The Pyxis power supply board consists of the following units;
- Analog ±16V (500 mA) supplies.
- Peltier supply 1 (0-12V, 2A).
- Peltier supply 2 (0-6V, 3A).
- Shutter driver (5-9V, 12-24V over voltage pulse).
- Filter wheel controller (12-28V).
- Digital 5V supply (1A).
All of these supplies require a 10-14V (12V nominal) unregulated DC input for efficient battery operation in the field. Switching efficiencies are generally 80% or better. The supplies are carefully filtered on both inputs and outputs to minimize switching noise coupling into the analog circuits. All noisy (high current draw) or critical supplies are connected to the 12V input through a common mode choke to avoid noise coupling through the ground line. The Peltier supplies can be automatically shut down during CCD readout to further reduce the noise. Operation off 115V AC mains requires a separate 12VDC supply; these are readily available commercially. The power supply is controlled by and ATTiny16 MCU that communicates with the main controller board using a simple serial interface.
Analog supply 
The ±16V analog supply employs a flyback switching with the switching controlled by an LM2588-ADJ (U8) controller operating at ~100 kHz. The circuit is not activated until the "ANALOG_POWER" line (see schematic) goes high; this line is normally directly set by the controller board MCU. In this way, the analog supplies can be shut down to save power when the camera is not in use.
The flyback transformer is fabricated from an Amidon pot core with the center post on both halves of the core ground down (approximately 0.3mm) until the correct inductance is obtained with the number of specified turns. It is essential that this be done or the ferrite will go into saturation, overheat and crack. The transformer should be filled with potting compound (any epoxy will do) or it may "buzz" - but don't do this until you've tested the transformer! Be careful when installing the transformer to ensure that the secondary wire are connected in the correct orientation. This will depend on how you wound the transformer (the dots by the transformer windings tells you the correct phase orientation - look this up if you aren't familiar with this notation). If you still aren't sure about the phases on the secondary, place a moderate load (200 Ohm, 3W resistor) across both ±16V outputs with the secondary transformer phases only loosely soldered to the board. Briefly power the circuit and observe if both outputs can be brought to rise above 16V when trimming R14. If not, then the phases are reversed and you should swap the position of the secondary wires.
The PCB layout assumes that inductors L1 and L2 (350 µH, 600 mA) are wound on Amidon pot cores, however any off-the-shelf inductor with the rated DC current handling and approximate inductance will work. Despite the loss of efficiency, the supply is post-regulated with LM317 (positive circuit) and LM337 (negative circuit) linear regulators. This is to ensure maximum noise rejection. The supply outputs can be set as low as 15.3V to minimize power consumption (the regulators used in the controller and camera boards have a 0.3V dropout) however it is better to leave at least 0.2V of additional headroom to account for voltage drops along the cables, etc. Setting the outputs to between 15.5V to 15.7V was the compromise taken. Note that regulation in the negative line of the switching supply is slaved to the positive supply output. The absolute value of the positive and negative outputs may therefore differ considerably. This difference can be minimized by ensuring that the inductance in both halves of the secondary are reasonably close. This also means that it is best not to operate the supply with strongly unbalanced loads on the two outputs when one half of the supply is drawing a large fraction of its maximum current.
The analog supply provides the +15.7V (nominal) used by Peltier power supply #2. For this reason, it is best to populate and test the analog supply circuit first. Remember that the analog supply will not operate unless the "ANALOG_POWER" line is set high. This is normally done by the controller board, but with the controller board disconnected (recommended during testing) one needs to manually tie this line to 5V. Note that tying pin 1 of U8 to 5V will not work. It is important that the voltage at the +17.6V node (marked in the schematic) not exceed 20V or the voltage rating of certain components will be exceeded, not to mention the additional load on the switching regulator and the flyback transformer. The trim resistor R14 should be set to produce the nominal voltage (within 0.1-0.2V) at the +17.6V node.
Peltier supplies
The Peltier power supply for camera 1 uses a flyback topology whereas the supply for camera 2 uses a buck topology. Both supplies use power MOSFETs (IRF1310) as switches. The MOSFETs are driven by the pulse width modulated (PWM) outputs of the ATTiny26 MCU. Regulation is provided by feeding back each output voltage through a resistive divider to two of the ADC inputs of the ATTiny26. The PWM is set to its maximum resolution of 256 clock cycles so that the operating frequency is 62 kHz (i.e. the PWM frequency is the clock frequency, 16 MHz, divided by 256). The pulse width is modulated according to the value of the feedback voltage read by the ADC. The ADC sampling rate is 1 kHz, so the transient response of the regulators is poor. This is of no consequence because the power draw from the Peltier elements is very stable. The regulation voltage is acquired from the controller board and depends on the CCD temperature feedback.
