Here is the description of a shutter we made for our Audine
camera. After we did some tests with electromagnets (often large, or little
but too weak, as in the relays), we had the idea to use a little motor,
which works like an electromagnet, and furthermore has a circular motion
which can be very usefull for a shutter device.
we use directly the motor's axle to carry the sheet of metal (in red on
the picture) which will close and open the shutter. On the picture, we
can see the following parts:
- The main stand, 4mm thick, milled on 3mm with the appropriate shape in order to permit the sheet of metal to move. We also milled a rectangular hole (but it can be a circular one) in front of the camera's window (dimensions of the hole: ~18x21mm). Remark that if you can't mill, you can build this part using two superimposed plates (more simple to make).
- The shutter (in red on the picture) made with a thin sheet of metal (brass for example); it's also possible to use plastic sheet.
- The closing sheet, which also carry the motor, realised in an aluminium sheet, 1mm thick; remark the different holes (rectangular one and 4 circular one to fix the shutter on the camera.
- The DC Motor, mounted on the closing sheet; It's alxe carry the shutter sheet.
The whole device is then 5mm thick, and one can mount
other parts (filter-mount, adapter for photo lenses,...)
the opposite photo, the milled stand, with (on superimposition) the shutter
sheet closed (yellow) and open (blue).
Remark here a little modification: the stand is higher and we milled horizontally in order to mount a spiral spring.
Operation of the device
The motor is feed with 5V DC supply. The opening is made manually with a switch. A DEL visualises the opening
When the motor is not feed (closed position), the spring holds the shutter sheet on a closed position. When you pull on the switch, the motor makes moving the sheet and the spring is bended... the shutter is now open. If the motor is no more feeded, the spring brings back the sheet to it's initial position and the shutter is now closed.
We use this shutter usually for 20 or 30 seconds exposure time. We didn't remarked any overheating of the motor, even during the focusing step, when the shutter was open several minutes long.
We made some measures of the minimum exposure time, using
an optic coupler and the chronometer function of our frequencymeter.
When using it manualy (pulling on the switch), it's easy to produce a 0.050 sec. exposure time (1/20 sec.), and sometimes 0.012 sec (1/80 sec). It would certainly be possible to have a lower exposure time using an electronic device for the shutter release.
An other solution we tested is to change the shutter sheet:
instead of opening and closing the shutter, it's possible to make a hole
or a slit passing by the CCD. We made an operational prototype and we measured
exposure times of about 0.003 sec (~1/350 sec.) with a 3mm thick slit.
It's of course possible to higher this speed (close to 1/1000sec) using
a thinner slit (about 1mm thick)
picture, the two parts of the shutter, ready to be assembled.
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