4.

Cheers,

Russ Pinchbeck

[Spyderman]

Thank you for your opinions guys :)

I don't know if a ISO setting is possible... it would need to be a pretty precise variable resistor... or a 3-position swich with set of trimmers for individual ISO values - like 100, 400, 1600 (nobody uses anything else anyway :D)

AFAIK different color LEDs have different parameters (current) and I dunno if they can be combined (correct me if I'm wrong).

Rich: a PCB would be a good idea, but I doubt it could be universal enough to fit all cameras...

[wolves3012]

Spyderman,

I can see a few problems with your circuit, sorry to pour cold water on it...

1) I can see what you were thinking in the circuit, but with no hysteresis the minutest changes would make the LEDs go on & off like mad at the "null" point. A has been suggested elsewhere, an op-amp , with feedback, would be a better choice.

2) Comparators are prone to latch-up . The resistance of a CDS cell can fall to a couple of hundred ohms in bright light, enough to take the inputs close to +ve supply and cause latch-up. Lack of supply decoupling in your circuit (presumably you'd want small batteries) would make that even more likely. You need to limit the input voltage swing by inserting suitable resistors in both sides of the input chains.

3) There is a huge variation in intensity to be catered for, which may mean that your "meter" is not useful in some situations.

4) A CDS cell may simply not be sensitive enough to be useful when using reflected light off a white patch on the curtains - I may be wrong on that since I've never tried it but it could be a deciding factor.

5) There will be sensitivity to voltage change as the batteries age. A bridge circuit to find a null point would be immune to such changes to a large degree. A dual op-amp would allow one amp to be used for the bridge and the other as an inverter to give the over/under indications. It would be easy enough to get a mixture of LED colours that way too. Be generous with supply voltage too, something like a 12V cigarette-lighter/car remote-locking battery would be better than a couple of cells (PX23 springs to mind but I may have that wrong). Comparators and op-amps generally have poor performance on 3V or so.

I'm not claiming to be an electronics expert, but it's been a hobby for many years. I'd be happy for you to prove me wrong on any of the above! Oh, and if you go ahead - good luck!

[rxmd]

Hi Ondrej,

here's a bump and a few suggestions. Disclaimer: I'm not much of an electronics guy apart from a hobbyist's interest, so my suggestions might be complete rubbish. Anyway:

- how about using an SBC phototransistor/photodiode instead of a CdS variable resistor? That way you get a much better sensitivity range. All camera vendors started to use SBC in the 1980s, there is really no reason not to follow their example. Especially when you want to read reflected light off the curtain!

- there should be a switch somewhere to switch the meter on and off.

- there should be some way of compensating for voltage drop. Either put Vcc through a small regulator circuit or use a dual opamp or both.

- there are dual-colour LEDs that are just two LEDs in one case with separate inputs, these would be easy to adapt for this purpose. Red for underexposure, green for overexposure. If you drive both at once, you get yellow.

- even though his microcontroller approach was rather complicated, you might still be interested in Huw Finney's project. He actually made two different versions. The first one used a light-to-frequency converter and read out the frequency using a microcontroller, that's the one you refer to. He later dropped that due to poor battery life. The second one (described here (http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=008bCG)) uses a phototransistor and an opamp/comparator circuit. He then hooks this up to the microcontroller. I guess it would be possible to do without a microcontroller as well. He uses the microcontroller (a) because he apparently likes microcontrollers, (b) because you can do some things more easily in software, such as linearity adjustments or possibly different sensitivity ranges (i.e. ISO selector). I got interested in fitting my Kiev with an TTL meter and asked him for his schematics files, which he sent me; I guess I could forward them to you if you're interested. I also like his idea of putting a thin PCB under the bottom; that way one approach would work for most FSU cameras.

I'm very much interested in your project, partly because it scratches an itch I've had for some time. If you decide to continue in this direction please keep us updated. After September when I'm back in Europe I might get in touch with you about it.

You're next on my list for the FED from Pass the RF III. Any FSU photo gear you might want in the same package? I can look around and see if I can find something.

Philipp

[Spyderman]

Thanks for comments guys...

