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John L.
02-04-2005, 12:40 PM
Hello everyone. I have a project in mind using the BASIC Stamp, but before I dive in I would like to outline what I am trying to do and get your hints, comments, suggestions, etc.

I have a plastic camera called a Holga. Although basically a toy camera, these are popular with art photographers because the Holga shoots medium format film and the cheap plastic lens produces atmospheric, vignetted, interesting pictures. People often modify their Holgas in weird and excessive ways and, well, that’s why I am here. I want to add a motor winder to the camera.

Here is what the system needs to do:
1. A two-position switch (“main power switch”) powers the system on and off.
2. The user specifies, via another two-position switch, if the roll of film will contain 12 or 16 frames.
3. When the camera back is opened, a contact switch closes, the system is powered on (even if the main power switch is not closed) and a (software) frame counter is set to zero.
4. The frame number is displayed on a small LCD panel.
5. The frame number is stored in non-volatile memory. If the system is powered off for weeks or months and then powered on again, it needs to still remember the frame number.
6. When the camera back is closed, the contact switch opens, and the system is powered off (unless the main power switch is closed).
7. If, when the main power switch is closed, the shutter button is pressed and then released, a circuit is closed which powers the electric motor that winds the film take-up spool via some reduction gears.
8. The motor is a small hobby motor. I haven’t picked the motor yet, but it will probably draw around 80-100mA at 5v DC (higher on stall). It will be powered by 4 AA batteries.
9. The system measures the number of rotations of a shaft in the geartrain, and also keeps track of the elapsed time since the motor circuit was closed.
10. The motor stops after the shaft has made a specified number of rotations, or after a specified time (like 1 second), whichever is sooner. The second condition is in case the motor or gears jam, or the film reaches the end of the roll.
11. The number of rotations is not a constant, but will be a function of the frame number and whether the user specified 12 or 16. If 12 was specified, the frames are wider and thus more film must be wound, than if 16 was specified. Also, as more film is wound onto the take-up spool, the spool’s effective diameter increases and it needs to rotate less with each successive frame. (This is why I want to use a Stamp for this project – otherwise the system could be all mechanical – in addition to the fun of learning something new, of course.)
12. After the motor stops, the frame counter is incremented by 1, the frame number is stored in non-volatile memory, and the frame number is displayed on an LCD.

The constraints on the electronics are:
1. All of the components (Stamp, LCD board, motor controller, and any other electronic components) must fit in a volume 2.0 inch x 5.0 inch x 1.5 inch.
2. The electronics will be powered by the 4 “AA” batteries that power the motor.

I was tentatively planning to use these components:
1. BASIC Stamp 2.
2. BPI-216 serial LCD from Scott Edwards (a smaller LCD would be fine, but I couldn’t find one).
3. A power MOSFET to drive the motor (as suggested in a BASIC Stamp FAQ I found). I’ll have no need to vary speed or direction.

I was tentatively planning to measure the shaft rotation by using a notched disk that will open and close a circuit every time it rotates 10 degrees or so. So the Stamp can measure the degrees of rotation by counting the number of signals sent to a pin.

My plan is to buy the BASIC Stamp Discovery kit, do some of the projects in the book to learn about programming the Stamp, build the system on a carrier board (wired to the camera which will have the motor and gears installed) to make sure it works, then mount the Stamp and other components inside the camera. There will be no room for the carrier board, so I presume I will have to solder wires to the Stamp’s pins – unless some other connection method is better? I have no experience with the Stamp, but I have experience with Visual Basic, and am looking forward to learning a bit about how to program a microcontroller.

However, I want to find out if my project sounds practical, before I launch into this. Also I would appreciate any suggestions on the components, and indeed on the whole plan. What do you think? Thanks!

Justin Pentecost
02-04-2005, 04:27 PM
It's an interesting project and certainly viable.···· I've built stepper motor drives for motion picture cameras, and I can confirm that the stamp is brilliant for these things.

