Pimping my Film Drum Rotation System
In the quest for more consistent 4x5 film development I have taken a few steps outlined in past articles. The next 'criticism' of might setup might be the use of a Cibchrome drum rotation system that only rotates in one direction. Looking at Jobo and other solutions I would like to target 25 RPM as a lower bound of speed and reverse every rotation or two. I timed the rotation unit and it already achieves the lower bound at 25 RPM.
I looked at the power supply and confirmed this was a DC power supply (12v on the nameplate) so I knew I could control the direction with polarity.
Next I opened the motor base by carefully prying it apart with a table knife. The two parts of the housing are held by interference fit of the 4 holes that hold the rods in place. Once inside I could measure the voltage and current (18v peak, 200mA peak). I also saw there was lots of room for electronics.
My approach which is simple, but massive technological overkill, is to use a small Arduino Pro Mini.
There really is not much to control here. I just need a DPDT (Double Pole Double Throw) relay to reverse the polarity as the speed seems suitable. The other problem is how to make a 5 volt power supply from the 18v (12v) power supply used for the motor to power the Pro Mini.
I managed to find a nice motor H-bridge controller (L298N) at Flux Workshop on eBay that meets the voltage and current requirements (by miles again overkill) and will also produce a regulated 5 volt power supply to power the Pro Mini. While this is over £8 compared to £2 for the relay I liked the simplicity of not having to wire another circuit. (You could use the relay and wire a 7805 regulator (plus capacitors) separately for the 5v to make it cheaper.) I did experiment with using PWM to control the motor speed but it seemed to not make much difference so I abandoned this feature (though it is still present in the schematic).
The schematic is simple as follows...
The 12 volt from the transformer goes into the L298 motor controller (red PCB) where it is shunted to the motor depending on the state of the IN1 and IN2 pins. An on-board 7805 regulator produces the 5v to power the Pro Mini. The ENA signal is wired up but not used.
The inside of the motor housing is spacious to easily fit the modifications. I glued the PCBs to the side of the case with silicone adhesive. Note I left the programming pins facing upwards in case I want to make a software modification. I covered the motor housing with plastic tape to reduce the risks of short circuits.
I wrote a simple program that runs the motor in one direction for 2 revolutions then halts for one second before reversing direction for 2 revolutions. The halt is to dissipate the inertia before reversing direction. The code is below (graphic). Link to txt file here... Short video of operation as well.
I looked at the power supply and confirmed this was a DC power supply (12v on the nameplate) so I knew I could control the direction with polarity.
Next I opened the motor base by carefully prying it apart with a table knife. The two parts of the housing are held by interference fit of the 4 holes that hold the rods in place. Once inside I could measure the voltage and current (18v peak, 200mA peak). I also saw there was lots of room for electronics.
My approach which is simple, but massive technological overkill, is to use a small Arduino Pro Mini.
 |
| Arduino Pro Mini |
There really is not much to control here. I just need a DPDT (Double Pole Double Throw) relay to reverse the polarity as the speed seems suitable. The other problem is how to make a 5 volt power supply from the 18v (12v) power supply used for the motor to power the Pro Mini.
I managed to find a nice motor H-bridge controller (L298N) at Flux Workshop on eBay that meets the voltage and current requirements (by miles again overkill) and will also produce a regulated 5 volt power supply to power the Pro Mini. While this is over £8 compared to £2 for the relay I liked the simplicity of not having to wire another circuit. (You could use the relay and wire a 7805 regulator (plus capacitors) separately for the 5v to make it cheaper.) I did experiment with using PWM to control the motor speed but it seemed to not make much difference so I abandoned this feature (though it is still present in the schematic).
The schematic is simple as follows...
The 12 volt from the transformer goes into the L298 motor controller (red PCB) where it is shunted to the motor depending on the state of the IN1 and IN2 pins. An on-board 7805 regulator produces the 5v to power the Pro Mini. The ENA signal is wired up but not used.
The inside of the motor housing is spacious to easily fit the modifications. I glued the PCBs to the side of the case with silicone adhesive. Note I left the programming pins facing upwards in case I want to make a software modification. I covered the motor housing with plastic tape to reduce the risks of short circuits.
 |
| Internal Arrangement |
I wrote a simple program that runs the motor in one direction for 2 revolutions then halts for one second before reversing direction for 2 revolutions. The halt is to dissipate the inertia before reversing direction. The code is below (graphic). Link to txt file here... Short video of operation as well.
 |
| Source Code (graphic see link above for txt file) |
Comments