I'm also not very good at electronics - that's why I posted it here - to getsome feedback and comments (bad AND good).

how about using an SBC phototransistor/photodiode instead of a CdS variable resistor?
I chose CdS cell, because they are cheaper and easier to find.

There is a huge variation in intensity to be catered for, which may mean that your "meter" is not useful in some situations.
That's why I thought of it as a substitute for Sunny16. Not a real meter for extremely low light...

there should be some way of compensating for voltage drop. Either put Vcc through a small regulator circuit or use a dual opamp or both.
I thought comparator comparing voltave drop on CdS and on a fixed calibrating resistor is supposed to be supply voltage independent... right?

Comparators are prone to latch-up
Sorry - I don't understand this...

You need to limit the input voltage swing by inserting suitable resistors in both sides of the input chains.
Please try to draw a scheme.. I just don't get it

A CDS cell may simply not be sensitive enough to be useful when using reflected light off a white patch on the curtains - I may be wrong on that since I've never tried it but it could be a deciding factor.
They are available in many ranges. I thought about using one with resistance up to 0.5Mohm in complete darkness. But the truth is that I have no idea how much light will actually reflect off the curtain and how much it will change the resistance of the cell... I'll have to find out empirically...

[wolves3012]

I thought comparator comparing voltave drop on CdS and on a fixed calibrating resistor is supposed to be supply voltage independent... right?

No - untrue - hence the superiority of an op-amp and bridge circuit. It may be near enough for the purposes you desire, however. In any case a potential divider consisting simply of a chain of resistors (effectively what you have) will always be voltage sensitive. The comparator will also have properties that vary a little. As I said, these factors may not be enough to worry over in reality, but worth considering as possible errors.

Sorry - I don't understand this...

Latch-up is a condition where the inputs are too close (voltage-wise) to the supply rails. In these conditions the output is unpredictable and "latches" to one rail, refusing to change until the circuit is powered off or the inputs change drastically.


Please try to draw a scheme.. I just don't get it

I'll have a go, be patient though!


They are available in many ranges. I thought about using one with resistance up to 0.5Mohm in complete darkness. But the truth is that I have no idea how much light will actually reflect off the curtain and how much it will change the resistance of the cell... I'll have to find out empirically...

I have no idea either, hence just pointing out that they *might* not be suitable.
Whoops - that quote wasn't quite as expected - hope it's clear though

[Spyderman]

Bump... :rolleyes:

Just wanted to let you know I just picked up the ordered parts: comparator (LM339) and some CdS photoresistors (VT93N2) ... and I hope I'll be able to move this project further...

Do you have any new ideas guys? Any schemes for me? ;)

[40oz]

IMHO, calibrating it to just one ISO is preferable. In other words, if the "meter" says I'm golden at 1/15 @ ISO400, doing the math for whatever film I'm actually shooting is cake. It's how I use my Canonet all the time. Considering which ISO to calibrate for depends on the sensitivity of your sensor. I'm not sure the world needs yet another meter that only works on sunny days, however :) But if you get it actually working in any configuration under any conditions at all, I'd think most of the work would be done. Then you can play with resistors and sensitivity until it's perfect and minimal :D

[Spyderman]

After an advice from my friend today I bought me a "solderless breadboard".

I already assembled the circuit but it's not working yet... :( It seems to work, but with supply voltage of 6V there is only 0.5V on one of the outputs and 0V on the other. Any help would be greatly appreciated...

[Spyderman]

Oh my God! it's working!

the problem was the connection of LEDs. They should be between supply voltage and comparator output, not between comparator output and ground! After seeing this page ( http://home.cogeco.ca/~rpaisley4/Comparators.html ) I reversed the LEDs and it's working!

[Spyderman]

Soo.. here is the final circuit - the one that actually works! :cool:

http://www.rangefinderforum.com/forums/attachment.php?attachmentid=46768&stc=1&d=1182802359

[thafred]

Great Ondrej!!
glad to hear your idea is taking form! :cool:

I´d be very interested to know how you will couple it to the shutter, I don´t have any idea how this can be done, oh and in what camera are you going to put it again?

congratulations anyways! sweet project :D

[Spyderman]

Any camera :D but I'm going to try it on a FED-3, then all the rest... ;)

[Spyderman]

I've got some good news.