As far as the motor is concerned I would use a small commercially availaibel motor with a reduction gearbox.·· It's good practice to mount an encoder on the motor shaft (you would probably only need to count the revolutions if you did that).

Some years ago I was looking into the viabilty of converting an RB67 into a camera capable of shooting time-lapse Imax footage.····· We were looking at useing steppers to drive the sprocket and a simple PWM for the takeup and a vacuum to activate the registration pins.

We dropped it when we realised how precise the regestration pins would have to be.

Could you read the "Stop point" from the paper useing an infrared sensor ?·· Is there an ISO standard for the paper markings ?

Justin
·

steve_b
02-04-2005, 08:18 PM
Neat project.· I don't know the size of the camera you're working with....but by the time you get an LCD/motor/stamp/components, you've got another box that's getting close to the size of some slimline camera's.

I don't know if there's an magnetic affects on film, but a hall affect shaft sensor might be an idea.· Will InfraRed affect film?· High speed film maybe?· Anyhow, hack an old mouse and steel the encoder out of it.· Lots have done that with some success.· there might be some code snippets out there.

If you use a stepper, you will be able to count how far you wind.· You have to be careful that you don't lose your position when cycling power (you could write the position to eprom).· OR, you might determine some gearing and use a 10-turn pot (maybe 20turn?!) that will keep a measureable reading through your entire film length.

I'm not camera savvy at all....but if you want the film to be loaded and unloaded...wouldn't you need two separate motors?

You need to pull the film from the spool to the take-up reel on the opposite side.· And when you're done, you have to pull back from the reel back to the spool.· Film is too flimsy to push, so pulling is the idea.· IS the gearing from the spool connected to the other side so that they both move the same amount?!· That would be ideal!

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Steve
http://members.rogers.com/steve.brady (http://members.rogers.com/steve.brady)
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

John L.
02-04-2005, 09:26 PM
These suggestions are great. I had not thought about using a stepper motor. The Stamp could command a precise number of small (e.g. 4 degree) rotations, and the system would not need to externally measure the rotations. This sounds promising. I will go look through some catalogs to see if anyone makes a stepper of appropriate size, power draw, with a reduction gearbox.

If I do use a conventional motor, I don’t think film is sensitive to small magnetic fields, so a Hall effect sensor is possible. I will also dissect a mouse (humanely, of course) to see how it works.

Nor had I thought about optically determining the film position. Unfortunately I don’t think the markings are standardized enough. All medium format film brands have frame numbers marked on the paper backing, and they are all at the same position (relative to the edges of the film). However, the size, font and look of the markings differs. Sometimes it is just a number, other times it is a number in a circle, etc. The size of those markings is similar to the desired spacing between frames, so variation (e.g. the width of “1” differs from the width of “10”) might cause inconsistent frame spacing. There are often other markings on the paper backing, which vary with the film make and type. Traditionally, the user manually wound the film while looking through a peephole in the camera back and watching for the frame numbers printed on the paper backing. So some film brands have “you’re almost there” markings before the actual frame numbers – and some don’t.

Oh, I should have clarified about the film – I’m sorry. Medium format film is not rewound after the last frame. The film has a paper backing, and when it is wound all the way on to the take-up spool, the last foot or so of paper backing protects the film from light. The take-up spool and film are removed from the camera as a unit, and the film is unwound in a darkroom.

Thanks for the great ideas. As you can see, none of them had previsouly occurred to me, so this is very useful.

Hulk
02-04-2005, 09:52 PM
John,

Are you going to do the dark room work?· Or, more to the point, can you prepare the unexposed film by punching small reference holes that your BS2 can use to determine film position?· I think you could use a bread and butter IR sensor to sense the hole.· I don't think your film will be sensitive to IR.· I am assuming B&W film, color may be exposed, however.

