Build an exposure Meter, Cheap, Easy & it Works

#1

Hi,
Following on from my 'Build a Shutter Tester, Cheap Easy & it Works,

I thought I would start a thread for my latest project, as the title suggests, to see how much interest there is.

The aim is to build an exposure meter for old cameras that either have a failed, or un-trustworthy meter.
It should be cheap & easy to build, using readily available parts & without the need for making or ordering circuit boards or mucking about with perf or vero board. Basic soldering skills will be required.

The unit should be small enough to mount on the hot/cold show if required, but be easily readable. I struggle a bit now to see my Zenit exposure meters.
It should cover the basic iso, aperture & shutter speed ranges & be easy to operate, no complicated features or multi-press buttons.

My prototype is working well. I am waiting for a slightly different processor board & battery to arrive & until they do, I cannot say for certain that it will work ok on a battery, although I cannot see any reason why not.

I will not repeat the setting up & getting started with the processor board & software, this is all covered in my build a shutter tester thread. I will detail the few additional steps required.

Parts count is low. At the heart is a Lolin D32 development board, which includes an ESP32 processor and battery charging circuit, a lipo battery (I have opted for a 1000mA one) a 1/3" Oled display, BH1750 light sensor, rotary encoder and a button.

Additionally some wire is required to connect the parts up and a nice box to put it in. Hopefully those with 3d printers will be able to help us out here.
If you want to use header pins to connect the individual parts, then dupont wires will be required, these are good for prototyping, but the finished meter will be far smaller without.

#2

Below are the parts required. Total price, excluding a box is £13.22
If buying from Aliexpress, try to get as many parts as possible from the same webshop, as hitting the magic £8.18 gives faster shipping.

Sourcing from local suppliers in your home country will of course arrive far quicker, but slightly more expensive.

I have shown a block of four push-buttons. Currently only one is required. The project may develop, requiring more buttons. You may wish to buy a one, two three and four button module.

The oled MUST have a SH1106 driver chip. The sellers description in my list does not specify the driver (some webshop ads do) but the reviews do say it has the SH1106 driver. It must also be the four pin IIC (not 6 pin SPI) version.

The encoder below with a round pcb is a better design than the more common ones with square pcb, as it incorporates RC filtering to help eliminate switch-bounce.

A word of caution with the battery. There is no standard for how the connector is wired & the polarity may be reversed, compared to your board. BEFORE plugging in the battery, ensure the polarity of the battery connector is correct.

This project looks to be over my head, but I’m very interested in the outcome. I have quite a few cameras with dead meters and would like something to slip on the flash shoe that is small, not expensive, and preferably not ugly.

KerrKid said:

This project looks to be over my head, but I’m very interested in the outcome. I have quite a few cameras with dead meters and would like something to slip on the flash shoe that is small, not expensive, and preferably not ugly.

Click to expand...

Hi,
I hope to make it as easy as possible so as many peeps that would like, can build one too.
Have you basic soldering skills? If not, then there are plenty of youtube tutorials, then buy a piece of cheap veroboard & practice soldering wires onto that.

To program the board, it will be a little different to that of the shutter tester. We will be using the expressif flash download tool


There is a guide how to use it here. Don't worry about all of the other stuff in the article, it is just the main screenshots you have to follow.


Here is a youtube demonstration

Happy to help & give assistance.

Thank you. Yes, I have fairly good soldering skills, but could be much improved upon. I love a challenge so maybe I’ll try to build this.

I did start to build one a while ago as I wanted to make a spot meter, but the project fell by the wayside.
Here's what I was basing my design on (https://github.com/vpominchuk/lightmeter).
I did breadboard it and it works very well. I had to rewrite the oled routing partly because the eBay seller sole me a 128x64 screen but delivered a 64x32 or something like that, and the other parts took so long to arrive that the complaint period was over.

Hi,
Yes I looked at that code. I could not work out if it was designed to be an incident or reflective meter. The code made reference to a dome multiplier, but photos showed a bare sensor.

It is also complex, covering speeds & apertures way outside that of a film camera.
I might take the code & update it to run on the same hardware as my meter, so peeps can try either that one or mine. The only addition would be more buttons, Mine has one, t'other has six.

My idea is to make a simple device that covers iso, shutter speeds & apertures found on a film camera & make it small enough to fit onto the hot/cold shoe.

Niglyn said:

The code made reference to a dome multiplier, but photos showed a bare sensor.

Click to expand...

Maybe the dome was removed for the photos, or installed after they were made and the code was changed to reflect this hardware change. He does use a DomeMultiplier constant that is apparently determined through a calibration process. The whole design suggests an incident light meter given how the buttons and display are laid out.

The code isn't too bad actually; it looks like a pretty decent starting point. If I were to do this project, I'd be very tempted to at least borrow a couple of the solutions he has worked into the code.

