Posting a major revision to this project, which has been migrated from Eagle CAD to KiCad.
Library and footprint creation and management is fairly easy in KiCad, and many parts have been created from the datasheet footprints. The Teensy footprint was something of a challenge.
The dew heaters are now controlled by two Murata mini 1.5A variable SMPS regulators. These 5 pin devices can be switched on and off at will and utilise a trim resistor to set the output voltage from 0.6 - 6V.
TEC control has not changed.
Tactile switch pads accept, top or bottom, several types of push button. The footprint is a hybrid.
I have also added 4 mounting holes to mount the board in other than a battery compartment. In which case camera power is provided by the addition of a 2 pin Molex connector to the CAMPWR pads. The plug part of an external supply can be connected to supply a regulated linear 8V - the 450D and 1000D accept 7.4 - 8.1V.
The board could also be adapted to include an intervlaometer using the SENSHT out pins controlled by the Teensy microprocessor and the pushbuttons for setting exposure time number of exposures, mirror lock up and delay between exposures with some fairly straight forward programming.
I have ironed out all known bugs and hardware issues - I think - and learned some things about revision control. It is frustrating looking for hardware problems generated by the wrong software revision - oh well... a learning experience on many levels for an amateur.
Welcome thoughts on the layout of the board - now attached