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http://a5.cpimg.com/image/C9/48/8184265-56bd-00410041-.jpg

Basic stamp Newbie

Jim McCorison
02-04-2005, 11:03 PM
John,

A couple of thoughts.

1) The stamp would be great as the program is stored in EEPROM and the EEPROM can also be read and written by the program. Your program should be small enough to have plenty of room left over in EEPROM to store frame count and other status information.

2) Most cameras that I'm experienced with use one of the alignment rollers on either side of the image area (sorry, don't know the technical term) to determine the film position. This is more accurate as the diameter of the take up roll isn't a factor. Perhaps you could hack the camera to put an encoder in this roller.

3) For a display it seems like you really only need two digits. Yes? What about a Varitronixs VI-201-DP-RC-S LCD display. It is a 2x1, digit height of 13mm, and is only 28mm x 30mm (WxH). They are $3.10 from Mouser. You'd have to add driver circuitry, but the end result would be much smaller.

Jim

steve_b
02-05-2005, 12:54 AM
Film usually has those track holes (like the old track-fed printer paper) so setting up an IR sensor/opto aligned to these holes would allow you to count how many have gone by.· I'm sure they're a standard distance apart.· so, in order to get to the next film 'spot' move the stepper until the IRcount = X (X being the number of holes to pass).



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Steve
http://members.rogers.com/steve.brady (http://members.rogers.com/steve.brady)
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

John L.
02-05-2005, 03:59 AM
My fault for failing to clearly describe medium format film.· Medium format film does not have sprocket perforations along the edges, as 35mm film does.· It is simply a long strip of film (appx 2.5 inches wide and roughly 4 feet long) with no perforations, with a long strip of paper backing that extends for several inches beyond the beginning and end of the film.· It is wound on a plastic spool.· You place the spool·on one side of the camera, pull the end of the paper backing·over the film gate and insert it into the take-up spool, wind·a few turns, then close the camera back and (in older cameras) keep winding while peering through the little hole in the back, at the paper backing,·until you see the printed·number·"1".

John L.
02-05-2005, 04:06 AM
Wow, that Mouser store is great.· Thanks for pointing me there.·

Justin Pentecost
02-05-2005, 06:30 AM
I was murfening around doing some other bits and bobs today and came accross the mechanism for a CD drive ..The motor and lead screw was pulled out to make a motion controled filter mover that is not 100% finished.·· Old 2 or 4 speed CD drives are ten a penny these days so it's easy to look at what I mean ..

The motor has mouse like encoders on the shaft and then a secondry reduction gear.· If you fitted that onto the front of the camera (fashioning it into a handle) and then·fitted a large gear·instead of the winder knob then you would have (in theory) a very slow but very accurate motor drive.

I've been thinking how you could avoid useing a flipping great lcd display ..· how about this? ..· texas instruments part no .. TIL311· ?··· It would light for 10 seconds after taking a photograph and "On demand".

·

John L.
02-05-2005, 12:52 PM
Well, I dropped by Radio Shack and bought the "What's A Microprocessor" kit.· Now it turns out that none of the working computers in the house have a serial port.· So I am spending the evening reviving an old P166 laptop that does have a serial port.· Ah well, there's always an intermediate step.

I·took apart a mouse and see what you mean·-·I could possibly adapt the mechanism·to measure shaft rotation.· Still, I'm intrigued by the idea of using a stepper motor and thus dispensing with the need to measure rotation at all.·

Well, I will start learning with this kit (after Windows finishes loading on this old laptop) and will surely be back for more help, support and encouragement when I'm further along.· In the meantime, I will be reading old posts to pick up more ideas.

By the way, I might end up using a largish LCD after all, because the logical next project would be to add a lightmeter, for which it would be useful to display a bit of·text, e.g. "f6.7 125iso".



·

Justin Pentecost
02-05-2005, 05:14 PM
My advice would be to forget stepper motor's.· I think they would be too heavy for this application the small motor and encoder idea is much better IMHO.