The circuit itself is still outside assembled only on a "solderless board", but I've tried to place the CdS light sensor inside of a FED-2 body, with a piece of paper placed on the shutter curtain (to simulate white paint on shutter curtain) and lens mounted where it should be, and . . . it's working!

By opening and closing the diaphragm of the lens I am able to find the point where the LEDs switch. There is no point where both are off, but there is a very limited range where both LEDs are "slightly on". No hysteresis problems as someone suggested earlier. Setting the switching point is just a question of calibrating reference resistor.

Now I'm looking for help from someone who knows how to use circuit board designing software (like Eagle or something else). I'd need to design a circuit board with SMD components the size of self-timer in FEDs and Zorkis - 38 x 16 mm.

If you can help, please send me a PM.

[Spyderman]

This is a post to sum up what has been said in earlier posts.


I'd like to make a built-in TTL light meter for Zorki and FED and possibly any other camera with enough space to fit it inside.
use CdS light-dependent-resistor because they are cheap, small and easily available (particularly I use VT93N2 (http://optoelectronics.perkinelmer.com/content/images/largeImages/vt900.jpg) )
place the sensor in similar way like Voigtlander Bessa series - facing the shutter curtain
the shutter curtain would need to be painted white (or grey, but we need as much light reflected as possible)
the circuit itself should be small enough to fit inside the body of FED or Zorki instead of the self-timer mechanism (which is 38 x 16 mm).
the circuit as I designed it is completely voltage-independent. As long as there is enough juice to turn on the LEDs the meter will give correct reading.
the self-timer release button would turn on the meter
meter readout would be by two LEDs of different colour in/near viewfinder (could easily be placed next to viewfinder eyepiece similar to "flash ready" indicators on P&S cameras to be visible from outside)
battery could be placed between lens and shutter (I suggest 3V lithium button cell CR1632 with 16mm diameter)



I suppose the meter would be calibrated for (shutter speed = film speed) thus it would work with all aperture values of the lens mounted. It would not be good for very low light, but it would help in those situations when sunny16 is difficult to use (late afternoon, in shade...).


For example I can use sunny16 with good light from f16 to f5.6. At dusk or in shade I'm lost without a meter. But this meter can measure light from f16 to f2 or even f1.5 if you have Jupiter-3 mounted!


Experimentally I found that with the cell I use VT93N2, with the amount of light for correct exposure (with sunny16 rule) hitting the shutter curtain, the resistance of the CdS cell is about 80-100 kOhm.


And this is the correct circuit:
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=46768&stc=1&d=1182802359



Note:
It would be possible to use more calibrating reference resistors for use of shutter speeds other than (shutter speed = film speed) and a switch somewhere on the camera body to select which resistor is used.
For example 2 additional calibrating resistors: 1 for shutter speed 4-times slower and one for 16-times slower. So with ASA400 film, you could use the built-in light meter with 1/500, 1/125 (4-times slower) and 1/30 (16-times slower). This is already the full range of shutter speeds of FED-2 !
The problem is there would have to be enough space for the extra variable resistors on the circuit board and the placement of the switch. If you could mount this into a Kiev-4's meter housing, it might be possible to incorporate the additional VR and the switch, thus widening the light conditions in which this meter would be usable.

[Spyderman]

Here's my "FED-2 test body":
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=55700&stc=1&d=1202580049


The circuit assembled on a solderless board:
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=55701&stc=1&d=1202580049


and the suggested battery holder and a CR2032 battery. This is slightly too large to fit inside so a CR1632 with apropriate holder should fit.
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=55702&stc=1&d=1202580049



I'm still looking for help with designing the circuit board !

[Spyderman]

... and I'd like to use this type of switch to turn on the meter (the scale is in cm):
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=55705&stc=1&d=1202581303


and this CdS cell:
http://www.rangefinderforum.com/forums/attachment.php?attachmentid=55706&stc=1&d=1202581406

[zuikologist]

Very cool project - I look forward to the results. Do you have a mean of calibrating/recalibrating the meter if needed?