Interesting project. If you are looking for a smaller processor with a low power mode and support for battery charging take a look at the SEEED XIAO nRF52840 (https://www.seeedstudio.com/Seeed-XIAO-BLE-nRF52840-p-5201.html). It's smaller than the display, but a bit slower than the one you are using.

koraks said:

Maybe the dome was removed for the photos, or installed after they were made and the code was changed to reflect this hardware change. He does use a DomeMultiplier constant that is apparently determined through a calibration process. The whole design suggests an incident light meter given how the buttons and display are laid out.

The code isn't too bad actually; it looks like a pretty decent starting point. If I were to do this project, I'd be very tempted to at least borrow a couple of the solutions he has worked into the code.

Click to expand...

Yep looks to be good, but there are some bits of code around the light measuring that I don't quite understand & that is the important part to know is working correctly.

It complies ok on the Nano, but not on the ESP 32 & I can't work put why. Error message and code segment below.

I have changed the oled to the SH1102 iiC & removed the battery check code.
Removing the bad lines below allows the code to compile & it is running here now.
Another issue us that it is not saving data to eeprom, it always starts at iso 8
Edit - of course on esp32 one has to add the line EEPROM.commit() after eeprom write. Doh )
updat -updat - have added EEPROM.commit(); and it still does not work )



Using library EEPROM at version 2.0.0 in folder: C:\Users\\AppData\Local\Arduino15\packages\esp32\hardware\esp32\2.0.9\libraries\EEPROM
exit status 1

Compilation error: 'float log2(float)' conflicts with a previous declaration

Niglyn said:

Compilation error: 'float log2(float)' conflicts with a previous declaration

Click to expand...

Probably because log2 is a pre-defined function/macro in the C++ version that's used by the ESP32 core. The default Nano core uses a different compiler, so these kinds of issues tend to pop up from time to time.
You could try simply commenting out that entire function declaration and see if it works OK.

Niglyn said:

but there are some bits of code around the light measuring that I don't quite understand & that is the important part to know is working correctly.

Click to expand...

Yeah, I can see what you mean. The documentation could have been more complete/explicit. Looks like he's grapping the lux value from the light meter and then using that to sequentially work out the photography-relevant parameters. It's a bit odd that he's buried most of the logic inside the refresh() function that's specified in the header file. Try pulling that one apart for a bit to see how it works. It's not extremely complicated; it's just a bit chaotic (well, IMO - I guess someone else's code is usually chaotic LOL).

koraks said:

Probably because log2 is a pre-defined function/macro in the C++ version that's used by the ESP32 core.

Click to expand...

Or just do a find/rename on all those names and replace them by something else.

Not being a programmer, my code always starts-out convoluted, but by the end, I've had to clean it up so much that it's actually quite minimalistic.

Yep, I commented out the offending two lines & it runs ok.

Some odd behaviour, it seems one can set the iso from 8 to 4000000 and similar outrageous values for f stop 0 to 3251.
Now whether this is happening as it is now running on an ESP32, I don't know, but can't see how it can. Also there is no f/8 in the list of else statements. Guessing this line just got deleted by mistook at some point.

Also the code only allows a maximum lux of 65534 and then the dome multiplier.
I know the sensor natively only goes to this figure, but can go up to 100 000 lux in hi-res mode., I assumed the library would spit out a number above 65534.
I only have the sensor with the dome, so can't get a reading that high anyway. When my battery arrives, I can take it out & about.

I have knocked up a quick piccy of the hardware, which will allow either the vpominchuck, modified to run on ESP32 code, or my simple exposure meter to work. Code available upon request.
No guarantee either code will work fully.

Looks interesting! I'm going to be starting with the shutter timer, but if that passes the smoke test, I'll move on to this one. More data is always good!

sbhathaway said:

Looks interesting! I'm going to be starting with the shutter timer, but if that passes the smoke test, I'll move on to this one. More data is always good!

Click to expand...

That's great ) but don't forget to post photos. We only have a few completed shutter testers and no exposure meters. If you are going for the ESP32 based shutter tester, code as a .bin file is sent on receipt of pictures )

For the ESP32 version, I'm planning to add connection the the camera pc sync socket, to ensure flash fires when curtains are fully open and also add a light sensor, measuring lux & EV. This way peeps can supply their own light (use an LED dimmable panel, for example) and test the cameras light-meter. it would even be possible to make the shutter tester automatically adjust the light to the users set ev level, I might also do this, but ideas for the future.

Exposure meter V1 code is all written, sadly, nobody has yet requested it. It seems to work ok, my battery arrived a few days ago, so have been adding battery monitoring & power reduction/sleep to the code. It needs a few beta testers, to try it alongside their cameras and light meters, to see how it compares.
Planning to update the thread in the next day or two (package from aliexpress due today) with photos of the bits required & explanation of the functionality (hint, press the button & tells you the exposure ) Said it was simple )

I don't know if that's in the code but it would be super nice to be able to test the delay between flash trigger and light detection. Though that might be better suited to the shutter tester?