Hi Grumpy Mike, thanks for pointing out some potential problems I didn't think about before.
The basic idea is to use uno as an interface to drive a downstream device upon receiving an input from a external power source.
The power source itself is a device that delivers -28V to the power output. In it's own circuit, the 0V is used as common, which I suppose is the ground for the circuit within the device. In some of the modules (made by the same manufacture) that can be operated by the power supply, the (+) input to the module is connected to the 0+ wire of output ( and (-) of module to -28V wire). This feature makes me think that drawing positive voltage output from the power supply can be done by the same principle.
In the diagram (apologize that if it's not clear to you),the 0V wire is connected to the input of 7805, and -28V wire to the 7805 ground (and thanks for restating the need of capacitors). Output of 7805 to uno input pin, uno output pin to a BNC connector that will be hooked up to the downstream device. The uno ground is wired to 7805 ground. The metal part of BNC is also grounded to the same ground. Don't know if the attached diagram will be better (and apologize in advance if it's not).
Any other thought will be greatly appreciated!!