[acheyj]

Hey Syderman.
First of all had a whole bunch of reasons why it wont work, however reading all the post just maybe.
Your idea of emulating the sunny 16 will vastly simplify things. Before I gave up photography late 60's I built a densitometer and later an enlarging exposure meter (I went insane on the Zone system !). Things to watch out for.
1. Stable voltage (will need a vr)
2. Spectral sensitivity the cds I had to work with were red sensitive and lacking blue.
3. Temp sensitive .
Size today is not so bad as surface mount components are really really small, get somebody to do the circuit board and stuff after you have proven up the design on a breadboard. You certainly would find it easier to use a micro-processer for a full blown TTL but thats not your intention. Find a local ham (amatuer radio operator) who dabbles in tracking satellites, we tend to be also slightly demented !. He/She will be a big help and wont cost you (I built my complete computer controlled tracking station from scratch).
I wish you luck and will watch progress with intense interest. You realise of course it will be much cheaper to use a ps digital thing and the prints will be better than 35mm, however thats a side issue and not relevant.

ron (gee I wish I was young again) VK5AKJ (acheyJ)

[Murray Kelly]

Ondrej, you don't tell us how deep the board can be. It sounds like plenty of room and remember there can be two sides to the board, given enough depth.
I will run this past my son who designs this stuff at his work. He made me a very nice digital speedometer for my car.
I will question him about linearity and voltage drop issues.
Using surface mount components it should be a shoe-in.
The switches are a common component - I know them well since I had to replace a bunch in one of my tranceivers.
Good luck with the project.
Murray VK4AOK

[Spyderman]

Well, no news since my last posting, but for Murray: the available height is about 10mm, maybe 8.

My friend was helping me with the board, but he has a lot of work now and no time for my crazy projects.

I'm attaching my friend's design of the board (of course the LEDs won't be on the board but there will be wires attached and LEDs will be inside or near the viewfinder). This is just the first version, so take it as such.

[lex]

looks promising!

take a look at the olympus OM2 shutter curtain (the pattern the meter gets reflected light from). the cds metering on this guy is renowned for its... well, lets just say it is a good meter!

[Spyderman]

take a look at the olympus OM2 shutter curtain (the pattern the meter gets reflected light from). the cds metering on this guy is renowned for its... well, lets just say it is a good meter!

IMHO it's not important whether the curtain is complete grey (Bessa), white or it looks like on OM-2. The important thing is that it reflects light to the metering cell.

[Murray Kelly]

I don't think you'd want something brilliant white inside the camera during exposure. That would contribute to flare? OTOH that would be the best color for low light sensitivity.
The Bessa has the outer shutter to play with. It folds away before the main shutter fires. (Hence all the noise).
It will have to be able to work with a 'spot' or similar.
My son is coming to dinner tonight - I will run it all past him then.
As I look at the PCB it looks like you've made space for the LEDs even tho they will live elsewhere?

Murray

[Murray Kelly]

I had a quick look at that Russian sitewith its very elegant multi resistor arrangement for shutter speeds. I think the 'sunny 16' rule would be easier in the long run.
1. If you want to look back at the shutter a brilliant white line down the edge that retracts into the blind roller first would cause as little flare as could be expected. Say, the width of the gap at highest speed?
2. I looked at my collection (FED1thru 3 and Zorkii-4 and Kiev4 & 4A) The Feds and Zorkii have, as been pointed out, a flat area above the film frame where a CdS cell could live, pointing forward. Very sensitive, too. Problem is, it's out of the frame when you shoot. You'd have to meter with the camera pointing down slightly before actually shooting.
3. The Russian has a bridge with a long-tailed pair consisting of darlington transistor pairs as the detector. Could be made smaller but the bridge and comparator sounds better. What intrigued me was the arrangement of the detector. It faces forward and must flick down as the blind crosses over?
My Russian is confined to yes/no/g'day/goodnight/ thanks. Does it explain how they accomplish this?
4. Nearly all the old cameras (not the Kiev) have this space above the frame and the oldest have this big space below the frame where a SMD PCB could live. No need to gut the selftimer. I think that is where you propose to put the battery? Such a little board could live there, too, possibly?

