DIY project: simple TTL metering

S

Spyderman

Well-known
Local time
10:15 PM
Joined
Apr 8, 2006
Messages
1,429
Location
Bratislava, Slovakia
View My Gallery
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:
attachment.php


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) (LM393N (click)) 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?
 

Attachments

  • schema s komparatorom.gif
    schema s komparatorom.gif
    10.6 KB · Views: 0
Last edited:
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!
 
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) 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.
 
Last edited:
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..
 

Attachments

  • quicky.jpg
    quicky.jpg
    4.1 KB · Views: 0
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!! 😎
 
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 ?
 
Last edited:
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?
 
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!
 
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
 
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 😀 But this analogue circuit with comparator seems like a usable and DIY-able 😉
 
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.'
 
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.
 
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.
 
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
 
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 😀)

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...
 
You must log in or register to reply here.
Back
Top Bottom