If you do go for a lightmeter then that's something I'd be interested in co-operating on.···· I have various projects that would be very usefull with a stamp based light meter.

Justin

Gadgetman
02-05-2005, 07:00 PM
Steppers aren't that heavy...

Take a look at an 3.5" floppy drive and you'll find two of them.
One to rotate the floppy at a constant speed, and another with a wormdrive to move the head.
I'd imagine that the one to move the head would be just about perfect for this job.
(Or maybe one from a 5.25" drive as they're slightly larger)

I wonder...
Is that camera anything like my Lubitel?
(it also uses 120 Roll film, but has a setting of either 12 6x6cm or 18 4.5x6cm pictures, and have two glass lenses, one to expose the film and one to act as viewfinder, not too unlike a Seagull)

Can your camera use 220 film?
(It's twice as long as 120 film)

While you're at it, why not stick another stepper onto the aperture control, too?

Do you need to 'wind up' the shutter any way?
(You don't want to know how many double exposures I've made by accident)

Bean
02-05-2005, 10:41 PM
For the display, I saw at Wal-Mart you can get a pedometer for a couple bucks.
I think you can also get one at McDonalds for a dollar.

It counts steps, so you should be able to control it by just clearing the count and then clocking it up to whatever number you want.

Bean.

John L.
02-07-2005, 01:16 PM
The Lubitel TLR, from what I know about it, is a much better camera that the Holga. It has glass lenses and can chose different shutter speeds and apertures. The Holga, in its stock form, has one shutter speed (appx 1/125) and one aperture (something like f11). And the lens is plastic and vignettes in 6x6, but that's considered part of the "Holga look". Basically the Holga is a $20 plastic toy camera, that people like for that "look", and that can be easily modified.. Some people have installed Compur shutters for full shutter/aperture control, Polaroid sells a back that make the camera a "Holgaroid", one guy has added a Nikon F-mount, etc.

I've modified my Holga to use the aperture assembly from a junk lens. Someday I might try the Compur route. I'm currently planning the motor drive modification, as discussed. Someday I'd like to add a lightmeter (I think basically that means a Cds cell, a capacitor, RCTIME or similar code, a switch or button to set film speed, and a display.to show the result, in f-stop or exposure value). Which is why I will probably use an LCD panel that would be overkill for merely displaying the frame count.

The shutter does not need to be wound, it is very a primitive camera - not any more sophisticated than a disposable film camera, except that the lens can be focused.

Post Edited (John L.) : 2/7/2005 4:32:09 PM GMT

Gadgetman
02-07-2005, 04:01 PM
If the Holga vignettes in 6x6, how can you choose 16pictures instead of 12, then?
(The Lubitel uses an inlay frame and a plate on the back can be turned to cover the usual frame-number window and expose another. Don't ask what happens when you only do one or the other...)

John L.
02-07-2005, 11:27 PM
The Holga's film gate can be unsnapped and swapped. There are two gate sizes, 6x6 and 6X4.5, and without a gate you get 6x6 (but with film flatness problems). The Holga doesn't actually vignette much at 6x4.5, which is why everyone shoots it at 6x6.

So, if I put a stepper on the aperture lever, combined with a lightmeter, I could in theory get an auto-exposure Holga, eh?· Well, I think I'd better chew one bite at a time - heck, I'm still working through the exercises in "What's A Microcontroller".

Post Edited (John L.) : 2/7/2005 4:30:10 PM GMT

kochiro
02-09-2005, 06:00 AM
John,
Interesting project you got going there. Just a quick question about the radio shack kit you got. Is the Stamp removable or is it one of the ones where it is permanently affixed to the board? I was thinking about getting it but didn't want to if the Stamp isn't removable.

Thanks,
Jay

John L.
02-09-2005, 07:29 AM
It is not removable. I didn't realize that, and would have bought a different kit if I'd known.

kochiro
02-09-2005, 09:07 AM
Thanks John. You just saved me some money.

Jay