Just some thoughts.
Murray vk4aok

[Murray Kelly]

As promised I spoke to my son, the electronics engineer.
Firstly he said that modern devices would show little if any drift with the sort of temperatures we're likely to meet.
As for a PCB it would be trivial to design and what you have done is fine, even over done with the LEDS mounted on the board.
I still haven't worked out quite where to put the LEDs.
The hard part would be getting them made (the boards) as commercial houses would expect to make a whole sheet of them. That would be hundreds, they are so small. Not inexpensive, however - some hundreds of dollars. Better learn to etch your own. Fiddly but not impossible.
My sugestion is to make the prototype in 'dead bug' style. Hotglue the IC onto a piece of copper clad upside down and solder the components to the legs. Cut little squares for islands where components are not connected to the ground plane or the IC. Just make sure you mark, clearly, which is pin #1. When its upsidedown it's easy to lose track of the pinout.
Use the smallest resistors you can find. Once upon a time you could get 1/10 or 1/20th watt devices but they have given way to SMDs. I see one can get 1/4 watt resistor that looks as big as 1/8th watt.
I am mulling over the idea of using a 2n5777 transistor - I have some if I can only find the darned things after moving house 5 years ago.
Other devices like diodes and LEDs are photosensitive. The green are best, IIRC. Some very early transistors were in a can to keep out the light. They were mounted in a glass capsule. Take off the can and you have a very sensitive detector. Some were only painted black - scratch the paint off.
I once removed the top of a TO18 transistor and it was very sensitive but it deteriorated over time as the air got at it. Seal a clear window over it and it might last years.

Just some ideas. I feel I haven't helped much. Sorry.
Murray

[Murray Kelly]

Found the 2n5777 light transistors. They are darlingtons too, and will be harder to test than just with the multimeter. More and more voltage needed.

Still don't know where you plan to install the leds.

Murray

[Spyderman]

Thank you for your ideas.

The manufacturing of the PCB is not a problem - there is a guy here that makes them in small batches - price based on size and number of holes to drill. Together up to $ 10 for one.

I'm not that much affraid of the white painted curtain... maybe I'll make it gray. Leica also uses something similar in M6 and newer.
And considering that the CdS are very sensitive to red light, maybe blue-ish spot on the curtain would compensate for the higher red sensitivity...

The project is sleeping at the moment, because both I and my friend have lot of work for school. But I hope I'll be able to work on it during the summer.

I'd be most grateful if you could design a PCB for me. I know the one from my friend is far from perfect. We could work on it a bit more, but we're both just beginners at this.

BTW: if I put the board between the lens and film gate, where could I place the battery ? Any ideas ?

[Murray Kelly]

A quick reply - your friend's board is fine if you just allow for pads to solder leads to the LDR and LEDs instead of mounting on the board. That would save a little space. 'Dead bug' is a very viable substitute for a board. Not in this case, but at high frequency it often performs better than a PCB.

Batteries are a bit of a problem. Maybe (I haven't checked) there's enough room in there for a battery pack with flying leads? Solder directly to the cell?

Or, maybe, on the bottom plate, covered by the carry case. Especially OK if the camera has a hinged back. Excludes the one I want - the FED-2. :-(
I still can't figure where the leds will go. Haven't been into the top plate of any of these. Only the Bessa-R, and that was 'packed'!

Murray

[Spyderman]

There's plenty space for the LEDs :) FSU tops are almost empty...

Especially on a FED-2 I was thinking the LEDs could go beside the viewfinder on the right-hand side. That way it would be visible even when not looking into the viewfinder, but still visible with your peripheral vision when you're looking through it. Also a red-green LEDs would make it easy to distinguish the moment when they're switching...

So basically I've got it all in my head, now just make it real :)

[Murray Kelly]

Found some really tiny LEDs for surface mounting today. Also tracked down the 2n5777 s. Will visit a better store tomorrow if possible for more stuff.

Just because I have them, I am tempted to go the darlington transistor circuit as at the russian site. Your'e right - the batteries are the headache.

Murray