The PCB design is pretty simple and comprises the following sections:
- 12V input
- Connectors for 5V regulator for Raspberry Pi and DC motors
- MCP23S17 12 bit IO chip connected to Raspberry Pi SPI port
- 2 x XPPower LED Driver modules + filter and control voltage circuitry
- Headers for 2 x Big Easy Driver stepper motor controllers
- Optically isolated shutter release for external camera
- Mosfet switches for power to 2 x DC motors + LED light
- 5 x GPIO header for unused bits on MCP23S17
- Headers for toggle switches for power to LEDs + Stepper motors
- Headers for 2x power LEDs, 2x DC motors, 2x 4wire stepper motors
- 26way header for Raspberry Pi GPIO port
The PCB is 10cm x 10cm, and is designed to connect to the Pi with a ribbon cable. It might be possible to connect it directly to the Pi - but a spacer may be required to clear the Pi's ethernet port, and allow space for the camera ribbon cable.
It is a pretty simple circuit - all through hole components, except for three power Mosfets. It uses modules for the 5V power regulator, LED drivers, and stepper motors. Power is designed to be provided by a 12V supply, rated at 3A+, that uses a standard 2.1mm DC jack plug.
5V is provided by an inexpensive LM2596 based module, soldered to headers on the PCB. It's also possible to use an alternative buck converter using the pads for an alternative header. It's important to use a module that can cope with 2A output. I found one of the two LM2596 modules I bought dropped the voltage too much when the DC motors were powered, causing the Pi to reboot. Not good. The other one works fine. On the prototype I also used a hacked 2A rated car-USB adapter.
The Raspberry Pi is powered through the 5V line on the 26way connector, so as this avoiding the protection on the Pi, it is very important to check the voltage provided by the regulator so that it exactly outputs 5V. These modules can be set to output anything from 1 - 12V. Adjust the trimmer to the correct voltage before connecting the Pi.
Very Important
Using a SPI controlled 16 bit IO expander (the MCP23S17) means that everything can be controlled simply using the Python bindings for WiringPi. It also means that the telecine control scripts can be run as a normal user.
LEDs take advantage of XPPower's LDU05/07/14 Series constant current drivers. I have tested with their 700mA, 350mA versions. Details and a link to the datasheet are available here. Datasheet PDF The input filter using a 68uH inductor is from the datasheet. The inductors are also available from Farnell
The LED drivers also allow for brightness adjustment using a voltage control. I implemented this using a trimmer potentiometer and voltage divider.
Stepper motor drivers are Big Easy Stepper drivers available from Pimoroni, Hobbytronics, Sparkfun and other places. They're really easy to use, and support 1/16th microstepping, which allows for fine tuning of the position of the film. The drivers are plugged into header sockets on the PCB. It's possible to use screw terminals on the BED boards, or use 2.54mm headers for the motors. As the BigEasyDriver boards are controlled by 3.3V logic, the 3V/5V jumper pad has to be bridged with solder.
Mosfets on the board are the only surface mount components - these are in SOT-223 format which is pretty easy to solder. IRLL024NPBF. From farnell or cheaper from CPC These switch the power to the DC motors and for the LED modules. These are logic level MOSFETS, and are rated to 55V and 4.4A, and require only 2.2V to switch on fully.
Camera release is based around a pair of 4N35 optocouplers. With a 2.5mm jack it can be used for Canon and Pentax DSLRs. Might also be suitable for other cameras. The optocouplers isolate the camera circuit from the Pi. This isn't used for my current project, but means the PCB is versatile, so could be used with an DSLR like the original Kinograph system - or allows the PCB to be used as a timelapse controller.
PCB was designed in Kicad, and I created modules and footprints for the BigEasyDrivers, LED drivers and DC module. It was made by Smart-Prototyping, as they were having a special offer on 10x10cm boards.
[Kicad project files are here](../hardware/RPI telecine circuit.zip).