On startup, the PWM outputs are disabled (target regulation voltage of 0). Unless testing firmware for the ATTiny26 MCU is written, the supply board must be connected to the controller board (and the controller to the PC) to operate the Peltier supplies. Never disconnect the voltage feedback lines or modify the ATTiny firmware involving the voltage feedback without fully understanding what this involves! If the feedback is disabled, then the software will try to compensate for the "apparently" low voltage by increasing the PWM duty cycle. If there is a light load on the supply, the voltage on the output could rise to dangerously high levels of 60V or more! I have cooked several parts in this way myself.
The custom transformer for Peltier supply #1 is somewhat simpler to construct than the analog supply transformer because it only has a single secondary winding. The transformer is wound on a pot core with a ground down center post to avoid magnetic saturation at the high current levels involved. As is the case with the analog supply transformer, one must pay attention to the phase of the secondary winding. On enabling the Peltier 1 supply from the PC software, it will ramp up to its full output voltage over the period of a few seconds. This will not happen (and you will hear scary noises) if the supply is under load, and the secondary wires are incorrectly oriented. If this happens, quickly shut down the supply and reverse the wires.
Peltier supply #2 is more foolproof, it uses a commercial inductor and requires no custom parts. The maximum voltage on both supplies is set by trimming resistors R13 (supply #1) and R16 (supply #2). Both supplies can be manually disabled by switches. The switches may be directly replaced by TTL/5V-CMOS logic level lines to provide an interlock if desired.
Shutter driver
Most electronic shutters use a solenoid activator. These solenoids typically require between 4 to 6 times the holding voltage of the solenoid to be briefly applied during the opening of the shutter. This is in part due to the greater force required to open the shutter than to hold it open, but mainly to overcome the inductive reluctance of the solenoid. The approach taken here is to use a boost regulator (LM2586-ADJ) to provide the initial high voltage pulse, followed by a linear regulator to provide the low-voltage holding level. While the shutter is being held open, the switching regulator is disabled so that the DC current into the linear regulator first passes through the switching inductor (L8) and the output diode (D13); this introduces no additional losses to the linear regulation. The holding voltage can be set from 5 to 9 V. The over voltage can be set between the input voltage (12V nominal) and 28V.
The shutter opening sequence after receiving the opening signal is as follows. The LM2586-ADJ is enabled and the output is allowed to rise for 80 ms, this charges the output capacitor C42 that provides most of the opening energy. After 80 ms, the MOSFET switch for the shutter power is closed allowing current to flow into the shutter solenoid. At approximately the same time, the 80 ms LM2586-ADJ enable pulse returns low, shutting down the high-voltage supply. The shutter remains open until the shutter control input goes low. This scheme allows a shutter rate of approximately 5 Hz. The shutter control accepts input from the controller board as well as from a manual switch.
Filter wheel controller
The filter wheel controller consists of a 12V-24V boost converter to supply power to the DC motor driving the filter wheel. The supply input can be bypassed through the input select pins if lower voltages are required. The controller uses a monolithic MOSFET H-bridge (MC33887) which greatly simplifies the driving circuit and ensures robust operation. The H-bridge is controlled by the ATTiny26 which sets the power polarity and the enables/disables power to the motor. A single digital line into the ATTiny26 provides feedback on the filter position. The current firmware expects one state transition per filter increment. Three lines (+5V, ground and digital input) are provided to implement the position feedback device. My prototype uses a reflective sensor and an encoder with alternating dark and reflective sectors to indicate the filter position. An electromechanical switch could also be used. The ATTiny26 applies a delay once a state transition is detected before any other polling is done on the position feedback line. This avoids problems with multiple transitions due to switch bounce or other transients. I highly recommend building a motorized filter wheel into your camera from the start because hands free operation of the filter wheel is a great help when doing colour photography on small, shaky instruments.
Digital supply
This consists simply of an LM7805 fixed output regulator (U1).